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1 Approaching a Field-Study of African Wildlife Personal note. The topic of this assignment is important to me for two reasons. First, it expresses my conviction that tropical habitats are special—and that, to me, Africa is the most special tropical place of all. I have tried to teach biology in North and South America, Southeast Asia, and southern Africa. I have liked all of these places, in different ways, but the first and the last found their way most deeply into my consciousness. South Carolina is, of course, my home; her Lowcountry will always be the place of my heart. But if I have a second home, it is in Africa, where the sounds and the smells and the laughter are a familiar benediction to any child of sand and palmetto. Besides, for a natural historian, Africa has to be special. In Africa, at the coming of the rains, frogs build nests in the trees. In Africa, termites taste like cashews. In Africa, pythons grow to Dream-Time size. And only in Africa can we still observe what the world of mammals was like at the height of the Pleistocene. When I have taught in Zimbabwe, my highest priority has been to convince my students that the biodiversity of their Africa is something truly extraordinary. But this is not a lesson for African biologists only. Rather, the special-ness of Africa is a lesson for all biologists—indeed for all people—everywhere. Second, when I’m a little sad—particularly on lonely evenings, when most folks can become somewhat melancholic—I often cheer myself up by considering the evolutionary processes that filled the world with its present biota. (Yeah, I agree that I’m a couple of standard deviations beyond “weird.”) During these pensive moments I do not usually conceptualize vertebrate natural history as a set of formal phylogenetic diagrams, complete with bootstrap values and estimates of divergence times. Instead, I visualize evolution as a great dance in which the world-music is ever-changing and the dancers swirl in intricate patterns—coming together, dividing, disappearing, arriving, dying, giving birth. I am thankful that I have available, for my most difficult times, this vision of Life. My dearest hope for Interim ’08 is that Africa will conjure up this vision for you. My objective in this assignment is to explain, in very dull words, what the African manifestation of that vision is all about. Introduction: Illustrating Tropical Biodiversity. The business of a biologist is to study life, and therefore every good biologist needs to live for at least a few years in the tropics, where life is seen in so many forms. Let me illustrate this statement with two tables. The first reports mammalian diversity in three geographical areas where I have worked— the South Atlantic USA, Vietnam, and Zimbabwe.1 As you can see, in all taxonomic categories, the tropical areas hold an enormous numerical advantage. 1 The South Atlantic USA is defined as South Carolina, North Carolina, Virginia, Maryland, and West Virginia. Although taken from field guides and other publications, the numbers in my table should be considered approximate. (Decisions had to be made about animals at the margins of geographic areas, about reintroduced species, about species recently extinct, and about the splitting or lumping of species and genera.) To be fair to landlocked Zimbabwe, neither cetaceans (whales, etc.) nor sirenians (sea cows) were included in the counts for Vietnam or the USA. 2 Place S.Atl. USA Zimbabwe Vietnam Mammalian Diversity: the Temperate Zone versus the Tropics Area (km2) # of Orders # of Families # of Genera 394426 8 16 48 390308 13 38 123 329566 11 33 79 # of Species 74 193 118 The second table considers another vertebrate group, the squamate reptiles (snakes, “lizards,” etc.). We’ll count varieties of these animals in South Carolina and in my second home, Old Mutare2 (in eastern Zimbabwe). Place South Carolina “Old Mutare” Squamate Diversity: the Temperate Zone versus the Tropics Approx. area(km2) # of Families # of Genera 70,000km2 7 29 7,500km2 15 61 # of Species 50 96 Note that Africa’s numerical lead in reptilian biotypes is as impressive as its advantage in mammalian diversity. I am less familiar with birds and insects, but I’m told that inter-continental comparisons within these groups show similar results. Indeed, African biodiversity is impressive across a wide range of living taxa—and although the northern continents3 surpass Africa in the diversity of some biotypes, these are exceptions, not the rule.4 Now, before we really get started with the scientific stuff, here’s the first sermon: I want you Interim students to go to Africa with a passion to observe biodiversity (and I don’t mean just elephants). Why Such Richness? Africa as a “Species Machine.” I. Listing reasons for African species-richness. If we admit that Africa’s biodiversity is impressively high, then perhaps we should ask why this is the case. To begin my attempt at an answer, I’ll merely list three related reasons. Then, as you probably expect, I shall discuss each reason in some detail. First, refugia in Africa have preserved more species than one might expect, judging by comparison with other continents. 2 Old Mutare is the site of Africa University, the United Methodist institution at which I have taught for several semesters. Data on squamates are taken from Branch, 1998, and Conant & Collins, 1991. A species was considered in “Old Mutare” if I thought Branch’s range map for the critter came within 50km of the place. If a Conant & Collins range map colored any part of South Carolina, the referent species was credited to the State. (Note that South Carolina has about 10 times the area of my Old Mutare 50km circle.) 3 Other tropical areas are typically about as rich as Africa—except in really big mammals. For many taxa, South American biodiversity exceeds Africa’s (at least on a per-area basis), and the species-richness of South and Southeast Asia is extremely impressive. 4 As a South Carolina herpetologist, I proudly note two particular exceptions. My State is blessed with many species of turtles and salamanders—while sub-Saharan Africa is not particularly rich in turtles and lacks salamanders entirely. 3 Second, evolution in Africa has created more species than one might expect, judging by comparison with other continents.5 Third, fine-grained niche-partitioning in Africa has allowed sympatric species to be packed more densely6 than one might expect, judging by comparison with other continents. II. Africa as species preserver.7 I shall begin with the preservation factor because it is super-important and is very easy to understand. As most of you know, Cenozoic (post-dinosaur) times have witnessed the flowering of countless mammalian, avian, and squamate8 species. Yet the vast majority of these species have become extinct, and in most biomes the faunas that we see today give only a glimpse of the diversity that characterized the pre-Holocene Cenozoic. So, if extinction has been the rule, then a continent capable of retaining a larger percentage of her ancient species would be especially rich in vertebrate biodiversity. I shall argue that Africa has been the species-preserver par excellence because she spans a wide range of both latitudes and altitudes and because African latitudes are approximately centered on the Equator. A. The role of latitude in species-preservation. In a sense the theme of this essay could be, “Why is Africa biogeographically bigger than Eurasia?”9 Eurasia, of course, is much larger in raw, geographical area. Furthermore, both Africa and Eurasia have approximately the same amount of latitudinal spread (about 70 degrees from north to south in each case). However, Eurasia runs basically from the extreme Arctic to about 10oN, with most of the continent lying between the Arctic and 35oN.10 On the other hand Africa extends between roughly 35oN and 35oS. Now consider the effects of the Pleistocene’s global coolings, and ask what happened in Asia and Africa during the Ice Ages. The most dramatic features of these times were the vast expansions of continental glacial sheets, which extended southward to about 40o—or even further in upland areas (see map below). During these Ice Ages11 much of the world—even beyond the glaciated areas—became colder and drier. 5 Historically and logically, Point B precedes Point A. However, these points are easier to understand if discussed in reverse order. 6 That is, in Africa, more species can co-occur in the same amount of area. 7 In the argument that follows I shall focus largely on mammals because, in southern Africa, mammals are often considered to be the very essence of wildlife. Nevertheless, the basic point holds for many non-mammalian taxa as well, and occasionally I’ll indulge myself by discussing reptiles. 8 But here’s some trivia: North America has preserved almost all of its squamate species! 9 The importance of absolute size is so obvious that I do not need to discuss it. 10 Leaving aside India and the Malay-Indochinese Peninsula (both of which are special cases, in many ways biogeographically similar to Africa), the southern latitude of Eurasia is not far different from the northern extent of Africa. Incidentally, you Wofford students should be familiar with the latitude of 35oN; that’s about where our college is located. 11 Of course there was not just a single ice age but rather a succession of glacial expansions and interglacial periods. Indeed, some climatologists argue that nowadays we are living within another warm time between ice ages. 4 However, such climate changes were least extreme in equatorial regions,12 which remained throughout all changes the warmest and wettest areas on Earth. In other words, during the Ice Ages, much of northern and central Eurasia was covered by 3-4km of ice, and most of unglaciated Eurasia experienced a climate roughly comparable to that of today’s Siberia.13 Only in the Indian and Indo-Malayan14 margins did Asian tropical habitats persist, and even there the tropical refugia tended to be small, patchy, and isolated. In Africa, by contrast, the north-to-south width of the tropical biomes did constrict, but near the Equator they always persisted. Thus, at no time was any major African habitat-type in danger of complete disappearance. So, here’s the take-home message: Africa’s vast latitudinal extent, centered on the Equator, helped preserve the continent’s many, diverse, habitat-biomes. And because Africa preserved her habitats, Africa also preserved her species. B. The role of altitude in species-preservation. Africa’s equatorially centered latitudinal breadth was probably the primary factor in maintaining the continent’s species diversity. But we must also recognize the role of altitudinal variability in preserving “islands” of habitat within a “sea” of change. As you know, both temperature- and moisture-regimes vary with altitude, and mountain ranges create microclimates on their slopes and in their shadows. Furthermore, African landforms vary tremendously in altitude, ranging from below sea level to almost 6000m. Thus, during a particularly dry time, moist environments might persist on mountaintops or along windward slopes. Conversely, when the general climate was exceedingly wet, residual patches of xeric Climate remained essentially unchanged at the poles, but that’s irrelevant to the present story. 13 Many parts of southern Eurasia were cold-temperate in climate. 14 This is why Indo-Malayan Asia is, biogeographically, a “little Africa.” 12 5 habitat might still be found within the rain-shadows of a mountain range.15 Given Africa’s complex matrix of altitudes, when Pleistocene climate-changes altered most habitats across a biome, patches of the old habitats would usually be preserved. In other words, during the Pleistocene’s dramatic fluctuations in temperature and rainfall, major habitat-types might disappear from vast stretches of Africa, but topographically localized habitat-patches would preserve every African biome—at least on a small scale. And within these microclimate “refuges,” many African species managed to survive. C. Summary of species preservation. Thus, as world climate shifted from warm to cold, many elements of the rich Asian mammal fauna were pushed into extinction by habitat loss—while African species were able to remain in habitat by adjusting their latitudinal or altitudinal ranges. So Africa retained more of its pre-Ice-Ages mammal fauna than did Eurasia.16 IV. Africa as species-creator. But Africa has been not only a species-preserver; she has also been an extraordinary species-creator!17 A. The general importance of allopatric speciation. I hope that you are already familiar with this concept from a basic evolution or high school biology course. The basic idea is that geographic isolation can be a primary step in species-creation: interruption of gene-flow across a parent population allows two daughter populations to “fine tune” their adaptations for their separate environments. If the interruption persists, the daughter populations can pursue separate evolutionary trajectories, eventually becoming separate species. To understand allopatric speciation in Africa, we need to know how various parent populations were split in two. This requires us to consider again the peculiarities of African climates—and to learn how these climates changed during the Pleistocene sequence of Ice Ages and Interglacials. B. Allopatric Speciation: the Continent-scale Big Picture. At a fine-scale level,18 patterns of change were intricate, as I shall eventually try to explain. However, as an 15 Our conventional view of the Pleistocene, based upon the geo-history of the northern continents, envisions climate-shifts between warm and cold. In Africa, however, the most dramatic alterations were between wet and dry. That is why, in this section, I have emphasized the preservation of moisture-defined habitats. 16 Of course there were many exceptions (Asian survivors, African extinctions) to this generality. Note that arguments for Eurasia also apply to North America, which might be considered as part of a greater Laurasian landmass. The isolated continents, South America and Australia, are special cases, never as rich in mammals as Africa (or as Eurasia at its height). 17 As we shall see, the phenomena of species creation and species preservation are closely related. 18 I shall not define the term precisely, but by “fine-scale,” I mean blocks of perhaps < 10,000km2. 6 initial simplification, let us consider two ecotype-belts that, as the Pleistocene climate fluctuated, bisected Africa at different times in different directions.19 (Please see the pair of maps immediately below and also the explanation that follows them. Then consider some examples.) 1. An east-west axis of rainforest expansion and contraction. During warm, moist Interglacials, rainforest extended as a broad belt from the Atlantic all the way to the Indian Ocean. But during glacial extremes, rainforest was limited to patches along Africa’s west coast and in the center of the continent—plus a few tiny refugia close to the Indian Ocean. Oversimplification: During wet times, a barrier of rainforest splits parent-populations of arid-adapted species into two big “islands” of dryness. 2. A northeast-southwest axis of arid-land expansion and contraction. During glacial periods, when so much water was tied up in ice sheets, a band of desert extended all the way from from Namibia to Somalia, sundering what at other times was a vast stretch of rainforest. During moist Interglacials, as explained above, the relevant deserts were confined to the Somali Arid Zone in the northeast and the Namib Arid Zone in the southwest. Oversimplification: During dry times, broad barriers of desert split parent-populations of rainforest species into smaller “islands” of moist habitat. 3. Examples of living biotypes20 with separate populations in 2 present-day “islands” of dryness. During our Holocene Interglacial, Africa’s rainforest presently dominates (slightly) and more or less bisects the arid-land belt. Consider in that light the distribution 19 When we discuss the distribution of African rainfall, we should remember that East Africa is usually drier than one might expect, given the region’s latitude. This is because of the “monsoon effect”: during most climatic epochs, the summer warming of the enormous Asian landmass turns moisture-bearing east-winds northward. This robs East Africa of much summer rain. 20Note that some of my examples are related species-pairs (or paired species-sets; Oryx and Gazella have what I’d call “isolated sub-genera” in north and south). Most are subspecies-pairs or genetically isolated populations without separate taxonomic status (depending on whom you ask). The range of one biotype (the giraffe) may not be completely bisected. If you have time, it would be instructive for you to examine range maps for these animals and also to read accounts of their preferred habitats. 7 of 8 African dry-land mammals listed below—all of which have disjunct populations in the Somali (northern) and Namibian (southern) Arid Zones. a. The genus Oryx (different species-sets in north & south; a northern variety is shown below) b. The genus Gazella (different species-sets in north & south; a northern variety is shown below) 8 c. The giraffe (one species with multiple subspecies or geographic races; the rangebisection for this species is less complete because giraffe are ecologically less strictly confined to the arid zone; a southern example is shown below) d. The “white” rhino (one species; 2 subspecies or geographical races; the very modern distribution of the white rhino has been disrupted by over-harvest and restocking; a southern example is shown below) 9 e. The Grevy’s zebra and the mountain zebra (two ecologically similar species; a Grevy’s zebra, the northern variety, is shown below) f. The aardwolf (one species; a southern example is shown below) g. The bat-eared fox (one species; a northern example is shown below) 10 h. The black-backed jackal (one species; a southern example is shown below) C. Allopatric Speciation: the local, or micro, level. As we have seen, on the continental level, Africa experienced vast habitat sunderings and reunions, caused by the alternating expansions and contractions of the two above habitat axes (the east-west wet and northeast-southwest dry). Simultaneous with these cycles, a similar phenomenon was occurring on smaller geographical scales throughout much of Africa. As the continent generally dried out, “islands” of moist habitat could persist on the tops and rain-slope sides of mountain ranges. As Africa became wetter again, these humid refugia would expand and dominate—while arid habitats would contract to “islands” within the rain-shadows of mountain ranges. Thus, throughout the general climatic fluctuations, populations of some taxa could cling to their habitat-niches by adjusting their distributions up and down the slopes of Africa’s numerous mountain ranges. In other words, throughout all the changes, Africa retained remnant habitat islands, strips, and pockets—all with varying degrees of isolation, all affecting the evolutionary trajectories of Africa’s biota. And don’t forget the basic point: isolation can lead to speciation! To illustrate the importance of small biotype refugia, we can consider briefly the snake fauna of southern Africa—at a slightly finer resolution. Today about 25 essentially South African snake species have relict populations far to the north, in Zimbabwe’s Eastern Highlands. The highly disjunct distribution of the rinkals cobra, Hemachatus haemachatus (right), is a particularly striking (but not atypical) example. D. Allopatric Speciation, Summary: Africa as a species-creator. In other words, with each Pleistocene climate shift, some African species expanded their ranges, some contracted their ranges—and some were forced into or released from genetically isolated 11 refugia. In a sense, the Pleistocene alterations turned Africa into a gigantic speciation machine.21 The intricacies of this process can be illustrated by considering a single species-complex, and that is the next task that I shall address. (What follows is complex and particular to one Tribe of hoofed animals. Biology majors interested in mammals should definitely read on, but my discussion of duikers may get a bit sticky for other folks. Also, duikers are not a hugely important component of the mammalian faunas in the areas we are likely to visit; only one species is found in Namibia.) V. Africa’s duikers: A more particular example of species-creation and preservation—and an introduction to the concept of species-packing. A. Introduction: what are duikers and where do they live? Duikers are small forestantelopes. Ecologically all are concentrate-selectors, which means that they choose dietary items carefully to maximize concentration of nutrients. Most duiker species in most habitats obtain the majority of their calories by eating fallen fruit, but all supplement this diet (particularly for protein) with leaves. The basic duiker body form is rounded, with proportionally massive hindquarters that are higher than the animal’s shoulders (below, center); heads are large and are usually held low. Duikers exhibit little sexual dimorphism; in most species both genders have small, short horns.22 Duikers are usually considered to be a separate Tribe of the Subfamily Antilopinae, within the Family Bovidae. They are given this separate taxonomic status in large part because of the structure of their preorbital glands. These consist of several secreting tissue-layers encapsulated into a single organ by connective tissue. Each preorbital gland opens into a hairless slit (illustrated below, right). One slit is on each side of the nose, beneath and forward of the eyes.23 In most bovids, (simpler) preorbital glands are rubbed against leaves and twigs to leave smell-markers that designate territory, thus warning other members of the species against trespassing. This is true for duikers too. In these small antelopes, however, the preorbital glands have an equally important function in cementing pair bonds (see “Sociality,” below).24 Duikers are split into two genera (the common duiker belongs to a monospecific genus) including about 16 species.25 As a group they are found in almost all sub-Saharan habitats except deserts and the most open grasslands. However, the epicenter of their current distribution is West Africa and the equatorial rainforest. 21 Of course this was true, to some degree, for every substantial piece of real estate (including many aquatic habitats) on Earth. I have argued, however, that for terrestrial vertebrates, the species-birth/species-death ratio was generally higher in Africa than elsewhere. 22 In a few varieties only the males have horns. 23The preorbital glands account for the cheek swelling that is particularly noticeable in males. 24 Apparently all duikers are monogamous. 25 Fifteen species comprise the genus Cephalophus; the common duiker is placed in the monotypic genus Sylvicapra. Some modern authorities also separate the blue duiker into its own genus. 12 B. Appreciating the general biological importance of duikers. Natural historians find duikers fascinating in their own right. Furthermore, to the theoretical biologist, these antelope are particularly important for two reasons: First, they partition their habitat more intricately than any other “large” terrestrial animal I know about. (For example, 6 duiker species live sympatrically in the rainforest of Gabon.) A study of duikers’ partitioning mechanisms could be important for general ecology, and it is clearly relevant to our eventual interest in African species packing.26 Second, the duiker-Tribe’s distribution provides insight into Pleistocene speciation processes—creation and preservation—in sub-Saharan Africa. Here the basic ideas involve (a) waves of cross-continent duiker colonization during Pleistocene wet periods and (b) stranding of relict duiker populations in separate refugia during dry periods. During such times of fragmentation, allopatric speciation could at least begin. C. Background in duiker natural history. Before we can fully appreciate hypotheses about the speciation of duikers, we need to learn something about the biology of these little antelopes. 1. Ecological strategy. Some duiker species are strict specialists, and others can persist under a wider variety of ecological conditions. However, as suggested above, all species in most habitats (and some species in all habitats) obtain most of their calories by eating fallen fruit. This diet is supplemented by leaves, which are usually selected carefully for their protein/fiber ratio. Most duikers eat a few fungi for calories. Probably all species eat some animal matter (mostly insects or carrion), and on rare occasions some species have been observed hunting small vertebrates such as birds or mice. (This suggests that the basic diet is not over-rich in protein.) Duiker mouths have a wide gape, and few forest fruits are too big to be taken in and chewed. Saw-like cheek teeth enable duikers to chew roots and bark; the animals 26Remember, we said Africa was so rich in mammalian biodiversity for three reasons. Thus far we have been discussing species-preservation and species-creation. Before our discussion of African biodiversity is complete, we must also tackle the phenomenon of species-packing. And can you imagine six species of little hoofed animals living in one North American forest? 13 also occasionally dig for special foods with their feet or even their noses. The precise nutritional reasons for digging and bark-chewing are not well understood. 2. Sociality. Let us first consider pairing and territoriality. All duiker species are basically monogamous.27 Pairs (mated, it appears, for life) defend small territories against same-sex conspecifics. Duikers generally live in low-visibility conditions, and their territories are apparently designated largely by chemical markers. Most of this marking is done by the male. Pair bonding is also chemically facilitated. In all species that have been studied, mated males and females spend a lot of time standing head to head, pressing their preorbital glands together. It is not known exactly how this cements pair bonding. (I suspect that gland-pressing mixes both animals’ scents so that “signposts” put out by the male also carry information about the female pair-member. Perhaps it’s almost as if the male is signaling, “P.S., I’m married.”) Although territorial boundaries appear to be marked largely by secretions from preorbital glands, duikers also leave scent-information from their pedal glands as well as in feces and urine. Probably, then, entire duiker territories are densely packed with chemical information—and for duiker family members within territories, the lasting matrix of smells may substitute for continuous visual contact (which, given the nature of duiker habitat, must be rare). In all duiker species that have been studied, breeding appears to be nonseasonal—even in strongly seasonal habitats. Neonate birth weight is about 10% of mother’s weight. Gestation periods are not well known for most species; lengths between 3 and 7 months have been reported. As far as I know, duikers do not commonly reproduce more than once per year. 3. A few notes on habitat partitioning. A general principle of ecology suggests that very similar species living in the same area should evolve partitioning-mechanisms to minimize inter-specific competition. That is, the various species should use different parts of the habitat or use the same parts in different ways or at different times. This is what I have called habitat partitioning, and duikers appear to be very good at it, though the full details of their success are incompletely known. Here are a few clues that biologists have recently begun to uncover. Even within habitat that appears uniform to the human observer, different animal species may restrict their activities largely to tiny patches of microhabitat. For duikers of similar size,28 the main dimension of such partitioning seems to be based on soil moisture. Some species select very wet areas; others prefer microhabitats that are somewhat drier. Such partitioning along a moisture gradient may reflect (or reinforce, or even cause) dietary specializations. Also, when duiker species exploit very similar microhabitats, some species are typically day-active while others are generally nocturnal. These two hypotheses about habitat-partitioning are interesting to me, but their importance is far from clear, and further research is needed. On the other hand, biologists are more certain about the importance of duiker size. 27 Generally rare among mammals, monogamy is particularly rare among artiodactyls. See the following paragraph for information on how duikers select their microhabitat according to species-size, which appears to be the most important factor in the Tribe’s habitat-partitioning. 28 14 In forests where duikers of different sizes co-occur, primary microhabitat is selected according to the adult body-size of each duiker species.29 Here is how that works. Although duikers are anatomically adapted to push their way through thick habitat, cross-branches of dense understory vegetation will nevertheless impede a duiker’s movement. So each duiker species selects microhabitat characterized by the “lowest” vegetation that the species can easily negotiate. The smallest duiker species can go almost anywhere, but they stay mostly in the low stuff to avoid competition with larger members of the Tribe. And, as you would expect, progressively larger species inhabit progressively “higher” understory.30 E. Duiker species accounts. In order to flesh out our understanding of duiker biology (and before we turn finally to the topic of duiker speciation) we should learn a bit more duiker natural history. In this context I shall write about two very different species, the blue duiker (Cephalophus monticola) and the common duiker (Sylvicapra grimmia).31 In my opinion, these are the two most different members of the Tribe; when you know how they operate, you will have a good handle on what duikers can and cannot do. Again, some of this material relates to a species that we will not see. Therefore, the casual reader (if she or he is perchance still reading…) might wish to skip to the boxed topic immediately above Section D, which concerns the more general topic of duiker speciation. Cephalophus monticola, the blue duiker. These antelopes are small creatures. Males average about 35.5cm high, with mass of about 4.6kg. As is usual among duikers, the females are slightly larger, averaging about 36.2cm high, with mass of around 5.4kg. As the common name implies, this species’ fur often has a bluish cast, but under some light conditions the color appears slate-gray or grayish-brown. Short horns (about 5cm) are usually borne by both sexes but are sometimes absent in females. Blue duikers are diurnal, open-forest fruit-eaters ranging throughout moist central Africa and into relict forest habitats to the east and south. Males and females live in monogamous pairs on tiny territories (typically about 2.5-4ha). Territorial boundaries are marked largely by the male, mostly using preorbital gland secretions. A pair’s entire territory is probably mapped by a matrix of chemical signals (from preorbital glands, from pedal glands, from urine and feces). Pair bonds in C. monticola are established after an intricate courtship involving aural, visual, and especially chemical signals. Pair solidarity between mated animals is periodically reinforced when the female and male press their preorbital glands together. Whenever possible, a pair of blue duikers likes to maintain immediate contact. They do 29 Of course all duikers are small antelopes, and size differences important to habitatpartitioning would probably not be noticed by a casual observer. (Also, in my experience, all duikers are highly secretive, and a merely “casual observer” would be unlikely to observe any duikers at all.) 30 Some research also suggests that vegetation “passage-heights” in African rainforest are distributed in discrete steps, which duiker species-heights (also discrete) have evolved to match. 31 Note that these two species are depicted above. 15 this by making short, soft moans (probably inaudible at more than 10m) and also by visually signaling with up-down flicks of their tails (which are white underneath). Gestation in C. monticola is probably about 4 months. Young are weaned after an additional 2.5-3 months. First-year mortality in one study was estimated at 34%. Annual adult mortality is about 10%; female mortality may be slightly higher. Young blue duikers usually out-migrate as yearlings, but unlike many other antelopes, blue duiker parents make little apparent effort to chase their offspring away. Females usually disperse first (at 1-1.5 years; males tend to depart their parents’ territories at about 1.9 years). Nearly anything will eat a blue duiker, but major predators include pythons and crowned eagles. The photograph below shows an intrepid white hunter who has faced Death and managed to slay the fearsome blue duiker. Sylvicapra grimmia, the common duiker. Compared to forest duikers (such as C. monticola), this antelope has longer legs, a flatter back, and straighter horns, which occur in males only. The color is red to gray. Sizes have been recorded as follows: Males Females Height (Zambia) Mass (Zambia) Mass (Botswana) 50cm 12.9kg 18.7kg 52cm 13.7kg 20.7kg The common duiker is found throughout sub-Saharan Africa except in rainforest.32 Almost its only habitat requirement is for patches of very dense cover, so the common duiker can live right up to the edge of villages, and in many African habitats it is the last antelope to become extinct. Despite the common duiker’s near-ubiquitous distribution, it has not been well studied. The animal is highly adaptable, eating fruits, leaves (perhaps more than most other duiker species), and flowers. In areas where tsetse-control programs attempt to kill 32 If we keep our eyes open, we should be likely to see this little antelope in some brushy part of Namibia. 16 off all wild ungulates, common duikers may actually increase in numbers—probably because the removal of major grass-eaters allows cover to grow extremely dense (this makes common duikers difficult for people and other enemies to find). Mammalogists assume that the common duiker is monogamous and territorial and that its preorbital glands play a significant role in courtship and pair-maintenance. Breeding is apparently year-round, even in highly seasonal South Africa. During courtship the male may follow the female for several days, making various signals and eventually licking her genitals before copulating. Gestation has been variously estimated at 3 months, 5.7 months, and 7months.33 One captive pair bore 11 young over about 8 years; mean inter-birth interval was 259 days. The single offspring is quite precocial; it can be up and running within 24 hours after birth. Common duikers are preyed upon by medium-sized mammalian carnivores. Pythons also take a great many. And here is another intrepid human hunter, this time with a common duiker. The main take-home point of my species-natural-history discussion is that different types of duikers might respond in slightly different ways to habitat-alterations caused by the fluctuating climates of the Pleistocene. If Africa was a speciation-machine, then duikers comprised excellent raw material upon which the “machine” could operate. This generalization is discussed at length below. D. Duiker radiations and speciation. As I have suggested above, duiker speciation was associated with Africa’s Pleistocene shifts between wet and dry times.34 During interpluvial35 periods various duiker populations were isolated in moist refugia—in which they began to follow separate evolutionary trajectories, adapting more precisely to local conditions. When rainy climates returned and the moist forests expanded once more, these isolated duiker populations were brought back into contact within a more or less uniform habitat. Within this wider, wetter, and friendlier world, the best-adapted duiker 33 These estimates cannot all be correct. Do you begin to understand that biologists do not know a great deal about this abundant animal? 34 These are often called, respectively, pluvial and interpluvial periods. Why don’t you make those your vocabulary words for the day? 35Literally, the word means between the rains. I know I should have used the term “dry periods,” but I really like the way interpluvial sounds. I just can’t help it. 17 forms spread most widely—and often displaced the less-adapted forms, driving them to extinction by competitive exclusion. Then, when the African world dried out yet again…. Metaphorically one could consider the pluvial duiker expansions as ocean waves— waves that would recede as the world dried, leaving isolated populations stranded on distant “shores.” When the next wet-times duiker “wave” arrived, it could wipe away (by genetic swamping) the strandings that had been left by the previous wave. However, if subsequent waves did not reach quite so far, then some ancient strandingspecies would remain in isolation for a very long time. Some population-strandings from the Pleistocene persist today as relict-species. Thus Cephalophus adersi (of Zanzibar and Kenya’s isolated coastal forests) is a northern relict derived from a very ancient radiation of red duikers: no subsequent “waves” ever reached the habitat-fragments in which it survives. Probably C. natalensis is a relict of a later “wave” that left this duiker stranded in the south. Fossils suggest that both of these ancient duiker types once had other populations more widespread throughout the heartland of Africa’s moist-forest block—but these populations were completely replaced by overflowing “waves” of duikers belonging to the C. callipygus biotype.36 V. Conclusion. I want to thank you for reading this essay. During an actual visit, only the dullest heart could find African wildlife less than exciting, personally, at the visceral level. I also want you to understand that naturalhistory-type observations, which you could make in Africa, can also have serious intellectual important. So, I have tried to give you some background information that I have found relevant in my own African work. I am sorry that I have been so wordy—and that, at times, I have meditates on esoteric examples more meaningful to me than to you. But please don’t forget the basic points, that Africa has been a species-creator, a species-preserver, and a species-packer. Namibia, of course, has played an important role in this Dance of evolution: it has been the unfailing refuge of aridity, the deep and ancient home of species requiring a particular sort of southern dryness. I have never been to this place, and I am so, so excited that you all are allowing me to explore it with you. 36 Our old friend, the blue duiker (C. monticola), was an exceptional case. This animal is the smallest and probably one of the most ancient members of the Tribe—and yet it is also one of the most widely spread members. I believe it became so extremely and so perfectly specialized (to such a widespread habitat) that none of the “waves” washing over it included an effective competitor.