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JOURNAL OF ANTHROPOLOGICAL ARCHAEOLOGY ARTICLE NO. 16, 189 – 225 (1997) AA970309 Hunter–Gatherer Foraging Strategies in Tropical Grasslands: Model Building and Testing in the East African Middle and Later Stone Age Curtis W. Marean Department of Anthropology, SUNY at Stony Brook, Stony Brook, New York 11794-4364 Received October 30, 1996; revision received February 24, 1997; accepted May 1, 1997 Hunter–gatherer adaptations to moist tropical grasslands are not well known from either the ethnographic or the archaeological record. This is unfortunate as grassland adaptations are clearly significant to human biological and behavioral evolution. The most effective strategy for remedying this problem is to develop models for grassland exploitation based on strong understandings of the ecological similarities and differences between cold, temperate, and tropical grasslands. Cold, temperate, and tropical grasslands are similar in that water and raw materials are often scarce and the most abundant large mammals are gregarious and mobile. Tropical grasslands differ from cold and temperate grasslands by having a greater diversity and biomass of edible above-ground plants and plants with underground storage organs, making carbohydrate availability greater and less seasonal. Large mobile mammals and resident large mammals are more diverse and have greater biomass in tropical grasslands. Overall, tropical grasslands are a richer and less seasonally punctuated environment than either cold or temperate grasslands. A comparison of ethnographic data regarding variation in foraging strategies in different cold, temperate, and tropical settings lead to the construction of three models for hunter–gatherer exploitation of tropical grasslands: a Generalized Grassland Model (no specialized tactical hunting—considered the favored model given modern African grassland conditions), a Seasonal Grassland Model (only seasonal use of specialized tactical hunting techniques—considered unlikely for Africa), and a Specialized Grassland Model (regular use of specialized tactical hunting strategies—considered highly unlikely for Africa). A preliminary test of these models shows the Athi-Kapiti Plains Holocene archaeological evidence is most consistent with the Generalized Grassland Model. The Last Glacial Maximum is most consistent with the Seasonal Grassland Model. A single MSA occupation also suggests that specialized tactical hunting strategies were used. These differences in hunting strategies were probably due to the differences in ecological conditions between the Holocene and the Last Glacial Maximum. q 1997 Academic Press Wissler (1927: 22 – 23) wrote that ‘‘all Plains tribes seem to have practiced cooperative hunting in an organized military-like manner.’’ By ‘‘military-like’’ Wissler is referring to the communally organized hunting that often used natural or modified features of the landscape to tactical advantage. The Eskimo practiced similarly complicated hunting maneuvers, and both Plains Indians and Eskimo aimed these military-like tactics against large gregarious mobile species, the bison and the caribou, respectively. Tropical grasslands in Africa team with large mobile prey that often pack closely into huge herds. Not one observation exists, however, of African hunter – gatherers in grasslands using the military-like hunting techniques described above for Plains Indians and the Eskimo. Why is this? Addressing this question is essential for understanding hunter – gatherer adaptations to tropical grasslands, and understanding human evolution overall. The idea that tropical African grasslands were specially significant to human evolution has been with us a long time. Dart (1925) thought that life on the grasslands provided the selection pressures that shaped the human intellect. Many models of human evo- 189 0278-4165/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved. AID JAA 0309 / ai04$$$$41 10-07-97 09:20:38 jaaas AP: JAA 190 CURTIS W. MAREAN lution identify grasslands as a likely ecological context for the development of bipedal locomotion and other physiological, anatomical, and behavioral characteristics of hominids (Isbell and Young 1996; Lovejoy 1981; McHenry 1982; Wheeler 1984, 1991, 1992). Grasslands figure prominently in most hominid foraging models (Blumenschine 1987; Bunn and Ezzo 1993; Foley 1987; Potts 1988; Rose and Marshall 1996). It is likely that African grasslands will play an important role in the debate over the origins of modern humans, as the variants of the Replacement Model argue for an origin in Africa sometime between oxygen isotope stages 8 and 6 (Aiello 1993). Though the paleoenvironmental data for this time are poor in Africa, data from the more recent Late Quaternary show a tendency for most of Africa to be colder and drier during glacial periods with a consequent expansion of grasslands (Deacon and Lancaster 1988; Hamilton 1982). Thus, grasslands were likely to have been abundant during isotope stages 8 and 6 when modern humans were evolving. From a distance grasslands seem similar; expansive landscapes of swaying grasses and slowly migrating ungulates. In reality there is great variability within grassland ecosystems and between grassland ecosystems, particularly in the availability of those items most significant to humans: water, plant foods, animal foods, and raw materials. I use the term ‘‘grassland’’ to refer to any herb-dominated vegetation community (De Vos 1969) — typically grasses but also including sedges. One similarity that crosscuts all grasslands is the dominance of the faunal biomass by large mammals (Coupland 1993: 479), those typically in the size range (above 5 kg) preferred by hunter – gatherers. Second, most of these large mammals are gregarious and mobile grazing species of ungulates. This similarity combined with the well-known ethnographic record for Great Plains Indians has structured most of our ideas about grassland hunter – gath- AID JAA 0309 / ai04$$$$41 erer existence. The diversity and density of these mobile mammals is highly variable, however, and tropical grasslands typically have an abundance of non-mobile large mammals that may be important sources of food. Equally significant, grasslands in different climatic zones vary in the density and availability of plant foods edible by people, as will be discussed below. Despite the recognized significance of grasslands, it is humbling to realize that we know very little about hunter – gatherer adaptations to tropical grasslands, though we know more about hunter – gatherers in cold grasslands and temperate grasslands. As defined here, cold grasslands occur roughly at 60 degrees latitude and above. These environments are often dominated by sedges or grasses with significant lichen and moss representation and the large mammal communities are dominated by migratory caribou (Pielou 1994). The two cold grassland ecosystems discussed here include the Barren Grounds steppe on the north-west coast of the Hudson Bay and the arctic prairie and tundra along the flanks of the Brooks Range in north-east Alaska. Temperate grasslands occur between 60 degrees latitude and the tropics, and my primary focus will be on the various grassland ecosystems within the North American Great Plains and Great Basin. Tropical grasslands occur between the tropical latitudes, and in Africa there are many grassland ecosystems of great diversity. While all three grassland types experience dramatic seasonal fluctuations in rainfall, only temperate and cold grasslands are subject to significant seasonal fluctuations in mean daily temperature (Ripley 1992). I will argue below that tropical grasslands differ from cold and temperate grasslands in ways that make adaptations to cold and temperate grasslands poor direct analogs for tropical grasslands. This tripartite classification, however, is not rigid but serves as a useful simplification. Most of the ethnographic data on tropical 10-07-97 09:20:38 jaaas AP: JAA 191 TROPICAL GRASSLAND HUNTER – GATHERERS hunter – gatherers derives from either forest hunter – gatherers or arid grassland hunter – gatherers, such as the various San groups and the Hadza. Forest hunter – gatherers are unarguably poor analogs for grassland hunter – gatherer subsistence due to the critical differences in forest and grassland flora and fauna. Arid grassland hunter – gatherers live in environments that are on the edge of grassland classification (areas that receive below 500 mm of rainfall; Lind and Morrison 1974; Pratt and Gwynne 1977). This leaves a gap between 500 mm and about 1500 mm that remains poorly documented ethnographically. It is in this gap that conditions exist for creating the classic moist tropical grassland ecosystems where large ungulates are diverse and dense and move long distances in large groups. This lack of data was recognized some time ago by Foley (1982). Thus, hunter – gatherer adaptations to moist tropical plains ecosystems of the kind represented by the Serengeti in Tanzania or Athi-Kapiti plains in Kenya remain an enigma. This is largely because hunter – gatherers were displaced or absorbed by pastoralists from these excellent rangelands long before the hunter – gatherers were ethnographically observed. The earliest sheep/ goat remains in East Africa occur in northern Kenya (Owen et al. 1982) and central Kenya (Marean 1992a) before 4000 BP. At this time East Africa was just entering a dry phase that resulted in an expansion of grassland and a decrease in woodland (Hamilton 1982). The prior early Holocene wet phase may have presented vegetative conditions that excluded pastoralism (Bower 1991; Marean 1990). These initial thrusts by pastoralists probably involved substantial burning to open the plains to grass and to drive back the tsetse fly, a pattern well documented among modern East African pastoralists. The middle Holocene dry phase, and the consequent expansion of pastoralists, combined to produce essentially modern AID JAA 0309 / ai04$$$$41 East African grassland ecosystems (Marean 1992a, 1992b). The big losers were hunter – gatherers who could not compete against the more militant and organized pastoralists. The result was that by the time systematic ethnographic observations were made, people practicing a hunter – gatherer existence were absent from the moist grasslands. This is both frustrating and stimulating. It is frustrating because archaeologists usually work from the present to the past when explaining the archaeological record, and clearly this is not possible as regards hunter – gatherers in tropical grasslands. However, the problem is stimulating because it leaves archaeology as practically the only viable means of investigating the tropical grassland adaptation of hunter – gatherers. The question, of course, is how do we build an understanding of adaptations to tropical grasslands on the basis of archaeological evidence alone? I believe the proper procedure is to construct a comparative understanding of cold, temperate, and tropical grassland ecology. First, we need to define the relevant ecological parameters in tropical, temperate, and cold grasslands that structure human foraging adaptations. Second, we must examine the differences and similarities in ecology among the three grassland types. Third, we should look for structural correlates between ecological parameters and human behavior in the ethnographically known cold and temperate grasslands. Fourth, we need to build models of foraging strategies for tropical grasslands based on points one through three above, and guide these models with behavioral ecological theory. Fifth, we can test those models with archaeological data. This paper will concentrate on the first four steps outlined above and finally apply some preliminary archaeological data from the Athi-Kapiti Plains to the developed models. Hunting strategies are part of an 10-07-97 09:20:38 jaaas AP: JAA 192 CURTIS W. MAREAN overall foraging strategy that includes a dynamic relationship between decision-making for plant-food collection and hunting, but the archaeological data available at this stage are restricted to zooarchaeological remains. My approach is guided by the basic tenets of behavioral ecology, but much of what I will discuss will be framed qualitatively as the quantitative information typically used by behavioral ecologists is lacking or inaccessible in the archaeological record. GRASSLANDS AND HUMAN SUBSISTENCE The most important ecological parameters to compare are those most closely tied to human foraging needs; they include diversity, biomass, seasonal availability, and predictability of plant and animal foods. Foley (1982) attempted to estimate these parameters for the purpose of building a model of human exploitation of tropical environments. Foley argued that hunter – gatherers in xerophytic plant communities (those below 500 mm of rainfall per year) should receive most of their calories from plant foods, particularly plants with underground storage organs (USOs). Drawing upon some basic ecological principles and the data available at the time, Foley argued that ‘‘woodland and forest hunter – gatherer populations, such as the Mbuti Pygmies, will subsist primarily on plant foods, as in the higher rainfall regimes these are more abundant’’ (Foley 1982: 398). Ethnographic and ecological data on forest hunter – gatherers has increased in quality and quantity since Foley’s paper, and the new data disprove the forest predictions of Foley’s model by documenting a scarcity of plant foods (Bailey et al. 1989; Bailey and Headland 1991; Hart and Hart 1986; Headland and Reid 1989). Foley hypothesized that tropical grasslands receiving between 500 and 1500 mm of rainfall would have few plant species edible by humans and that hunter – gather- AID JAA 0309 / ai04$$$$41 ers in these environments would rely almost exclusively on large mammal hunting. Examining this assertion in the light of new data is prudent. In the discussion below I examine the basic animal and plant ecology of tropical grasslands by comparing tropical grasslands with several cold and temperate grassland ecosystems. The comparative sample derives from those areas where good ethnographic data are available on hunter – gatherer adaptations. I am restricting the sample in this way because the ultimate goal is to relate plant and animal ecology to known foraging adaptations. The cold grassland sample is restricted to the ecosystems of the two major Eskimo groups that focused entirely on terrestrial resources: the tundra ecosystem to the north of the Brooks range in Alaska (Nunamiut Eskimo), and the ‘‘barren grounds’’ ecosystem west of the Hudson Bay (Asiqmiut or ‘‘Caribou Eskimo’’). The temperate grassland sample includes the Great Plains and the Great Basin-Plateau. The Great Plains vary from tall grass to a short grass prairie, but in all areas grasses are the dominant plants (Coupland 1992). The Great Basin-Plateau is sometimes classified as an ‘‘Intermountain Grassland’’ because it is used as rangeland (Brown 1989); however, sagebrush (in the daisy family) is the dominant plant species while the predominant grasses are bunch grasses. I include the Great Basin-Plateau in this analysis as it provides a useful comparison to the arid grasslands of the tropics. Mammal species richness (number of species) increases with decreasing latitude (Krebs 1978; Pianka 1966; Rosenzweig 1992), which suggests that large mammal richness would also be greater in tropical grasslands than in temperate and cold grasslands. Figure 1 and Table 1 show the large mammal species richness in several tropical, cold, and temperate grasslands from the sample regions. The sample of African grasslands includes two types of grasslands: edaphic 10-07-97 09:20:38 jaaas AP: JAA 193 TROPICAL GRASSLAND HUNTER – GATHERERS grasslands (Kafue and Rukwa) and secondary grasslands (Serengeti and Athi-Kapiti Plains). Edaphic or natural grasslands form when the succession to more woody growth has been arrested by biotic mechanisms such as water-logging or root-resistant soil formations. Secondary or derived grasslands are typically maintained by fire, often resulting from the activity of people (Vesey-Fitzgerald 1963, 1973). Tropical African grasslands have more diverse large mammal (Cole 1986), reptile (Barbault 1983) and bird (Fry 1983) faunas than other tropical grasslands and thus may not represent tropical grasslands worldwide. I have not placed extinct grassland ecosystems on Fig. 1 because it is not possible to place all species in the habitat categories confidently. One of the best known extinct grassland ecosystems, the Mammoth Steppe, had a species richness above both extant cold and temperate grasslands but still significantly below tropical African grasslands (Guthrie 1990). Figure 1 shows that tropical African grasslands have more species of both mobile and resident large herbivores (5 kg or greater in body weight) than higher latitude grasslands. The residents may be important to hunter – gatherers as their yearlong availability would dampen the wide seasonal variation in mammal availability, resulting from migration, that is so typical of temperate and cold grassland ecosystems. Some predominantly migratory species, such as Thomson’s gazelle (Gazella thomsoni), also have subpopulations that do not migrate. Such territorial individuals have shorter flight distances (the distance between prey and predator that results in prey flight) than nonterritorial individuals (Walther 1995), making them easier targets for predators. The migrations in tropical African grasslands typically involve a succession of animals spread over several months, facilitating niche separation and accommodating the high diversity of migratory species. Successions of this type are present in both second- AID JAA 0309 / ai04$$$$41 FIG. 1. The number of migratory/mobile and resident large ungulates in a sample of cold, temperate, and moist tropical grasslands that were inhabited by hunter – gatherers. ary (Bell 1969, 1971) and edaphic (Sheppe and Osborne 1971; Vesey-Fitzgerald 1964) grassland ecosystems. These successions begin with one species that can tolerate the more fibrous tops of grass plants and then other species move in as their particular plant part is made available through the actions of the prior feeders. Succession systems increase the time that a region can support a migratory population and may thus increase the temporal availability of large herbivores in tropical grasslands relative to migration systems that lack successions. Successions are absent or poorly developed in modern cold and temperate grasslands, though Guthrie (1990) has argued for such a succession in Pleistocene steppes. Tropical grasslands have a higher biomass and net primary productivity than cold or temperate grasslands (Whittaker and Likens 1973) and consequently tropical African grasslands also have a greater biomass of large ungulates than temperate grasslands (De Vos 1969; Coupland 1993). As the majority of this biomass is composed of large herbivores (Coupland 1993), it is reasonable to assert that the biomass of both mobile and resident large mammals is higher in tropical African grasslands than either temperate or 10-07-97 09:20:38 jaaas AP: JAA AID JAA 0309 / ai04$$0309 Brown (1989) Weaver (1956) Sinclair (1972) Sinclair (1977) Great Plains Serengeti 10-07-97 09:20:38 27 6 4 Brown (1989) Great Basin 3 2 Campbell (1968) Pielou (1994) Citation Barren Grounds Steppe Birket-Smith (1929) Pielou (1994) Brooks Range Tundra Locality Number of species Buffalo Thomson’s gazelle Wildebeest Zebra Bison Pronghorn antelope Pronghorn antelope Caribou Caribou Major migrants Eland Elephant Hartebeest Oryx Topi Wapiti Wapiti Minor migrants Highly mobile Black rhino Buffalo Grant’s gazelle Giraffe Hartebeest Oribi Ostrich Thomson’s gazelle Topi Warthog Wildebeest Zebra Bison Pronghorn antelope Muskox Muskox Resident or smallscale movement on grasslands Bohor reedbuck Hippopotamus Waterbuck Resident Edaphic grassland Bushbuck Bush duiker Bushpig Dik dik Steinbok Mule deer White-tailed deer Mule deer White-tailed deer Wapiti Resident Riparian woodland Klipspringer Mountain reedbuck Dall sheep Specialist of hills and inselbergs Resident or small-scale movement TABLE 1 The Mobility Status and Presence in Microhabitats of the Large Animal Species in a Sample of Grassland Ecosystems Buffalo Bush duiker Bushpig Dik dik Impala Steinbok Mule deer Wapiti Mule deer White-tailed deer Wapiti Moose Resident woodland/ grassland edge 194 CURTIS W. MAREAN jaaas AP: JAA AID JAA 0309 / ai04$$0309 10-07-97 09:20:38 jaaas Sheppe and Osborne (1971) Vesey-Fitzgerald (1964) Foster and Coe (1968) Stewart and Zaphiro (1963) 22 18 16 Lechwe Wildebeest Zebra Buffalo Elephant Topi Zebra Hartebeest Wildebeest Zebra Buffalo Eland Impala Puku Roan Eland Lichenstein’s hartebeest Roan Eland Thomson’s gazelle Å Edaphic Å Edaphic Giraffe Grant’s gazelle Ostrich Rhino Warthog Hippopotamus Common reedbuck Sitatunga Warthog Waterbuck Bohor reedbuck Hippopotamus Puku Waterbuck Bohor reedbuck Hippopotamus Waterbuck None present None present Bushbuck Bush duiker Steinbok None present None present Klipspringer Mountain reedbuck Bushbuck Bush duiker Bushpig Hartebeest Impala Kudu Oribi Sable Steinbok Bushbuck Bush duiker Giraffe Impala Steinbok Warthog Bushbuck Bush duiker Bushpig Impala Steinbok Note that a species may be both migratory and resident as some species, such as Thomson’s gazelle, often have two subpopulations; one migrates and the other does not. Some flexible species may occur in several microhabitats. The classification in this table is collapsed into Fig. 1. Kafue River Floodplain Rukwa Valley Athi-Kapiti Plains TROPICAL GRASSLAND HUNTER – GATHERERS 195 AP: JAA 196 CURTIS W. MAREAN FIG. 2. The number of edible plant species used by several hunter – gatherer groups that inhabited or inhabit cold, temperate, and tropical grasslands. cold grasslands. In summary, tropical African grasslands have a greater abundance of large herbivores, greater diversity of large herbivores, greater abundance and diversity of residential large herbivores, and a longer temporally extended grazing succession. In Fig. 2 and Table 2 I have extracted from the ethnographic literature the number of plant species used for food by hunter – gatherer groups occupying cold and temperate grasslands and African tropical grasslands. This is not an estimate of the total number of edible plants available, but a list of those plant species productive enough to warrant inclusion in the diet of the hunter – gatherers present in those environments. As noted before, we have few observations of hunter – gatherers in moist tropical grasslands and several observations of hunter – gatherers in arid grasslands. I have included these arid grassland hunter – gatherers (the various San groups of the Kalahari) and also one hunter – gatherer group (the Suiei Dorobo) that use an East African grassland that receives about 620 mm of rainfall annually. The plant food collection of Eskimo is well documented as negligible. For example, the Tuluaqmiut Nunamiut used only 12 species of plants AID JAA 0309 / ai04$$$$41 (Campbell 1968), Stoney (1900) records the use of 22 species of plants among Kuuvakmiut Eskimo (these are neither Nunamiut or Caribou Eskimo, however), and BirketSmith (1929) records the use of just 6 species of plants among the Caribou Eskimo. Figure 2 shows that the African hunter – gatherers generally use a larger variety of edible plant foods than hunter – gatherers in cold and temperate grasslands. The Great Plains groups are remarkably consistent in the variety of species used, which suggests collection at the maximum available species richness in those temperate grasslands. The Great Plains Indians exploited far fewer USO plants than tropical grassland hunter – gatherers. A more effective measure would be the total calories exploited, as the Great Plains Indians clearly obtained large quantities of Psoralea esculenta (the Plains turnip, Reid 1977; Kaye and Moodie 1978), but those data are not available. The arid tropical grassland hunter – gatherers use and presumably have access to more USO species than in moister grasslands, though these are environments more arid than the environments of interest here. The Suiei Dorobo, however, do provide some useful observations. Suiei collect 122 plant species (Ichikawa 1980), 69 of which are fruits, and plant foods make up most of their diet, unlike most other Dorobo/Okiek (Blackburn 1982). Of the 122 plant species, 92 are found in the grasslands. Of the 10 staple plant species, 6 are found in grasslands (Ichikawa 1980). The Suiei data are inconsistent with Foley’s expectations of a diet lacking in plant foods in tropical grasslands receiving more than 500 mm of annual rainfall. USOs (rhizomes, tubers, corms, and bulbs) are favorites of hunter – gatherers because the often massive storage organs are large carbohydrate packages (Gott 1982; Harrington 1967; Vincent 1985a and 1985b). Figure 3A shows the species richness of USOs in several African regions as summarized by Vincent (1985b) plotted against 10-07-97 09:20:38 jaaas AP: JAA 197 TROPICAL GRASSLAND HUNTER – GATHERERS TABLE 2 The Number of Plant Species Exploited as Food in the Great Basin, the Great Plains, and Africa Greens Fruits, berries, nut, plum Gum Sum Reference Steward (1938) Steward (1938) Steward (1938), Chamberlin (1911) Steward (1938) USOs Melons Pods, beans 5 9 8 0 0 0 0 0 0 41 13 47 4 ? 12 5 7 12 0 0 0 54 29 81 Lemhi Great Plains Cheyenne Blackfoot 20 0 0 13 2 3 0 38 10 8 0 0 1 0 0 0 2 10 18 15 0 1 31 34 Comanche 13 0 1 0 0 20 0 34 8 0 2 4 5 15 0 34 Grinnell (1915) Hellson and Gadd (1974) Carlson and Jones (1939) Gilmore (1919) 24 41 35 18 2 2 2 0 2 3 3 4 4 2 0 5 4 6 24 23 22 29 12 69 11 18 9 5 69 101 85 122 Marshall (1976) Lee (1979) Tanaka (1976) Ichikawa (1980) Great Basin Owens Valley Reese River Gosiute Missouri River Africa !Kung of Nyae Nyae Dobe !Kung Central Kalahari Suiei Dorobo Seeds Note. Not all cited sources classified plants in the same way so the author moved some species to other categories to increase consistency. The data from the Missouri River Region are a compilation of the foods eaten by several groups. The ‘Sum’ includes species not classified on the table. mean annual rainfall, and Table 3 gives the raw data with further contextual elaboration. These data show that no tendency exists for USO species richness to decline where rainfall lies between 500 and 1000 mm (contra Foley 1982), and the relative frequency of USOs as a component of all plant species (Fig. 3B) increases linearly with rainfall values between 100 and 1000 mm. These data also indicate Foley’s (1982) 500 mm rainfall threshold is not substantiated. In addition, the data reveal a soil nutrient effect. Localities on soils derived from granitic parent material, and thus low in nutrients, tend to have more USO species for a given amount of rainfall than localities with high nutrient volcanic-derived soils. Thus it appears that soil nutrient status and rainfall interact to determine USO species richness. Though few data exist on the edibility and AID JAA 0309 / ai04$$$$41 density of these USO species, most tropical African USOs are edible either without processing or with minimal processing (Vincent 1985a, 1985b). As mentioned earlier, grasslands are heterogeneous ecosystems that contain many small microhabitats conditioned by variations in geology, soil catena, and drainage. USOs typically require well-drained soils and thrive in dry conditions. In a study of the distribution of USOs near Lake Eyasi and Lake Manyara in Tanzania, Vincent (1985a, 1985b) found that USOs are most abundant high on the catena on the slopes of escarpments and inselbergs, and least abundant in moist soils of forests and edaphic grasslands. Though well-drained secondary grasslands may have several species of USOs, the density and species richness of USOs clearly declines from the well- 10-07-97 09:20:38 jaaas AP: JAA 198 CURTIS W. MAREAN FIG. 3. The number of USO plant species plotted against mean annual rainfall (A), and the percentage of all plant species that have USOs plotted against mean annual rainfall (B), for several African ecosystems. The circles represent localities on low-nutrient basement rocks, and the triangles represent localities on high-nutrient volcanic rocks. drained top of the soil catena to the poorly drained bottom. This is also the case for the distribution of USOs on temperate grasslands (Kaye and Berry 1978; Kuhn 1987; Reid 1977). USOs typically occur in distinct patches forming from compound clusters or root propagation. Importantly, tropical USO species can be edible year-round although they do undergo changes in their nutritional value (Gott 1982; Vincent 1985a, 1985b). Temperate grassland USOs have much stricter seasonal cycles that depress their palatability and visibility (Kuhn 1987). Vincent found that East African USOs tend to be nutritious and palatable year-round, and importantly they retain their above-ground AID JAA 0309 / ai04$$$$41 shoots year-round, allowing them to be discovered more easily and consistently (Vincent 1985a, 1985b). The less seasonal character of USOs in tropical African grasslands is a key ecological distinction relative to higher latitude grasslands. Plant foods that grow above ground (AGPs), such as fruits, berries, nuts, greens, and seeds, are generally found on woody growth, and woody growth typically requires well-drained soils with regular access to moisture. In grasslands woody growth occurs on levees near seasonally flooded streams and lakes (gallery forest) or higher on the soil catena. Sept quantified the abundance of edible plant foods near the Voi River in Tsavo East (Kenya), an arid channel that drains into Lake Turkana (Kenya), in the Parc National des Virunga in the upper Semliki Valley (Zaire), and along the Ishasha River in Zaire (Sept 1984, 1986, 1990, 1994). Sept has found that fleshy fruits occur in abundance along rivers and streams. More specifically, on the proximal margins of streams the soils are well drained and covered with gallery forest that includes many species of fruit-bearing plants. On the poorly drained distal margins edaphic grasslands are present while collectable fruits are rare. Overall, Sept’s data show that within tropical grassland ecosystems microhabitats contain significant quantities of edible plants that grow above ground. The data discussed above, derived from ethnography and Vincent’s and Sept’s studies of edible plant availability, are qualitatively summarized in Table 4. These data show that tropical African grasslands are much richer in plant foods than originally portrayed by Foley (1982). Tropical grasslands have a higher species richness and higher biomass of mobile and resident large mammals than either temperate grasslands or cold grasslands. The mobile large mammals are a patchy resource spatially (they occur in dense herds), temporally unpredictable (the herds have regular routes but the 10-07-97 09:20:38 jaaas AP: JAA 199 TROPICAL GRASSLAND HUNTER – GATHERERS TABLE 3 The Number of USO Plant Species by Locality in East Africa Locality Habitat type Annual rainfall Mau Forest Mbeya Range Gombe National Park Mahale Mountains West Usambara Lake Manyara North-East Serengeti Meru National Park Ruaha National Park Simanjaro Plain Tsavo East National Park Marsabit South-west Marsabit Lake Turkana Montane Forest Montane Forest Brachystegia Woodland Brachystegia Woodland Montane Forest Acacia, Woodland Wooded Grassland Wooded Grassland Wooded Grassland Bushed Grassland Bushland and Acacia Semi-desert Semi-desert Semi-desert 1750 1750 1500 1500 1425 950 900 710 650 600 546 250 150 250 Soil parent material Number of USO species Percentage of all species that are USOs Volcanic Basement Basement Basement Basement Volcanic Volcanic Volcanic Basement Volcanic Basement Basement Volcanic Volcanic 31 14 57 29 2 74 68 54 255 29 93 119 55 40 6.3 5.9 10 4.6 2 13.1 19.7 11.9 19.3 14.7 11.3 10 8.9 7.8 Note. Soil parent material data are taken from maps of the Geological Survey of Kenya and Tanzania. All other data are from Vincent (1985b). timing and exact placement are typically variable), and seasonal in availability. The resident large mammals are also patchy (most occur in bushy or wooded micro habitats in grasslands), but temporally and spatially predictable, and not seasonal. Tropical African grasslands, at least up to 1000 mm of annual rainfall, have a higher species richness of plants with USOs than either temperate or cold grasslands. These USOs are a rich source of carbohydrate, they are patchily distributed, temporally and spatially predictable, and not as seasonal as AGPs. AGPs are also diverse in tropical African grasslands. They are patchily distributed, temporally and spatially predictable, and very seasonal. Overall, carbohydrate and protein availability in tropical grasslands is less seasonal than in temperate and cold grasslands and this is of critical importance to under- TABLE 4 Qualitative Summary of the Major Ecological Parameters Relevant to Foraging in Tropical, Temperate, and Cold Grasslands Large mammal biomass Large mammal species richness Resident large mammal species richness Mobile large mammal species richness Seasonality of large mammal availability Edible plant species richness USO species richness AGP species richness Seasonality of plant availability AID JAA 0309 / ai04$$$$41 Tropical Temperate Cold High High High High Moderate High High High Low Moderate Low Low Low High Moderate Low Moderate High Low Low Low Low High Low Low Low High 10-07-97 09:20:38 jaaas AP: JAA 200 CURTIS W. MAREAN standing hunter – gatherer adaptations to tropical grasslands relative to cold and temperate grasslands. Cold, temperate, and tropical grasslands experience dramatic seasonal fluctuations in precipitation, but tropical grasslands experience seasonal variations that are near zero degrees in mean daily temperature while temperate grasslands experience fluctuations up to 407C (Ripley 1992). This may be the causative mechanism underlying the greater diversity and less seasonal character to plant and animal availability in tropical grasslands. HUNTER–GATHERER HUNTING STRATEGIES IN GRASSLANDS The discussion above highlighted the biotic distinctions important to foraging strategies in cold, temperate, and tropical African grasslands. These distinctions can be used to develop models of hunter – gatherer plant and animal exploitation in tropical African grasslands. One way to accomplish this is to focus on the way that temperate and cold grassland hunter – gatherers adapted to the particular problems posed by their grasslands. If these problems are similar to those posed by tropical African grasslands, we may hypothesize a similar adaptation modified to account for other contingencies. Any attempt to model hunting strategies must consider the important distinctions between cold/temperate grasslands and tropical African grasslands in plant food availability. Cold and temperate grasslands pose difficult problems. From early winter to early spring few if any plant foods are available and thus the potential menu is low in high quality calories (Speth 1983; Speth and Spielman 1983). Most of the plant foods that are available during the warm months are small items (nuts, berries, seeds, etc.), patchily distributed, productive for only a few months, often unpredictable, and may require intensive collecting or processing strategies. An obvious strategy to compen- AID JAA 0309 / ai04$$$$41 sate for seasonal scarcity of plant foods is to switch to animal foods. However, in grasslands prey animals are typically migratory and thus only available in quantity during short periods of the year. Many of these animals are in poor condition and lack the necessary fat to compensate people for the loss of plant carbohydrate (Speth 1983; Speth and Spielman 1983). Many cold and temperate grassland hunter – gatherers designed similar strategies to cope with such problems. The data discussed above, however, suggest that tropical African grasslands do not suffer the striking seasonal dearth of carbohydrate so typical of temperate and cold grasslands. Before discussing the strategies, it is useful to break apart the various components of a hunting strategy so that each component can be directly related to specific ecological conditions. Components of a Hunting Strategy A hunting strategy is usefully understood as a composite of encounter techniques, prey choice techniques, and organizational techniques. The encounter technique can include various combinations of routed or encounter hunting, intercept hunting, passive use of stationary traps such as snares and pitfalls, and an active use of traps that I will call tactical landscape methods. These traps may be natural features of the landscape such as gullies, cliffs, or box canyons, or they may be stationary installations people have built on the landscape, or a combination of the two. Traps used in tactical methods may even be mobile, such as strings of nets held by people. Tactical methods are an approach to hunting where the landscape becomes a weapon, people have a dynamic role in the operation of the weapon, effective operation usually involves high search costs (particularly high when people modify the landscape) and handling costs, and de- 10-07-97 09:20:38 jaaas AP: JAA 201 TROPICAL GRASSLAND HUNTER – GATHERERS tailed planning and organization is critical for success. The prey choice technique spans a continuum from specialized hunting, which includes a very narrow choice of species relative to the natural species richness of a region, to generalized hunting, which includes a wider choice of species. Hunters may oscillate seasonally between specialized and generalized hunting. Specialized hunting typically involves a strategy where one species, or two, is targeted as the main prey item, while other species are ignored. Specialized hunting in grasslands is often, perhaps always, joined to tactical methods, as is discussed below. The reverse, however, may not be true as tactical methods could result in kills of several prey species, particularly in environments where grassland species co-associate (such as in many African grasslands). The Mbuti pygmy net surrounds do not result in a specialized species yield, yet these are clearly tactical methods (Ichikawa 1983). The organizational technique can include a single hunter, several hunters, or communal hunting by large groups. Grassland hunters using tactical methods typically employ communal hunts, as is discussed below. However, communal hunting need not always result in specialized prey choice. Communal hunts may not always be executed with tactical methods. Thus, some of the three components of a hunting strategy are casually linked in an inflexible way, while most components occur in various permutations. The ethnographic literature discussed below suggests that specialized hunting may be an example of the former — it may typically be implemented with communal hunting using tactical methods. My primary concern here is the relation between different components of a hunting strategy, the character of animal prey (biomass, species richness, and movement patterns) the strategy is meant to exploit, and the affects plant food availability have on the hunting strategy hunter – gatherers choose to use. In other words, are there regu- AID JAA 0309 / ai04$$$$41 larities in the relationship between prey characteristics, the techniques used to capture the prey, and the availability of plant foods? Such regularities can be used to construct models for hunting strategies in tropical African grasslands when one takes into consideration two important points discussed earlier: (1) plants foods are more abundant and less seasonal in tropical grasslands, and (2) species richness and biomass of migratory and residential ungulates are higher in tropical grasslands. The significance of this will become clear below when I construct likely hunting models for tropical African grasslands. Hunting Strategies in Cold and Temperate Grasslands The rich ethnographic literature on hunting strategies among cold grassland hunters in North America shows that tactical landscape methods were preferred for killing the primary grassland animal, the caribou. These methods are the same as those referred to by Burch (1972) as ‘‘head-’em-offat-the-pass’’ techniques. All Eskimo groups used various tactical landscape methods such as driving caribou into rivers, lakes, and streams and then dispatching them from a boat (Birket-Smith 1929; Boas 1888; Cantwell in Healey 1889; Gubser 1965; Spencer 1959; Stoney 1900) and intercepting and driving caribou into box canyons, valleys, constructed traps, and corrals (Binford 1978; Birket-Smith 1929; Boas 1988; Cantwell in Healey 1889; Gubser 1965; Ingstad 1954; Jenness 1928; Spencer 1959; Stoney 1900). Our main interest here is on those groups that relied solely on the food resources of the cold grasslands (the Caribou and Nunamiut Eskimo) and the ethnographic literature is clear that tactical techniques combined with communal hunting were the primary methods for killing caribou. Many authors are explicit about the communal nature of these hunts, for example: ‘‘The great spring hunt 10-07-97 09:20:38 jaaas AP: JAA 202 CURTIS W. MAREAN was undertaken by means of one or two well-defined methods involving activity on the part of virtually every member of the community’’ (Spencer 1959: 29). The many descriptions of corrals, enclosures, and fences show that a huge effort was expended to make and maintain such installations (Birket-Smith 1929; Cantwell in Healey 1889; Ingstad 1954; Spencer 1959; Stoney 1900). Routed or encounter hunts, referred to by Burch (1972) as ‘‘search-and-destroy techniques,’’ were used occasionally, more so among groups more dependent on marine resources, but most of the sources are clear that this method was a minor contributor to the diet of the Caribou and Nunamiut Eskimo. Though caribou formed the major component of the diet of Nunamiut and Caribou Eskimo, other terrestrial species were significant. The Nunamiut commonly hunted other large herbivores, including the Dall sheep (the second most hunted species), the moose, and probably the musk ox before its virtual extinction. Bears (black and grizzly) were a rare but important component of the Nunamiut diet, as were a variety of smaller mammals including snowshoe hare, red squirrel, beaver, and arctic ground squirrel (Binford 1978; Campbell 1968; Gubser 1965; Spencer 1959). The Caribou Eskimo did not have access to Dall sheep and moose and therefore were more dependent on caribou (Birket-Smith 1929). The ethnographic literature on temperate grassland hunter – gatherers is rich but complicated. One complication is that many eastern Plains Indians, particularly in the Missouri valley, also practiced agriculture and traded extensively for agricultural products (Lowie 1963). Another complication is that the introduction of the horse had a significant impact on the bison procurement strategies of the Plains Indians (Anell 1969; Ewers 1958; Roe 1955). My main interest here is on the pre-horse strategies of the nonagricultural groups. Although most obser- AID JAA 0309 / ai04$$$$41 vations of Plains Indian bison hunting were made after the introduction of the horse, there are several observations of pre-horse hunting in the northern Plains, where the horse was introduced the latest. Also, in many areas researchers interviewed individuals who had hunted before the introduction of the horse (for example the Cheyenne studied by Grinnell [1915]), or knew tales of the pre-horse times. The Plains Indian literature clearly shows that tactical landscape methods were the preferred encounter techniques for bison before the introduction of the horse. Though Plains Indian tactical landscape methods may superficially appear homogeneous, a variety of methods were used under different contingencies. Grinnell (1915) described foot surrounds by many hunters armed with bows, from discussions with Cheyenne in the late 1800s. He also described the use of a bluff or cutbank where the wall was used as one side of a pen with the pen being finished off with bush and wood. Lines of bushes extended from the entrance of the pen like wings of a chute onto the plains. Ewers (1949, 1955) described several Blackfoot tactical techniques for hunting bison, all of which had one distinguishing feature; the bison were driven between converging fences made of rows of cairns or people acting as beaters. These V-shaped lines ended in either a corral on level ground, a sunken corral meant to break the legs of the bison, or a very steep precipice. Anell (1969) compiled observations of the Plains Indian use of pounds and enclosures for killing bison. The pound was a substantial structure whose construction entailed significant labor costs. It was usually circular and made of logs, stones, and other materials and was built high enough to keep the animals from seeing over the top. The pound was reached by only one entrance of converging fences designed to be high and dense enough to conceal the hunters. The bison were driven in by beaters or lured in. 10-07-97 09:20:38 jaaas AP: JAA 203 TROPICAL GRASSLAND HUNTER – GATHERERS Once inside, those animals that survived the tumult were shot by arrows. Such substantial pounds were reportedly used by the Blackfoot (Grinnell 1915), and Hind (1860) provides a graphic description of the results of a pound-hunt by the Cree. Grinnell (1915) also describes Cheyenne hunting pronghorn antelope using pits placed near the convergence of two streams, supplemented by bush borders. Antelope were driven between the streams into the pits. The evidence discussed above shows that tactical landscape methods were the predominant encounter technique when hunting mobile grassland species such as bison, caribou, and pronghorn antelope in cold and temperate grasslands. Communal hunting was the preferred organizational technique for such tactical hunts. Many hunts included substantial landscape installations that required significant labor investments to build and maintain, thus driving up search and handling costs. Centering a hunting strategy around tactical hunts, with the associated costs of building and maintaining landscape installations and aggregating large numbers of people, is probably risky for several reasons (Frison 1991). First, migratory species can be prone to a boom and bust demographic pattern, and often the precipitous drop in population occurs suddenly. The large Alaskan caribou herds have undergone several population crashes (Burch 1972; Haber 1977) that placed enormous stress on the Eskimo (Burch 1972). The populations of African migratory species such as wildebeest can be quickly decimated by droughts, flooding rains, and disease (Foster and Coe 1967; Stewart and Zaphiro, 1963). Second, migrations pass through a region over a limited period of time, and the ethnographic literature discussed above suggests that there is a narrow window of time within which such hunts can occur. Failure to be successful in a tactical hunt expends a large portion of this window of time, and the costs associated with the failed attempt are high. AID JAA 0309 / ai04$$$$41 If tactical methods are risky, then this raises a question critical to building models for tropical grassland hunters. Why did temperate and cold grassland hunters use tactical and communal techniques when these techniques were clearly expensive and risky? Three different explanations can be suggested; two based in behavioral ecology and the third more social in causation. The first possible explanation is that tactical techniques are the optimal strategy for procuring mobile large prey in open highvisibility environments. If this is true, then tropical grassland hunter – gatherers may also have used such techniques simply because of the similar predominance of large mobile herbivores in open environments. The alternative ecological explanation is that tactical strategies are a response to nutritional deficits caused by the low species richness, low biomass, and seasonally punctuated availability of plant foods in temperate and cold grasslands. It could be argued that tactical communal hunting strategies were designed to overcome the nutritional problems caused by the complete lack of collectable plant foods winter through spring. Low animal species richness, combined with highly mobile prey, results in one or two seasonal windows of opportunity for attaining enough food for the year. Given this, people in cold and temperate grasslands had no choice but to use a hunting strategy that involved tactical hunts with the chance of decisive success and consequent surplus. This need for surplus is a common argument for explaining Great Plains communal hunts (Frison 1991). As shown earlier, tropical African grasslands do not share with cold and temperate grasslands the problem of highly seasonal food availability as tropical African grasslands have a regular availability of a diversity of residential large mammals and a less seasonal quality of USO availability and palatability. If tactical techniques are a response to seasonal caloric deficits, then we 10-07-97 09:20:38 jaaas AP: JAA 204 CURTIS W. MAREAN may not expect tactical hunting in tropical African grasslands. A third explanation for large communal hunts in grasslands is that these hunts are necessary to provision large aggregations of people that congregate for social reasons. Fawcett (1987) has proposed this as an explanation for Great Plains communal hunting in response to alleged inconsistencies in the more ecologically grounded explanations of Frison (1970, 1991) and Speth (1983). This explanation is not necessarily unique to grasslands as any large aggregation of people in any environment may stimulate a need for communal hunting. This interpretation may be correct for specific cases, however I find it unpersuasive as a general theory. The vast majority of temperate and cold grassland hunter – gatherers regularly practiced communal hunting, and for this hypothesis to be compelling as a general theory one must posit that all these groups were driven to aggregate for social reasons and only hunted communally during such special aggregations. Caribou and Nunamiut Eskimo practiced communal hunts but these were not associated with special seasonal aggregations of people — they were timed to coincide with the movement of caribou and typically carried out with the core group. Some specific cases of communal hunting are likely to result from the need to provision large aggregations of people for social reasons, but a general explanation for the prevalence of communal tactical hunts in temperate and cold grasslands most likely is found in one of the two ecological explanations. THREE MODELS FOR TROPICAL GRASSLAND FORAGING The discussion above leads to several alternative models of adaptations for hunter – gatherers in moist tropical African grasslands. I will describe these below, beginning with what I think is the most likely model AID JAA 0309 / ai04$$$$41 based on the similarities and distinctions between moist tropical African grasslands and cold and temperate grasslands. I will assume that the hunter – gatherers are exploiting the grasslands all year, however each model could easily accommodate a seasonal mobility component where the hunter – gatherers move off the grasslands into the woodlands. The models have many behavioral predictions with archaeological test implications. Here, however, I will focus on those aspects of the models that can be examined with the zooarchaeological data that are currently available. The Generalized Grassland Model is derived primarily from the biotic dissimilarities between moist tropical African grasslands and cold and temperate grasslands, particularly in regards to the availability of edible plants and residential ungulates. It differs from the pattern documented by ethnography for hunter – gatherers living in cold and temperate grasslands. This model predicts that plant foods dominate the diet breadth and contribute the majority of calories to the diet. The lack of dramatic seasonal variation in plant food availability, and the abundance of residential mammals, combine to ameliorate seasonal fluctuations in food. Thus this model predicts that tropical grassland hunter – gatherers will never use tactical landscape methods. The model assumes that tactical methods are used to overcome seasonal caloric deficits, and that tactical methods are not necessarily the most efficient open habitat hunting techniques. The lack of tactical hunts will cancel the need for large communal aggregations, and the primary organizational hunting technique will be individual or small group hunting. Large aggregations have distinct costs in that plant and animal foods are rapidly exhausted within the foraging radius of an aggregated group. However, it is possible that people may aggregate due to environmental constraints, such as around water holes during the dry season, or for social 10-07-97 09:20:38 jaaas AP: JAA 205 TROPICAL GRASSLAND HUNTER – GATHERERS reasons. The primary hunting techniques will include various combinations of routed hunting, passive trapping, and intercept hunting at water holes, riparian woodlands, salt licks, and other natural magnets for mammals. The species choice will be very broad, sampling migrants and residents, large and small animals. Residential sites should show high species richness as these sites are the end points for transportation from many encounter sites. We should not find open-air sites with the characteristics typical of mass-kills. The generalized strategies well described for the various arid grassland groups, such as the Khoi San and Hadza, fall within this model. This strategy is within the forager range of Binford’s (1980) forager/collector continuum. Given the ecological conditions of tropical grasslands, this is the preferred model for East Africa. The Seasonal Grassland Model differs from the Generalized Grassland Model in that it suggests that tactical hunting techniques should be used at least seasonally. This model assumes that tactical techniques are the most efficient technique for killing large mobile ungulates in grasslands. During certain seasons of the year large herds of ungulates pass through on their seasonal round, as in the Serengeti, or concentrate in wellwatered areas if the ungulates practice an aggregation-dispersal strategy, as in Amboseli. During these seasons of encounter, the use of tactical techniques raises the return rates of migratory animals so that migratory animals are regularly hunted. Other hunter – gatherer groups may move to converge on these natural points of animal aggregation and thus natural aggregations of people may form. Alternatively, people may aggregate for the specific purpose of producing larger more effective groups for communal hunting. Smaller resident ungulates will probably be ignored as their return rates are typically lower than for larger ungulates (Hawkes, O’Connell, and Blurton-Jones AID JAA 0309 / ai04$$$$41 1991). It is likely that large aggregated groups would rapidly exhaust plant foods within the foraging radius of the aggregation sites as even small groups of Central Kalahari San (Tanaka 1980) and Hadza (Woodburn 1968) exploit the plant foods within their foraging radius sufficiently to stimulate a residential move within days or weeks. For this reason plant foods would probably not be a major food item. During such aggregation seasons the hunting strategy should be characterized by a single species focus and communal hunts. During the rest of the year, when migrants are not concentrated in the region, the foraging strategy should shift to the Generalized Grassland Model and plant foods and resident ungulates will once again rise in resource rank. Thus the Seasonal Model posits a seasonally shifting forager/collector strategy. With the Seasonal Model at least some hunting sites will be locations characterized by very low species richness because drives of animals in grasslands are typically focused on one species (Frison 1991). In Africa, however, migratory species have a tendency to co-associate (such as wildebeest and zebra, Thomson’s gazelle and Grant’s gazelle), and thus mass kills are likely to have several species, unlike the bison and caribou mass kill sites in North America. These hunting sites will also display a seasonal signature in the mortality profiles as animals are killed at that location at restricted times of the year. Such sites should be strategically placed to make the driving process easier, and landscape modifications may be present. Residential sites occupied only during the tactical hunts will show a specialized species choice. Residential sites occupied throughout the year will show a generalized species choice. The Specialized Grassland Model, considered the least likely for tropical grasslands, is based on the basic abiotic and biotic similarities between the temperate grasslands and moist tropical African grasslands. In es- 10-07-97 09:20:38 jaaas AP: JAA 206 CURTIS W. MAREAN sence, this is a Great Plains Indian strategy transferred to the African plains and adjusted for the ecological dissimilarities between tropical and temperate grasslands. This model assumes that grassland ecosystems are best exploited with a specialized hunting strategy including tactical landscape methods, communal hunting, and specialized prey choice. Specialized hunts employing tactical and communal methods would be used whenever viable and these hunts would be a major determining factor in the yearly land-use strategy. This strategy would posit a subsistence system based around the drying and/or smoking of massive quantities of meat. Communal tactical hunts would occur during seasons when the migrations pass through or stabilize within a region. The prolonged grazing successions in tropical African grasslands would increase the time span, relative to temperate grasslands, during which communal and tactical hunts could be effective. Most kill sites should be dominated by a few co-associating migratory species and the kill sites should be at places strategic for tactical hunts. During seasons when the migrants are absent the hunter – gatherers should either switch to residential mammals and plant foods, or move off the grasslands. Residential sites should show a very narrow species richness focused on migratory species though residential sites may be more species rich to account for occasional opportunistic kills. ARCHAEOLOGICAL EVIDENCE FOR GRASSLAND EXPLOITATION IN EAST AFRICA Archaeological research on the Middle Stone Age (MSA) and the Later Stone Age (LSA) is very spotty in East Africa, with some centers of concentration in the Naivasha-Nakuru-Elmenteita basin and at Lukenya Hill (Fig. 4). Most other MSA and LSA data in East Africa comes from scattered AID JAA 0309 / ai04$$$$41 FIG. 4. Map showing East Africa and the location of Lukenya Hill. sites (such as Kisese II, Nasera Rockshelter, Mumba-Höhle, Loiyangalani). The isolated nature of these sites makes them poor candidates for investigating the hunting models discussed above. Lukenya Hill is the only locality that has several reasonably contemporaneous sites that were clearly occupied by hunter – gatherers within a grassland ecosystem. The Lukenya Hill Environmental Setting Lukenya Hill is a gneissic inselberg that rises to about 200 meters above the AthiKapiti Plains (Fig. 5). The inselberg measures about 8 km long to a maximum 2 km wide. It varies from being a coarse-grained soil-covered hill in some areas, to a rocky jumble of detaching and eroding rocks that form many cliffs, overhangs, shelters, and 10-07-97 09:20:38 jaaas AP: JAA 207 TROPICAL GRASSLAND HUNTER – GATHERERS FIG. 5. Map showing the Lukenya Hill (in the dotted-line box that delimits the extent of Fig. 7) and Athi-Kapiti Plains region; the position of major drainages, rivers, and streams; and the most common path of the large mammal migration. water trapments. Archaeological remains are abundant on and around the hill, attesting to its attractiveness as a place of stone age settlement. The Athi-Kapiti Plains and Lukenya Hill lie in the semiarid region of Kenya. Most of the region receives about 550 mm of rainfall per year, and the annual potential evaporation is high at 1500 to 2000 mm (Reed 1983). The immediate region surrounding Lukenya Hill includes the Athi-Kapiti Plains (1690 km2) and the Nairobi National Park (112 km2). The Ngong Hills and the eastern wall of the Rift Valley form a western boundary to the region. To the south is a rocky area of closed bush vegetation and the Pelewa Hills, and the eastern boundary is formed by the Selenkai River. The region surrounding Lukenya Hill has four distinct physiographic units: the high ground of the AID JAA 0309 / ai04$$$$41 eastern flank of the Rift Valley including the Ngong Hills (about 30 to 40 km west of Lukenya Hill), the Athi and Kapiti Plains (surrounding Lukenya Hill), the central hill masses of the Machakos district (within 10 to 40 km east of Lukenya Hill), and the partially dissected peneplain including the Yatta Plateau (about 50 to 60 km northeast of Lukenya Hill). The Athi-Kapiti Plains surround Lukenya Hill and would have been critical to the occupants of the sites. Most of the plains consist of flat volcanics that lie between 1000 and 1500 m a.s.l. The Athi Plains are underlain mainly by phonolite, lava, and tuff and tend to be flat and end in a bluff just west of the Athi River. Several seasonal rivers and streams traverse the Athi Plains flowing in an easterly direction. The Kapiti Plains occur east of the Athi River and the main underly- 10-07-97 09:20:38 jaaas AP: JAA 208 CURTIS W. MAREAN ing bedrock is an ancient metamorphic basement rock except along the western margin underlain by the end of the Kapiti Phonolite (Fairborn 1963). The topography of the Kapiti Plains is more varied than the Athi Plains, being gently undulating to flat with many small hills. No permanent rivers exist on the Kapiti Plains, just seasonal streams with short periods of flow. This geomorphic variety results in corresponding biotic diversity. The rolling topography of the Athi-Kapiti Plains shows a classic catenary pattern that determines much of the variation in vegetation (Stelfox 1985; Stelfox and Hudson 1986). There are three basic vegetation types in the Athi-Kapiti plains, and the occurrence of these types is primarily a product of topography and soil character. On the flatter plains various grassland types occur but they are dominated by dwarf tree ’whistling thorn’ (Acacia drepanalobium) grassland. Scrub lands with scattered bush with very little grass occur on the flanks of hills and on the exposed edges of lava flows where one often finds a thin line of bush vegetation. Riparian bush and woodland occur near the incised river courses. The predominant vegetation types, using rangeland classification (Pratt and Gwynne 1977), are primarily grasslands and wooded grasslands. The Athi-Kapiti Plains was once home to huge numbers of animals. The Plains represent a complete ecological unit with very little movement of animals in or out, but there is considerable internal movement stimulated by the degradation of rangeland conditions during the dry season (Stewart and Zaphiro 1963). The populations of mammals and the migration have been disrupted in recent years by many factors related to human settlement (Foster and Coe, 1968). Despite the radical modifications to the local region and the disruption of the migration, reconstructing the migration pattern with reasonable confidence is possible. Before fencing in 1947, mammals migrated freely in the dry season to permanent water in the AID JAA 0309 / ai04$$$$41 FIG. 6. Census data from the Nairobi National Park of the major grassland species on the Athi Plains. The Nairobi National Park census samples only a small fraction of the mammals that comprise the Athi-Kapiti Plains ecosystem, though the proportions are probably reasonably similar. Nairobi Park, the Ngong Hills, and to Thika (Fig. 5). In the wet season the game dispersed to the plains. Formal game censuses of Nairobi National Park have been taken since 1961 (Fig. 6, from Foster and Coe, 1967). A less rigorous census was taken before 1961 (Stewart and Zaphiro, 1963). The evidence from these two sources clearly shows that wildebeest (Connochaetes taurinus) was the dominant ungulate in the Athi-Kapiti Plains followed by Burchell’s zebra (Equus burchelli), hartebeest (Alcelaphus buselaphus), and impala (Aepyceros melampus). Since 1961, due to increases in the population of livestock, the effects of fencing, and the drought of 1960–1961, the relative numbers of animals in the Nairobi Park and in the plains have changed. Most of the animals shown in Fig. 6 are seasonal migrants. Members of the migratory species sometimes remain as residents as is true of Thomson’s gazelle. Species that are residents at the inselbergs include mountain reedbuck (Redunca 10-07-97 09:20:38 jaaas AP: JAA 209 TROPICAL GRASSLAND HUNTER – GATHERERS fulvorufula), klipspringer (Oreotragus oreotragus), and steinbok (Raphicerus campestris). Bohor reedbuck (Redunca redunca), waterbuck (Kobus ellipsiprymnus), hippo (Hippopotamus amphibius), and crocodile (Crocodylus niloticus) occur in or near the Athi River. This high density and species richness of large animals supported a large carnivore population, including lions, leopards, hyenas, and jackals. The density of lions and large herbivores encountered by Roosevelt (1910) attests to the richness of this area for a hunter, both human and animal. The Lukenya Hill Archaeological Record Archaeologists have repeatedly investigated aspects of the prehistoric record at Lukenya Hill since 1970 (Barut 1994; Bower et al. 1977; Bower and Nelson 1978; Gramly 1976; Gramly and Rightmire 1973; Marean 1990, 1992b; Marean and Gifford-Gonzalez 1991; Merrick 1975; Miller 1979; Nelson and Kimengich 1984) and both LSA and MSA occupations have been identified. The LSA in East Africa dates between about 40,000 B.P. and 1000 B.P. (Robertshaw 1995), and much of this time is sampled at Lukenya Hill. All of the LSA lithic assemblages at Lukenya Hill are microlithic, but those dating earlier than about 21,000 B.P. lack geometrics and the backed blades tend to be large. After 21,000 B.P. geometric microliths, backed blades, outils écaillé, and a standardized end scraper (fan scraper) become common. This sequence is similar to others in East Africa where the predominant raw material is a poor quality quartz or other intractable raw material. No formal names exist for these LSA assemblages, but I have divided them into three basic groups based on major technological features, following J. Deacon’s (1984) recommendations: Late Pleistocene Non-Geometric (40,000 to about 22,000 B.P.), Late Pleistocene Geometric (broadly equivalent to the period including the Last Glacial Maximum; 22,000 to 10,000 B.P.), and Holocene Aceramic (10,000 to AID JAA 0309 / ai04$$$$41 roughly 5000 B.P.). MSA assemblages are present but undated. I have studied four Lukenya Hill sites where faunal assemblages produced by hunter – gatherers are preserved. Other sites either lack faunal assemblages, have very small ones, or are clearly pastoral occupations. Site GvJm19 is a rock overhang that forms a small sheltered area. Three archaeological components are present: a Late Pleistocene Geometric, a Holocene Aceramic in the early Holocene, and a middle Holocene assemblage with ceramics and domestic animals. GvJm22 is a rockshelter with two major components: a Late Pleistocene Geometric that dates to the Last Glacial Maximum, separated by a disconformity from a late Holocene occupation with ceramics and domestic fauna. GvJm46 is an extremely large open-air site sampled by eleven 1-m2 pits. About 30 – 50 cm below the surface begins a dense Late Pleistocene Geometric occupation that varies between 30 cm to 1 m thick, much of which dates to the Last Glacial Maximum and perhaps earlier. Faunal and artifactual material is nearly absent at 170 cm below the surface where a thin lag deposit of eroded rock is found. Below 170 cm is a dense MSA occupation. GvJm62 is a small open-air site with three components: a Late Pleistocene Non-Geometric, a Late Pleistocene Geometric, and a Holocene pastoral occupation at the top. Excavation details, radiocarbon dates, and artifactual descriptions for these four sites can be found elsewhere (Barut 1994; Bower et al. 1977; Bower and Nelson 1978; Gramly 1976; Gramly and Rightmire 1973; Marean 1990, 1992b; Marean and Gifford-Gonzalez 1991; Merrick 1975; Miller 1979; Nelson and Kimengich 1984). The best sampled time unit is the period between 20,000 and 12,000 years BP; GvJm19, GvJm22, GvJm46, and GvJm62 all have occupations dating to that time, and all four sites are within several hundred meters of each other (see Fig. 7). A wide range of zooarchaeological evi- 10-07-97 09:20:38 jaaas AP: JAA 210 CURTIS W. MAREAN FIG. 7. Map showing a close-up of Lukenya Hill and the location of the four Lukenya Hill sites discussed in the text. dence bears on the models of grassland adaptations, including species richness, mortality profiles, and skeletal element representation. I have described the Lukenya Hill zooarchaeological and taphonomic data elsewhere (Marean 1990, 1991, 1992b; Marean and Gifford-Gonzalez 1991), so I will briefly review the pertinent interpretations here. None of the Lukenya Hill sites show much evidence for carnivore involvement as either accumulators or ravagers. This is shown by the low frequency of carnivore tooth marks and the lack of carnivores in the assemblages. Rodents (such as porcupines) are also weakly and inconsequentially implicated in the taphonomic history of the assemblages as documented by the low frequencies of rodent gnawing. Together with the abundance of stone tool cut marks and hammerstone percussion marks on the bones, and the abundant lithic artifacts, the AID JAA 0309 / ai04$$$$41 data suggest that people were the primary accumulators of these assemblages. Taphonomic processes differentially affected the preservation of bones from these sites. The two sites with the largest samples, and thus the most useful for examining skeletal element abundance, differ significantly in bone survival. The fauna from GvJm46 suffered severe destruction after hominid discard by abiotic processes. Few shaft fragments were longer than 2 cm, and only the most dense portions of bone survived. A completeness index developed to measure postdepositional destruction documents extraordinary breakage due to abiotic processes at GvJm46 (Marean 1991). GvJm22 suffered much less postdepositional destruction. The most prudent conclusion is that the skeletal element representation at these two sites cannot be usefully compared, while GvJm19 and GvJm62 have skeletal ele- 10-07-97 09:20:38 jaaas AP: JAA 211 TROPICAL GRASSLAND HUNTER – GATHERERS ment sample sizes too small to be analytically useful. For this reason I will focus my attention on species representation and mortality profiles, and exclude a discussion of skeletal element representation. Species Richness The raw data on species recognition and representation at Lukenya Hill have been reported elsewhere (Marean 1990, 1992b; Marean and Gifford-Gonzalez 1991). The most abundant large mammal in the MSA and Late Pleistocene LSA deposits at Lukenya Hill is an extinct small alcelaphine antelope. This alcelaphine was smaller than any extant East African alcelaphine, but similar in body size to the modern bontebok/blesbok (Damaliscus dorcas) found in southern Africa. The extinct alcelaphine had an extremely high hypsodonty index, suggesting an abrasive diet. I have not assigned it to a precise extant taxon nor named a new species as the material is simply not diagnostic enough to warrant a specific assignation. Other taxa that are well represented in the undated MSA and Late Pleistocene LSA deposits include species currently abundant on the Athi-Kapiti Plains, such as Thomson’s gazelle, wildebeest, burchell’s zebra, and warthog. Many other extant species are also represented but in small numbers. Several other rare but notable occurrences include the extinct giant buffalo Pelorovis antiquus, and two species that are currently not found in the region: Grevy’s zebra (Equus grevi) and oryx (Hippotragus oryx). The faunal representations suggest both similarities with and departures from the modern system. Some features present today in the AthiKapiti Plains were clearly also characteristic of the past: bush habitat was present on and immediately beside the inselberg, and the Athi-Kapiti Plains were predominantly grasslands. The presence of species specialized to dry grass feeding, such as Grevy’s zebra and oryx, suggests that the sur- AID JAA 0309 / ai04$$$$41 rounding grasslands may have been more arid for longer periods of the year than is the case today. This would account for the rareness of the wildebeest, a moist grass feeder, and the greater abundance of the extinct alcelaphine, a possible dry grass specialist (Marean 1992b). Alternatively, the lack of wildebeest and abundance of the extinct alcelaphine may reflect differences in their migration patterns and how these correlated with the scheduling of visits to the inselberg by hunter – gatherers. Figure 8 shows the species richness for all Lukenya Hill hunter – gatherer occupations plotted against sample size. It is well known that species richness increases as a function of sample size, and this relation is typically a logarithmic function (Grayson 1984). Plotting species richness against sample size allows one to compare richness values of similar sample size visually, but also facilitates empirical corrections to the sample size affect with an analysis of standardized residuals (Rhode 1988, Jones et al. 1983). Clearly, as sample size in the Lukenya Hill assemblages increases, so does richness. The overall correlation, however, is insignificant (r Å .51, p ú .05) due to the presence of two distinct linear relationships. One relationship, composed of most of the sites, displays a greater intercept with a similar slope. The second relationship, composed of the two GvJm46 occupations plus the small Holocene sample at GvJm62, displays a lower intercept. Both occupations at GvJm46 clearly show much lower species richness than other sites of comparable sample size, and remains of the extinct small alcelaphine comprise all but a small fraction of the sample. GvJm62 is dominated by domestic cattle. To summarize, the LSA and MSA occupation at GvJm46 are unique at Lukenya Hill in that they have large sample sizes and very low species richness. GvJm62 also has low species richness but the sample size is very small and it is clearly a pastoral occupation. 10-07-97 09:20:38 jaaas AP: JAA 212 CURTIS W. MAREAN FIG. 8. The log of species richness (number of taxa) versus the log of sample size for the Lukenya Hill sites. Rockshelters are shown as circles, open-air sites are shown as triangles. The Late Pleistocene LSA occupations at GvJm19, GvJm22, and GvJm46 all are roughly contemporaneous and date to the Last Glacial Maximum, have similar sample sizes, and are close together. However, they differ in that the rockshelters (GvJm19 and GvJm22) are species rich while the open-air occupations (GvJm46) are species poor. We can conclude that the GvJm46 occupations show a specialized species representation that reflects a focus on one species to the exclusion of the other species that were clearly present, as documented at GvJm19 and GvJm22. Mortality Profiles Mortality profiles are graphical representations of estimates of the age-at-death of fossil taxa. Three basic mortality models are current. The catastrophic model (Klein and Cruz-Uribe 1984), or life-structure model (Stiner 1990), has a frequency of age groups AID JAA 0309 / ai04$$$$41 resembling a living population. A profile where very young and very old individuals surpass other age classes is called the attritional model (Klein and Cruz-Uribe 1984), or U-shaped model (Stiner 1990). The third model, the prime-dominated model, has a dominance of prime-age adults (Stiner 1990). These models all assume that the mortality profile preserved in an archaeological site closely resembles the age representations of prey killed by people in the past. Mortality profile analysis has grown increasingly sophisticated in its theory addressing the ecological meaning of the profiles. However, theory that relates to the formation of mortality profiles in the archaeological record is less robust. More simply put, we do not know how closely the profile we generate from archaeological sites, what I will call the archaeological profile (Fig. 9), resembles the actual mortality profile of prey species as killed by the people in the past (the death profile). It is typically the death 10-07-97 09:20:38 jaaas AP: JAA 213 TROPICAL GRASSLAND HUNTER – GATHERERS profile we wish to reconstruct. The two main processes that stand between the archaeological profile and the death profile are differential destruction of teeth, and differential transport of heads of different size/age prey. Klein and Cruz-Uribe (1984) have argued, due to the less dense and fragile nature of deciduous dentitions compared with adult dentitions, that the archaeological profile is likely to be biased against juvenile individuals still without adult dentitions. These are juveniles typically in the first 10% age interval. Thus our estimates of the relative representation of juveniles will nearly always be underestimates, and the relative representation of these individuals will vary widely as a function of the extent of destructive processes. Heavily comminuted assemblages, such as GvJm46, will have a greater loss of juveniles compared with adults than a less comminuted assemblage, such as GvJm22. This means that the discarded profile (the profile discarded at the site by the occupants) will differ from the archaeological profile. I have argued that a check can be made on this loss by estimating the number of juveniles from bone fusion and comparing that to the dentition-generated mortality profile (Marean 1995). Unfused bones are less dense than fused bones (Brain 1981) so this is not a perfect test; however, it does provide a second estimate of the number of juveniles present. The problem of differential transport is potentially more manageable. A wide range of ethnoarchaeological studies has shown that many contingencies affect hunter – gatherer transport of carcass parts (Binford 1981; Bunn et al. 1988; O’Connell and Hawkes 1988; O’Connell et al. 1989). Several basic patterns have emerged from these studies. Other contingencies being equal, hunter – gatherers transport smaller mammals more completely than larger mammals. The significance for mortality studies is that animals may pass through several body size AID JAA 0309 / ai04$$$$41 categories on their way to adulthood. Any animal in the size 3 category (which, as defined by Brain 1981, includes red deer, caribou, reindeer, wildebeest, hartebeest, and many others) begin their lives in size 1 with deciduous teeth, pass through size 2, and end up in size 3 with a fully adult dentition. Thus zooarchaeologists should anticipate that decisions for the transport of heads should vary as the animal ages, and the shape of the mortality profile will be sensitive to these decisions. The data on body part transport by Hadza hunter – gatherers shows that the heads of size 2 animals are transported more often than the heads of size 3 animals (Bunn et al. 1988, O’Connell and Hawkes 1988, O’Connell et al. 1989). A second pattern is that, other contingencies being equal, large groups of people transport animals more completely than small groups of people. This observation is particularly problematic for researchers attempting to compare mortality profiles between sites that sample different settlement systems. For example, if one settlement system was occupied on average by large groups of people, then the residential base may have a more comprehensive mortality profile because the large group of people can reduce transport bias for large animals. A settlement system occupied on average by a small group of people may show greater selectivity of large heads, thus biasing against older animals at the residential sites. Hunter – gatherers also transport animals more completely the shorter the distance to be transported. Thus if the encounter site (the place where an animal was killed or scavenged) is close to the residential camp there may be less bias in the archaeological mortality profile than if the encounter site is far from the residential site. This means that the transported profile (the sample of heads transported from encounter sites to residential sites) may differ from the death profile. Importantly, the transported profile may or may not differ from the encounter- 10-07-97 09:20:38 jaaas AP: JAA 214 CURTIS W. MAREAN FIG. 9. Flowchart showing the taphonomic stages of alteration that mortality profiles pass through before interpretation by the archaeologist. This set of stages is equally applicable to skeletal element profiles, as recognized by many researchers. point profile (that sample of heads discarded at the encounter site). Figure 9 illustrates these points. One way to attempt to account for these problems is for zooarchaeologists to sample the mortality profiles from various site types within a hunter – gatherer settlement system. By sampling prey encounter sites and residential sites, we can control for the varying transport behaviors that were in operation. This is at least partially possible with the Last Glacial Maximum occupations at GvJm22 and GvJm46, because, as I will discuss later, GvJm22 is a residential site and GvJm46 is a kill site. AID JAA 0309 / ai04$$$$41 Figure 10 shows the mortality profiles for the extinct alcelaphine from GvJm22 and GvJm46 plotted as a histogram and Fig. 11 shows the age profiles plotted as a ternary diagram with the three models illustrated following Stiner (1990). All three mortality profiles show a dominance by the first and second adult age classes (prime-age adults) and the frequency of individuals declines rapidly after that. All three profiles would classify as ‘‘prime-dominated profiles’’ following Stiner’s criteria, and all are statistically indistinguishable (Kolmogorov – Smirnov Test, p ú .05). 10-07-97 09:20:38 jaaas AP: JAA 215 TROPICAL GRASSLAND HUNTER – GATHERERS shown by the dentitions. Including these juveniles makes the profile look much more similar to a ‘‘catastrophic profile.’’ It is possible that even more juveniles were originally present than recorded by the preserved unfused bones. If the GvJm22 and GvJm46 mortality profiles are sampling the same population of hunted antelope then we would expect that the seasonal signatures between the two sites would be similar. It is not possible to estimate season of death with cementum increments from the Lukenya Hill dentitions because the teeth are nearly all isolated and weathered. If there is a strong seasonal signal to the killing identifying seasonally sensitive peaks and troughs in crown heights may be possible. Figure 12 shows the incidence of adult teeth in metric units equal to about two months of growth for GvJm22 and GvJm46. Both sites show a weak tendency to have synchronous peaks of abundance at 12-month intervals, but the small sample sizes certainly warrant caution in the interpretation of these data. FIG. 10. Frequency histograms showing the mortality profiles with the ages broken into 10% increments of the estimated ecological longevity of the extinct small alcelaphine. The thick bars show the number of individuals by teeth, and the thinner black line on top of the bars shows the correction provided by bones. As I noted earlier, most mortality profiles will be biased against younger individuals due to the greater susceptibility of deciduous dentitions to destruction. Each of the mortality profiles shows a correction of the mortality profile derived from the bone fusion data. These corrections are taken from early fusing unfused bones. As we do not have fusion tables for the extinct alcelaphine, I have used ages from goats, which are roughly the same body size as the extinct alcelaphine. The bones show that many more juveniles are represented than are AID JAA 0309 / ai04$$$$41 DISCUSSION The Last Glacial Maximum occupations at GvJm19, GvJm22, GvJm46, and GvJm62 show interesting patterns of similarity and contrast. Given the close proximity of the sites and the overlapping dates of occupation, it is likely that these sites were used as parts of the same settlement system. Several lines of evidence suggest that GvJm19 and GvJm22 were residential sites in this settlement system: (1) the abundance of lithic artifacts, (2) the presence of hearths and burnt bone, (3) the rockshelter location, and (4) the high species richness of the fauna sampling several nearby habitats. Despite a diversity of animals, the small extinct alcelaphine was clearly the preferred prey animal at GvJm22, while hartebeest was the preferred prey at GvJm19. This could potentially represent a difference in the seasons 10-07-97 09:20:38 jaaas AP: JAA 216 CURTIS W. MAREAN FIG. 11. Ternary diagram showing the mortality profile models as defined by Stiner (1990), the position of the extinct small alcelaphine from GvJm22 LSA (black circle), GvJm46 LSA (open star), and GvJm46 MSA (black star) without correction by including bones (above) and with correction by including bones (below). of occupation, though we currently have no data to test this idea. GvJm46 has a different topographic context than the other sites. GvJm46 is on a slope AID JAA 0309 / ai04$$$$41 at the base of a steep cliff (Fig. 7). No natural shelter exists over the site, so it is directly exposed to sun, rain, and wind nearly the entire day. GvJm46 is a very large site 10-07-97 09:20:38 jaaas AP: JAA 217 TROPICAL GRASSLAND HUNTER – GATHERERS FIG. 12. The frequency of the extinct small alcelaphine by 2 month age increment (estimated metrically from crown height) in the LSA of GvJm22 and GvJm46. The lines connecting the two graphs are placed at 12month intervals. though the exact dimensions are not yet documented. The 11 excavated pits sample only a fraction of the deposit, suggesting that the unexcavated assemblage is immense and vast numbers of this small alcelaphine antelope likely remain unexcavated. A ravine formed by a seasonal stream runs to the north of the site, starting at the inselberg and running southwest onto the plains. This ravine is shrouded by a gallery forest in an otherwise grassland habitat. The wooded ravine and the long cliff face form a natural box with the site of GvJm46 within. As Fig. 7 shows, the migration of animals is split by Lukenya Hill, and the right split passes directly through the bottleneck formed by Lukenya Hill and the Mua Hills to the east. It is this excellent hunting location that likely accounts for the density of sites on the eastern face of Lukenya Hill. AID JAA 0309 / ai04$$$$41 Several lines of evidence converge to suggest that GvJm46 was a mass-kill site where the small extinct alcelaphine antelope was repeatedly killed in Late Pleistocene LSA and MSA times: (1) the open location in a natural topographic trap situated in a bottleneck along a well-documented migration route, (2) the concentration of one species of grassland antelope compared to high diversities of large ungulates at contemporary nearby residential sites, (3) the catastrophic/ life-structure mortality profile, and (4) the likelihood that GvJm46 represents many different kill events. If the site were formed by just a few kills, the volume of animals necessary to account for the kill, including the unexcavated material, would be enormous. Animals were probably driven between the wooded ravine and the cliff face as they migrated north, and if the ethnographic literature is any guide, placing people in the wooded ravine to keep animals from escaping the trap would have been effective. The landscape may even have been modified, though no obvious evidence survives to attest to that possibility. Alternatively, it is possible that animals were driven off the cliff above onto the slope, though I think this is less likely. The mortality profiles from GvJm46 closely resemble the catastrophic/life-structure model, and this is the anticipated profile shape when a mass kill of animals has occurred. However, it is important to note that there is very little taphonomic data to support this assumption. The mortality profiles from GvJm22 and GvJm46 are visually similar and statistically indistinguishable. Above it was argued that mortality profiles will be affected by differential transport decisions that in turn are affected by distance between the encounter site and the residential site, number of people available for transport, and size of the animal. If differential transport were occurring, then mortality profiles from transported assemblages (GvJm22) and encounter assemblages (GvJm46) should 10-07-97 09:20:38 jaaas AP: JAA 218 CURTIS W. MAREAN differ. The close similarity between GvJm22 and GvJm46 suggests that differential transport was not occurring, assuming these sites were part of the same settlement system. This is not surprising, however, because the distance between these two sites is only several minutes walk and the animal being transported not particularly large. When the evidence from GvJm19, GvJm22, GvJm46, and GvJm62 is considered together, the Seasonal Grassland Model best describes the exploitation of the Athi-Kapiti Plains during the Last Glacial Maximum. It posits a seasonally shifting strategy where tactical landscape use is employed during seasons when migrations or aggregations of mammals result in a situation conducive to mass killing. GvJm46, as showed by its low species richness, catastrophic mortality profile, and topographic location, is best explained as a tactical kill site. The Seasonal Grassland Model posits that during other seasons of the year hunters will switch back to a generalized strategy of killing a diversity of large mammals and collecting more plant foods. GvJm19 and GvJm22 represent residential sites where this greater species richness of prey animals is represented. I argued earlier that the Seasonal Grassland Model was unlikely to apply to tropical African grasslands. Thus, the Late Pleistocene evidence from Lukenya Hill is inconsistent with my original predictions. Though there currently is no excavated MSA rockshelter site at Lukenya Hill with an informative faunal assemblage, the close similarity between the MSA and LSA occupations at GvJm46 suggest similar interpretations for these occupations. This indicates that as early as the MSA, people on the AthiKapiti Plains were using tactical techniques, and probably the communal hunting strategies necessary to make tactical techniques function. Thus, the GvJm46 occurrence is, so far, the only MSA site in Africa where tactical techniques are directly implicated, though Klein has argued, from the evidence AID JAA 0309 / ai04$$$$41 at several cave assemblages, that MSA hunters in South Africa killed eland with tactical techniques (Klein 1989). Several Middle Paleolithic sites in Western Europe also display evidence of tactical hunting (Coudoulous, La Borde, Mauran, Le Roc; see discussion in Mellars 1996) and the Mousterian record from the Caucasus may also have evidence of tactical hunting (Hoffecker et al. 1991; Baryshnikov and Hoffecker 1994). These data suggest that Middle Paleolithic/Middle Stone Age hunters in Europe and Africa were capable of highly organized hunting behavior. The Holocene occupations at GvJm19 and GvJm62 are much less informative due to the small samples and the lack of site variety. The early Holocene occupation at GvJm19 shows a species rich fauna similar to the Last Glacial Maximum levels of GvJm19 and GvJm22. This is surprising as East Africa was colder and drier during the Last Glacial Maximum and warmer and wetter during the early Holocene (Hamilton 1982). Given these differences we would anticipate a richer fauna in the early Holocene, but this is not documented at Lukenya Hill. The Holocene sites represented at Lukenya Hill show species rich faunas that are most consistent with the Generalized Grassland Model. There is no evidence for the presence of a Holocene mass-kill site at GvJm46, nor anywhere else at Lukenya Hill. This could be a sampling problem. There is no other Holocene locality in East Africa, however, that resembles a mass-kill locality despite many excavations at numerous sites in diverse regions. In the earlier discussion it was argued that East African grasslands are sufficiently rich in plant foods and resident animals that risky time-intensive strategies such as tactical landscape use simply are not needed. If the Seasonal Grassland Model was employed as a hunting strategy during the Late Pleistocene, then either something is amiss with the models and the ecological parame- 10-07-97 09:20:38 jaaas AP: JAA 219 TROPICAL GRASSLAND HUNTER – GATHERERS ters as I have defined them, or the relevant ecology differed sufficiently from the present state in the Late Pleistocene of Africa as to make the Seasonal Grassland Model the favored hunting strategy. The lack of evidence for tactical landscape use in the ethnographic record, and in the Holocene archaeological record, argues against the former. However, paleoenvironmental data from East Africa suggests that ecological conditions were, indeed, much different during the Last Glacial Maximum. The Last Glacial Maximum climate in East Africa was roughly 5 to 7 degrees centigrade colder than present as shown by the placement of glacial moraines on several East African mountains (Osmaston 1989a, 1989b). It is not yet known how this temperature difference was distributed by season. Rainfall in the wooded areas of East Africa was less as indicated by the general lowering of lake levels, increase in grass pollen, and changes in vegetation zones on mountains (Hamilton 1982). Paleolimnological evidence suggests that rainfall may have been more seasonal during the Last Glacial Maximum (Richardson and Richardson 1972), occurring in one season as opposed to two, and it is possible that some grassland ecosystems received similar levels of rain as today but more tightly restricted in time. The cooler temperatures would also have lowered evapotranspiration rates, keeping grasses moist and green longer after the rains. These different climatic conditions may have lowered the return rates during the Late Pleistocene for several classes of food items compared with the Holocene. More seasonal rainfall and lowered temperatures are likely to have lowered the species richness and density of AGPs, thus increasing search costs and making them more susceptible to patch depletion. USOs would also have been less diverse and dense in cooler grasslands (see the discussion above), so it is likely that search costs were higher and AID JAA 0309 / ai04$$$$41 return rates were lower for USOs during the Last Glacial Maximum. Large mammal biomass in Africa correlates positively with rainfall (Coe et al. 1976), and thus if rainfall were less during the Last Glacial Maximum then resident mammals would likely have been less diverse, more widely spaced, and less abundant due to the lowered forage quality of the vegetation. This would have increased the search costs for resident mammals and made them more susceptible to patch depletion. Migratory mammals may have been less abundant and less diverse due to the lowered rainfall and lowered forage quality. When these animals are closely packed into a mobile patch during a migration, however, it is likely that search costs would not change dramatically because the animals would still be passing by in large numbers. Also, it is unlikely that even intensive stone age hunting strategies would deplete migratory herds to the point that return rates would be significantly decreased during the period that a herd was being preyed upon (see discussion in Kelly 1995). The grasslands of East Africa during the Last Glacial Maximum may have been transformed in the direction of temperate grasslands with less diverse and less dense plant foods and greater seasonality of all food items. The lowered species richness, lowered density, and increased seasonality of these resources may have made tactical landscape strategies a more effective strategy, just as they were in temperate grasslands. CONCLUSIONS Cold, temperate, and tropical grasslands are similar in important ways: water and raw materials are often scarce, rainfall is highly seasonal, the most abundant mammals are large gregarious and mobile herbivores. Tropical grasslands differ in some important ecological parameters that should affect hunter – gatherer foraging strategies. For example, in the tropics there is a greater 10-07-97 09:20:38 jaaas AP: JAA 220 CURTIS W. MAREAN species richness and biomass of edible AGPs and USOs making carbohydrate more readily availability and less seasonal in availability. Large mobile mammals are more diverse and have greater biomass and the migrations are characterized by a succession of complementary feeders. Resident large mammals are also more diverse and have greater biomass. Overall, tropical grasslands are a richer and less temporally punctuated environment than either cold or temperate grasslands. These ecological parameters led to the construction of three models for hunter – gatherer exploitation of tropical grasslands. The model deemed most likely for East African grasslands was the Generalized Grassland Model. In a preliminary test of these models the Holocene archaeological evidence is most consistent with the Generalized Grassland Model. The archaeological evidence dating to the Last Glacial Maximum on the Athi-Kapiti Plains is most consistent with the Seasonal Grassland Model. These differences in hunting strategies were probably due to the different ecological conditions of the Last Glacial Maximum that made the East African grasslands more similar to temperate grasslands in several ecological parameters. ACKNOWLEDGMENTS This paper benefited from the helpful comments of David J. Bernstein, Robert J. Blumenschine, John R. F. Bower, Jeanne Sept (referee), John J. Shea, and an anonymous referee. 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