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Journal of Archaeological SCIENCE Journal of Archaeological Science 31 (2004) 753–762 http://www.elsevier.com/locate/jas The grasshopper or the ant?: cultigen-use strategies in ancient Nubia from C-13 analyses of human hair H.P. Schwarcz a*, C.D. White b a b School of Geography and Geology, McMaster University, Hamilton, Ontario L8S 4M1, Canada Department of Anthropology, University of Western Ontario, London, Ontario N6A 5C2, Canada Received 28 July 2003; received in revised form 12 November 2003; accepted 17 November 2003 Abstract Sections of human hair from naturally desiccated Sudanese Nubian mummies representing X-Group (AD 350–550) and Christian (AD 550–1300) periods in the Wadi Halfa area were analysed by Journal of Archaeological Science, 20 (1993) 657. These data can be interpreted in terms of a model of annual variation of food consumption that apparently remained stable for more than 1000 years. The diet oscillated annually between consumption of 75% C3 foods (wheat and barley) in winter, to as much as 75% of C4 foods (millet and sorghum) in the summer. There was little use of stored summer foods in the winter, whereas a small amount (c. 25%) of winter foods were present in the summer diet; part of this may be a carry-over in the form of a C3 isotopic signal in the flesh of C3-fed animals. This evidently small use of stored grains suggests that grain storage facilities (granaries) were largely used as emergency measures. During normal years, the diet in a given season was dominated by freshly harvested crops. ! 2003 Elsevier Ltd. All rights reserved. 1. Introduction Isotopic analysis of human tissues allows us to reconstruct the nutrient sources of ancient populations [11,25]. This is possible because, in many regions, available nutrient sources, such as diverse cultigens, differ substantially in their natural stable isotopic compositions: !13C, !15N.1 Since these differences are transmitted faithfully to the living tissues of the consumers, they provide a basis for establishing the relative proportions of the various foods that were actually utilized by the peoples studied. For example, pre-Columbian populations of North America have been shown to have shifted from a diet of wild, C3 (low-!13C) plants and C3-consuming herbivores to a diet containing progressively more of the C4 plant maize, starting around AD 700 and reaching a climax around AD 1500 [13,25]. * Corresponding author. Tel.: +1-905-525-9140; fax: +1-905-546-0463 E-mail address: [email protected] (H.P. Schwarcz). 1 !13C is defined as follows: !13C=({[13C/12C]sample/[13C/12C]standard} "1)#1000 in per mil (‰). !15N is similarly defined for 15N/14N. 0305-4403/04/$ - see front matter ! 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2003.11.004 Typically, such isotopic paleodietary studies are based on the !13C analysis of collagen in human bones. The residence time of carbon atoms in collagen of living bone is on the order of 10 years [15]. Therefore isotopic paleodiet studies of this material give us only mean consumption patterns averaging over the last ten or more years of life. It would also be desirable to show dietary variations on a shorter time scale, especially where seasonal variations in food availability are suspected to have existed. For this purpose, it would be necessary to analyse tissues which had a shorter turnover time, either because they regenerate quickly (such as muscle or organ-tissues) or because they are fast growing components of the body such as skin, hair or nails. In earlier studies [29,31] we showed that naturally mummified human remains from the hyper-arid Nile Valley in Nubia contain preserved soft tissues which can be used to test shorter time-scale dietary variations. White [29] showed, in particular, that hair segments revealed large variations in !13C which she attributed to changes in the relative use of C3 grains (barley and wheat) vs C4 sorghum and millet through a seasonal 754 H.P. Schwarcz, C.D. White / Journal of Archaeological Science 31 (2004) 753–762 Table 1 Planting and harvesting seasons in the Sudan (modified after US Department of Agriculture, 1960 [17]) Table 2 Isotopic composition of modern plants used in ancient Nubia and sampled from the Nile Valley (after White and Schwarcz [31]) Commodity Planting season Harvesting season Scientific name Common name Plant part !13C (‰) C4 plants sorghum millet July–August July–September October–November October–December C3 plants wheat barley November–December November–December March–April March–April C4 plants Setaria viridis Digitaria sanguinalis Pennisitum divisum Panicum coloratum Sorghum durra S. sudanensis millet millet millet millet sorghum sorghum seed seed seed seed seed seed October–November October–November November July–August July January–mid-April February–March February–March March October October mid-May–June August–November "10.8 "10.6 "12.1 "13.1 "11.0 "12.4 Mean"11.7 SD 0.9 C-3 plants Triticum vulgare Hordeum vulgare Vigna membranaceae Cajanus cajan Acacia nilotica A. albida Hibiscus esculentus Eruca sativa Raphanus sativus Allium sepa Solanum melongena Corchorus olitorius Hibiscus sabdariffa Balanites aegyptiaca wheat barley cowpeas pigeon peas acacia beans acacia beans okra garden rocket radish tops onion eggplant jew’s mallow rosella date seed seed seed seed seed seed leaf leaf leaf skin leaf leaf fruit fruit "27.0 "22.1 "25.1 "27.2 "25.0 "27.5 "21.0 "30.3 "30.9 "28.0 "28.4 "28.2 "23.3 "27.1 Mean"26.5 SD 2.9 broad beans beans, dry chick peas groundnuts sesame onions dates raisins lemons oranges grapefruit September September–August September–December September–December cycle. In this paper we shall look further at these data in order to extract additional information about the seasonal cycle of cultigen use vis a vis the importance of food storage in this area. The samples analysed represent members (N=17) of the so-called X-Group (AD 350–550) and Christian (AD 550–1300) populations, excavated by the University of Colorado near Wadi Halfa, in northern Sudan. The site is now submerged beneath the waters of Lake Nasser. The plant foods available to these people and the probable times of the year when they were harvested are shown in Table 1. The isotopic compositions of the plant foods are determined by the photosynthetic pathway which they utilize. The winter-harvested grains (barley and wheat) are C3 plants which today have isotopic compositions averaging around "26‰, while the summer-harvested crops (millet and sorghum) are C4 plants whose !13C values are today around "12‰. These values would have been about 1.5‰ larger (less negative) in pre-industrial times [16]. Table 2 shows the isotopic composition of the major cultigens, corrected for this shift. A sample of sorghum collected by P. Rowly-Conwy at a nearby Christian site gave a !13C value of "9.7‰. The isotopic composition of eggs (of duck and geese) and meat from animals would be close to the !13C values of the diets of their source animals, because there is not much fractionation between these materials and diet. Milk, due to its higher lipid content, would be up to 2‰ lighter than the total diet. Because they were produced over limited time spans, the isotopic compositions of eggs and milk would have varied seasonally, as a function of the composition of available fodder, whereas the !13C of animal flesh would presumably represent a longer-term average of food consumption by the animal. 2. Analytical procedures Hair was isotopically analysed according to analytical procedures described by White [29]. Briefly, bundles of 15 or more strands of hair were cut into 2–4 segments of 20 mm length. The normal rate of growth of hair is about 0.35 mm/day [23], so the segments would represent about 50–60 days of growth. The samples were cleaned of lipids with methanol, dried, reacted with CuO at 550 (C in sealed, evacuated pyrex tubes to produce CO2, and analysed on a VG602 mass spectrometer to determine !13C, with a precision of about 0.2‰. The isotopic data are reproduced in Table 3 from White [29]. 3. Temporal analysis of the data The data are shown in Fig. 1, plotted as a function of distance from the scalp. The data for single bundles of hair were graphically presented by White [29, p. 663], who noted that the isotopic trend from hair tip (T) to scalp (S) was most commonly in the direction of increasing !13C, terminating with values that indicated greater levels of C4 food consumption. We have subsequently 755 H.P. Schwarcz, C.D. White / Journal of Archaeological Science 31 (2004) 753–762 Table 3 Analyses of hair bundles Site/burial Age (y) Hair segments (scalp to tip, !13C (‰ VPDB)) Sex th b S1 S2 S3 S4 2 4 6 8 $!s1"sx$a X-Group (AD 350–550) NAX 26 3 3 20 25 22 9 21 29 18 25 35 F ? ? M M F ? M F M F F "13.2 "18.8 "18.5 "13.8 "18.7 "19.6 "18.0 "14.6 "18.8 "13.5 "15.4 "17.1 "13.4 "19.5 "18.5 "15.7 "17.3 "19.2 "18.3 "14.6 "18.7 "15.6 "15.0 "16.4 Christian (AD 550–1300) 6G8 2 ? 9 6 33 ? 6B13 7 19 29 1 ? F M F ? "15.5 "11.7 "15.9 "18.5 "17.2 "15.5 "13.0 "17.8 "19.5 "18.1 24I3 a b 551 552 580 611B 640B 645 659 671A 676A 5 21 43 0.2 0.7 0.0 1.9 2.3 1.4 0.9 3.3 0.1 2.1 0.9 3.8 "16.4 "18.2 "18.9 "17.9 "14.5 "13.3 "15.6 "13.3 "19.2 "15.9 "14.9 "19.3 0.4 3.2 3.4 1.0 0.9 $!s1"sx$=absolute value of difference between first (s1) and last (sx, x=2,3,4) segment. th=estimated relative age of hair segment (months). "16‰) statistically have less variable ($!s1"sx$, Table 3) values along their length than bundles with higher (less negative) values (Student’s t=2.2, df=15, P>0.05). This suggests that these samples represent different phases in the agricultural year at this site. We have therefore attempted to treat the analyses of each bundle of hair as a record of the history of change in !13C in actively growing tissue through time, and we have tried to synthesize these data into a more-or-less complete picture of changing diet through time for this site. This approach is based on the following assumptions: Fig. 1. Isotopic ratios (!13C) of hair segments of Nubian mummies, plotted as a function of distance from scalp. The open circles are the analytical value positioned at the centre of the segment. The solid points at the end of each line represent the ends of the analysed hair segments. noticed that these data can be organized in a different fashion than shown by White [29]. Specifically we note that hair bundles with low !13C values (i.e. below 1. the average !13C of hair growing on any given date in a solar year was always about the same over long (multi-century) time periods. Although bone, skin and muscle register greater consumption of C4 foods during the X-Group period [31], the means of hair samples for these periods do not show a significant difference in the consumption of C3 versus C4 foods (Student’s t=0.87, df=42). 2. the samples of hair come from individuals who died throughout the entire year (although the mortality rate may have been higher in some seasons). 3. the rate of growth of hair was approximately constant in the population; 4. there was a constant isotopic fractionation between hair and diet (!hd). 756 H.P. Schwarcz, C.D. White / Journal of Archaeological Science 31 (2004) 753–762 2. There are no bundles whose data represent periods of slow change and high !13C values. 3. The samples with the highest !13C values exhibit steep positive and negative gradients. 4. Two pairs of hair bundles from different individuals (24I3 # 21, 6G8 #2) give closely matching data for their three sample points, or segments, indicating that these individuals had consumed similar diets for a similar period of time shortly before death. A third individual (NAX 551) showed no change in !13C over two segments, although its average !13C was relatively high ("13.4‰). 5. During the Christian period (dashed lines) !13C dominantly increases from T to S (toward the time of death) whereas values for X-Group samples increase as commonly as they decrease. 6. The total range of !13C values from "12‰ to about "19.5‰ suggests that the diet varied by as much as 50% in the proportions of C3 and C4 foods through the year. Fig. 2. Data from Fig. 1, replotted with an arbitrary origin for distance, and with hair bundles arranged to form a continuous curve. This appears to be a unique arrangement; no other method of arrangement gives a single continuous curve. Segments NAX 551, 580, 676A have been omitted (see text). Solid lines=X-Group; dashed lines=Christian. Based on the first assumption, we have arranged the individual records on a single plot in such a way that, as closely as possible, they define a single roughly continuous function (Fig. 2). The T-point on each bundle is plotted to the left, so that the graph shows date of growth increasing from left to right. The overall pattern is a “U”-shaped curve which could be interpreted as a single year’s sample of a continuous periodic function, representing the long-term variation in !13C in the diet, offset by !hd. The graph includes the data for the X-Group and the Christian period. This method of plotting shows that, although the data are taken from two temporally and culturally distinct populations, they seem to form a single homogeneous data set. Inspection of Fig. 2 reveals certain characteristic features: 1. The majority of the bundles display !13C values that are low (<"16‰) and slowly varying; a smaller number of bundles present data from periods of gradual to rapid change in isotopic composition, with !13C values both increasing and decreasing. We believe that the U-shaped trend of Fig. 1 corresponds to a seasonal trend of variation in !13C through most of a yearly cycle. Since the individual samples represent deaths occurring over hundreds of years, the data must represent a persistent, continuous trend which was repeated every year for centuries. Based on the foregoing assumptions, we infer that this annual cyclicity in !13C corresponds to a seasonal shift in the isotopic composition of the foods consumed by these people. If we assume that the rate of growth of hair was constant throughout the year, then the x-axis scale of length is proportional to the time of growth. From Fig. 2, the complete cycle from one maximum to the next takes place during the growth of about 140 mm of hair. Assuming that the complete series represents approximately one year of growth, the hair grew at 0.38 mm/day which is comparable to the average rate of 0.35 mm/day [23]. 3.1. Variation in !13C Hair is made of the protein keratin. Like other proteins, the isotopic composition of a sample of hair represents the average !13C of the diet, biased toward that of the protein component, due to the tendency of the body to construct proteins from pre-formed dietary amino acids rather than to synthesize the non-essential amino acids from other dietary sources, especially carbohydrate and lipid [1,14]. However, the main protein sources for people at this site, as indicated by !15N values of human tissues, would likely have been domesticated animals (mammals, birds) which were fed agricultural products [31]. There is very little offset in !13C H.P. Schwarcz, C.D. White / Journal of Archaeological Science 31 (2004) 753–762 (<1‰) between the flesh (or other edible products) of animals, and the plant foods which they are fed [26]. Also, we know that these variations in !13C are not in any sense intrinsic to hair growth. For example, sequential analyses of hair from individuals from Egyptian oases showed no systematic trends in !13C along hair bundles, even though both C3 and C4 cultigens were available [7,30]. We suppose therefore that the variations seen at Wadi Halfa are entirely due to seasonal variation in the proportion of C3 (wheat, barley, fruits, vegetables, pulses) and C4 (sorghum, millet) plants consumed by these individuals, as described by White [29]. In order to calibrate the changes in !13C of hair with respect to the amounts of C3 and C4 foods eaten, we must know the isotopic fractionation between diet and hair. O’Connell et al. [20] found that hair was depleted in 13C by 1.4‰ with respect to bone collagen. Assuming a diet-collagen fractionation of 5‰ [25], this implies that hair is enriched by about 3.5‰ with respect to diet. Other controlled feeding studies show an offset of 1 to 1.4‰ between diet and hair [6,10,18,27]. Estimates of the hair-diet fractionation (!hd) for humans in noncontrolled feeding contexts range between 2.6 and 3.2‰ [8,12,19,21]. We will assume an average enrichment of 3‰. Fig. 2 shows that !13C of hair varied between "19.0%0.5 and "11.5%0.5‰. This corresponds to a variation in diet between "22 and "14.5‰, which would correspond to a shift from dominantly C3 (low !13C) to C4 (high !13C) plant foods, either consumed directly by humans as bread and other plant-based foods, or consumed by animals and then transferred to humans as meat, milk and eggs. If we neglect the possible minor role of wild grasses in the animal diet, then most of the cyclic (seasonal) variation in !13C appears to be a shift from dominant use of C4 plants to dominant use of C3 plants and then back again. That is, the maximum and minimum !13C values correspond to the times of maximum consumption of C4 (13C-rich) and C3 (13C-poor) crops, respectively, offset by the lag between the time of consumption of a given food and the appearance of its carbon atoms into the growing hair. The actual seasonal chronology of this progression in !13C values cannot be independently determined since we do not have the dates of death of these individuals. In any case, the date of appearance of a given diet-based isotopic signal will be offset from the date of consumption of that nutrient by a delay corresponding to the time elapsed between consumption and protein synthesis [20,29]. The times of harvest of the C4 and C3 dominant crops are in early summer (June, the seifi crop) and late-fall to mid-winter (November–January, the shitwi crop) respectively. These two harvests are almost exactly six months apart, consistent with the symmetrical form of the curve in Fig. 2. A third crop (dameira), which combines seifi (C4) plants with fruits and vegetables (C3) 757 is harvested in October. Assuming a constant lag between consumption of a food and appearance of its isotopic signal in hair, the dates of appearance of signals will be temporally spaced in the same way as the dates of consumption of the respective crops. We can use the inferred isotopic composition of the diet at the isotopic extremes of Fig. 2 to calculate the range in proportion of C3 and C4 foods in the diet with the formula from Schwarcz et al. [25]: %C4! "!c!!3#!d!h$"100 "!4!!3$ where !3 and !4 are the !13C values of the C3 and C4 foods, respectively. This formula assumes that the quality of the respective C3 and C4 foods (i.e., proportions of various amino acids, lipids, carbohydrates) were similar. If we assume that the C3 foods (wheat, barley) had !13C values of about "26‰ while sorghum had a value of "10‰, then the most 13C-depleted diet consisted of about 75% C3 foods, while the most enriched diet consisted of about 25% C3 foods. Referring to the percent C4 food in the diet as PC4, the hair data suggest that PC4 varied between a low value of 25% and a maximum of 75%. 4. Discussion We have shown that !13C of hair at Wadi Halfa varied through the course of a year reflecting change in the average !13C of diet through the year. The same extent of variation was seen both in the earlier X-Group and the later Christian population at this site. This degree of variation was not clearly shown by analyses of either skin or muscle [31], although both of these tissues should turn over their carbon atoms on a time scale of less than one year. Presumably the continuous secretion of keratin by hair follicles provides a better running monitor of changing diet, although its variation too would be somewhat decreased by the buffering effect of the protein stores in the body. The minimum and maximum PC4 values which we have inferred are therefore upper and lower limits, respectively, to the actual seasonally varying PC4 values of the dietary intake. Even in the absence of stored C4 plants, and at the point in time farthest removed from the harvest of C4 plants, it seems unlikely that PC4 in this population would ever have dropped to zero. This is because the diet would have continued to include the meat of animals whose year-round diet included some millet and sorghum, which are major fodder crops today [2,17]. These crops would have imparted more 13C-rich values to the meat, for which the turnover rate of C atoms is much slower than in human hair. Consumption of meat from a C4-consuming animal would result in dietary 758 H.P. Schwarcz, C.D. White / Journal of Archaeological Science 31 (2004) 753–762 !13C values equivalent to that from direct consumption of some C4 plant foods, and a PC4 value greater than zero. The seasonal alternation between C3 and C4 foods is determined by the climatic tolerance of these plants and their consequent times of planting and harvest. Although today a variety of C3 foods are planted and harvested at various times throughout the year, the staple grains are scheduled as opposite season crops (Table 1). Sorghum and millet (C4), wheat are planted between July and September and harvested between October and December. The C3 staples, wheat and barley, are planted after the annual flood from November to December and harvested in March and April. Animal-derived foods would track these seasonal isotopic shifts to the extent that the animals were fed harvested grains. This would influence !13C of eggs, milk and the flesh of domestic animals such as ducks, geese, goats and sheep. The seasonality of the !13C signal in each of these foods would depend on the size and turnover rate of the carbon pool that is represented. It appears, in any case, that the residents of Wadi Halfa tended dominantly to consume C3 plants and plantderived foods in the season of harvest of these plants, and tended to consume C4-labelled foods during the season of harvest of C4 plants, either directly or through the consumption of rapid-turnover tissues of animals (milk, eggs and meat of smaller animals). In constructing Fig. 2 we were able to combine the data from X-Group and Christian burials into a single homogeneous curved array, indicating that this pattern of seasonal consumption persisted over a span of more than 1000 years. This suggests that the choices of possible cultigens and the manner in which their harvests were scheduled were extremely consistent over these long intervals even though cultural and climatic factors (extent of Nile flooding, droughts, etc.) may have affected other aspects of life in Nubia. This consistency in choices and management of cultigens is brought out in long-term studies of food consumption in Egyptian populations [5], and continues in the descriptions of life in Coptic Egypt [4,22]. From Fig. 2 we also can note that there was no significant inter-populational difference in the most probable time of death. White [29] has noted the significantly higher mortality rate in individuals with isotopic values of hair near the rising (late spring) portion of the cycle. She concludes that the actual times of death would be a few weeks after the ingestion of these foods, due to the delay time needed for the body to equilibrate to the isotopically enriched foods. Given that the 13C-rich, C4 foods are typically harvested in June, there was a preferential time of death in late July–August, a time of increased climatic stress due to higher temperatures [17]. These effects were as prevalent in the X-Group as in the Christian Group. Three exceptional individuals (one X-Group, the other two Christian), as noted earlier, appear not to have shifted to a relatively pure C3 diet in the winter season, but rather exhibit intermediate !13C values. Presumably these were two individuals whose diet included significant amounts of both C3 and C4 foods, possibly because they lived at a time when stored C4 foods were required to supplement recently harvested winter grains. It is also possible that these individuals had differential access to foods or perhaps personal preferences in diet which resulted in their anomalous isotopic behaviour. Let us now consider the significance of the alternation in dominance of food sources through the year. Under normal conditions, the steady flow of the Nile would have guaranteed the availability of crops throughout an annual cycle. However, availability of grains could be affected by both environmental change, such as fluctuations in the level of the Nile, and political or economic policies involving trade or taxation. Trade in foods and other commodities existed along the Nile, from earliest Pharonic times [5,28], but we assume that imported food never formed a mainstay of the subsistence base. Storage technology was probably developed during the preceding Meroitic period (350 BC to AD 350) when this area was an agricultural hinterland for the Kingdom of Meröe to the south. The X-Group populations, however, represent a period of disintegration of centralized authority and were autonomous chiefdoms. As there is no current evidence of the necessity to store and submit grain as a tax, most storage at this time was likely an insurance against famine. A population with access to varying sources of food can be assumed to have various options with which to handle these supplies. Two extreme possibilities would be as follows: Immediate consumption (IC): As each seasonal crop becomes available it is consumed as the major source of nourishment for humans and animals. Storage/pooled consumption (S/PC): As each crop is harvested, the majority of it is stored in granaries, from which it is drawn upon as needed. If the latter (S/PC) model describes the most frequent behaviour of people living at Wadi Halfa, then we would expect that !13C of diet would remain almost constant through the year, as was observed at oases [7,30]. Depending on the extent to which the granaries homogenized the returning flow of food back to the consumers, the S/PC model might partially approximate the IC model. In the case of complete IC behavior, on the other hand, we would expect a seasonal variation in diet and therefore in !13C of hair (and other rapid-turnover H.P. Schwarcz, C.D. White / Journal of Archaeological Science 31 (2004) 753–762 759 Fig. 3. Interpretation of data from Fig. 1 showing possible temporal sequence in relation to possible temporal sequence. The solid bars represent possible times of harvest of the respective crops (wheat, barley in November to January; millet and sorghum between June and September). Winter consumption of 13C-depleted C3 grains leads to falling !13C values in growing tissues (i.e., hair), reaching a minimum in late spring. As consumption of C4 grains begins in summer, !13C of tissues begins to rise. Lack of a period of constant high !13C values is discussed in text. A lag time of a few weeks between consumption and appearance of an isotopic signal in the hair might have further shifted the position of these curves with respect to the corresponding dates of harvest. tissues) that would reflect the range of !13C in the nutrient sources. The data shown in Fig. 2 suggest that the Wadi Halfa population was at least partly following the IC model. Very few of the hair bundles showed constant !13C throughout their length, and most of those that did displayed !13C values close to the minimum of the population, as would be expected for those samples that happened to record the diet during the period of least availability of C4 foods (i.e., late winter to early spring). The remainder of the samples record transition from C4-dominant to C3-dominant diets, or the reverse. We propose therefore that the isotopic records of hair are transformed representations of the record of harvests of C3 and C4 plants. On Fig. 3 we show schematically how these two series could have been related in time. The falling limb of the population (decreasing !13C) would represent people who died in the spring while data in the rising limb come from people who died in the late summer. Only a small number of individuals show high PC4 values. The distribution of data is partly a function of the mortality trends in this population. The lack of individuals with high, constant !13C values presumably reflects the fact that few individuals died during the period of C3 harvests. This would have been a period of cooler climate and abundant food, possibly leading to lessened morbidity and is the time of the year when fewer people die even in modern times [17]. According to this model it is likely that at some time during the year the !13C of the hair of a significant number of people at Wadi Halfa was greater than "14‰, but they are not represented in these data because none of the sampled population had died while their hair had this composition. The following fable captures what we have found here: one sunny day in late summer the grasshopper observes his friend, the ant, busily storing away grain in his underground storage bins. The grasshopper laughs and chides his friend for such a waste of energy and goes back to nibbling on tender stalks of grass. But a few months later, when the grass has withered and cold winds are blowing, the grasshopper wonders where he can find something to eat; meanwhile, the ant is contentedly munching on his stores of summer produce. In our treatment of the data, the IC model represents the grasshopper community: all harvested resources are immediately consumed and none are stored. Ant communities follow the S/PC model: some fraction of the available food is always put into storage and drawn upon throughout the year. This would tend to even out the !13C values of the hair of consumers. Note that in some of the individuals we analysed, !13C is almost constant, but lying at intermediate !13C values of "15.5 to "16.5‰. These individuals appear to have followed the S/PC model for unknown reasons. The intermediate position of these persons’ isotopic data is consistent with the notion that when grains were stored they consisted of approximately equal amounts of C3 and C4 foods. In general it seems reasonable to suppose that, during good times, all individuals would tend to follow the IC model, consuming crops as they become available. Use of stored crops would tend to occur during times of crop failure or insufficiency. The model presented here and the data on which it is based inevitably leave many matters unknown: 4.1. Relative crop yields Although we know that both C4 and C3 crops were harvested at Wadi Halfa, we do not know the relative 760 H.P. Schwarcz, C.D. White / Journal of Archaeological Science 31 (2004) 753–762 sizes of these two crops. If the IC model is a good description of behaviour at this site, then we suppose that both crops were of comparable size, or else stored C3 (or C4) crops would have been required to see the populace through the agricultural year. That is, some partial S/PC behaviour would have been entailed by unequal crop yields. There is no clear isotopic evidence to support or reject this model. Although no granaries or other storage facilities have been reported at this site, the presence of tetracyline-labelled bone in this sample does indicate that consumption of stored grains occurred [3]. Tetracyline is produced naturally from Streptomyces infestation of stored grain and has also been found in bones at other Nubian sites [9]. We are currently carrying out further studies of the distribution of tetracycline labelling in bones from various sites in relation to other skeletal indicators of life status. We may make some comparison with crop harvests in modern Nubia. Today, the main subsistence crop is that harvested in the winter (sitwi: [4,17]). The spring crop (seifi) is dominated by C4 staples, whereas the late summer (dameira) crops combine both C3 and C4 plants. The existing literature on Nubian agronomy does not make clear the relative sizes of these seasonal crops, although the sitwi (winter) crop is considered the most important. Clearly, however, we cannot assume that these cropping practices have persisted unchanged since ancient times. 4.2. Buffering of isotopic composition One of the unknowns in the interpretation of our data is the extent to which protein and fat stores in the body were being called on to provide amino acids for keratin synthesis. If, for example, nutrient levels were generally low because one of the annual crops (presumably the summer crop of sorghum and millet) was skimpy, then keratin molecules would tend to be built from stored nutrient, and therefore dampen the shift to higher !13C values. The opposite trend is also possible, although the data suggest that it did not occur. 4.3. Effects of age and sex We would expect that juveniles, because of their smaller body mass and higher growth rates would turn over their carbon pool faster and therefore show values of !13C closer to the actual diet (i.e., less buffering). Consequently, in the summer months their !13C values would be higher than for adults and would place them closer to a pure C4 diet. The present data set shows no such separation, but one juvenile individual (6B13#29) is probably pre-weaning and its !13C values would track those of its mother. The other four children (ages between 3 and 9 years) had !15N values indicating that they had been weaned and, therefore, would have had !13C values indistinguishable from the rest of the population. The difference between $!S1"Sx$ for males and females in Table 3 indicates that male diets swing more widely and more consistently between C3 and C4 foods than do female diets (x̄$!S1"Sx$ for males=2.6%7‰, for females=1.5%1.4‰). This difference verges on statistical significance (Student’s t=1.57, df=10) and could reflect differential consumption behaviour between the sexes where males tend to use the preferred IC model and females rely more on the S/PC model. Differential access to food by males is also indicated in the larger sample by "15N values showing male consumption of different and/or more abundant protein [31]. Other differences in socio-economic status could not be tested in this sample as there are no archaeological indicators for any status differences in the population. 4.4. Dominance of C3 foods A large proportion of individuals exhibited !13C values less than "16 ‰ This appears to be a consequence of the mortality trends in the population (that is, most likely season of death). Nevertheless, it is striking that there are no individuals with constant, high (C4dominated) !13C values. Indeed, all the line segments on Fig. 1 for individuals with high !13C values are characterized by steep positive or negative slopes. The highest !13C values are between "13 and "11 ‰. This suggests that, although C4 grains were the dominant food source of humans and animals for some part of the year, C3 foods continued to comprise up to 25% of the nutrient base. 4.5. The routing problem We have indicated above that proteins are preferentially utilized as a source of non-essential amino acids, even though the body is capable of synthesizing them. However, Schwarcz [24] has noted that when diets are deficient in protein, non-essential amino acids will be synthesized from the global carbon pool (with the exclusion of some lipids). At Wadi Halfa, we see that the isotopic composition of a human protein (keratin) is apparently tracking changes in the abundance of C3 and C4 plant foods. This does not necessarily mean, however, that the diet was protein deficient, since rapidturnover animal proteins such as dairy products, eggs, and the flesh of short-lived animals (birds) could be reflecting a month-scale turnover in sources of feed grains. These data do, however, indicate that the flesh of wild (or domesticated) animals (dependent on dominantly C3 wild plants) did not constitute a major source H.P. Schwarcz, C.D. White / Journal of Archaeological Science 31 (2004) 753–762 of protein in most people’s diets. This is consistent with modern ethnographic data indicating that domesticated animals are dominantly fed C4 plants (millet and sorghum) [2,17]. Indeed, it is possible that these grains may have been stored for use as fodder. 5. Conclusions From a re-analysis of the isotopic data on human hair recovered at Wadi Halfa, we have demonstrated that although granaries or storage facilities that may have integrated one or more years of production were probably used, the populace at this site also tended to consume crops soon after they were harvested. These practices, which we have referred to as “grasshopper”like behaviour, persisted over more than a thousand years at this site, and through major transitions in social organization. The arguments presented here, based on isotopic analyses of hair, help to refine our understanding of food utilization practices, and provide a model for seasonal food consumption and storage behaviour at other Sudanese Nilotic sites. We have noted that none of these snapshot records of diet indicate either pure C3 or C4 composition. Such mixed consumption could result from: (1) storage availability of foods with an isotopic composition different from the dominant foods immediately available; (2) storage availability of foods that are both C3 and C4; or, (3) immediately available C3 and C4 foods. The shortterm fluctuations observed between hair segments and their arrangement in a seasonal cycle strongly suggest a significant amount of immediate consumption. Possible evidence for use of stored grain, however, is most evident in three individuals whose data failed to cohere to the cyclical pattern of Fig. 2. The !13C values of the successive segments of these persons’ hair were uniform, and close to the midpoint of the total !13C-range. Perhaps these hair samples came from individuals who had died during years of food shortage, when the community was subsisting on a homogeneous supply of grain. The intermediate !13C value of these samples also suggests that the individuals were consuming a mixture of summer and winter grains. As a possible explanation for the paucity of individuals who had homogeneous diets indicative of yearround consumption of stored grains, we can consider the real significance of the story of the grasshopper and the ant. Societies may prepare for times of hardship and yet seldom experience them. As long as society is stable and undisturbed by war or internal conflict, we may expect it to have some mechanisms for creating reserves of food, whether or not crises arise necessitating their use. There may be long periods in which no shortages are experienced and grain-storage practices fall into disuse. In the biblical story of Joseph’s interpretation of the Pharaoh’s 761 dream (Genesis, 41:1–57), it appears that the Pharaoh of that day lacked a plan to deal with years of decreased food supply (a grasshopper). But when the crisis arose, he and his vizier were quite capable of constructing storehouses (executing ant behaviour) to withhold large parts of the current, overabundant crop for future use. Sequential analyses of hair are ideal data to test for ant/grasshopper behaviour because the growth sequence from tip to scalp has a “clock” built into it which allows us to reconstruct the temporal variation in food consumption. Unfortunately sites at which hair is preserved for studies such as this are extremely rare. There are no other rapid-turnover tissues which are commonly preserved in burials. To further our understanding of the nature and consequences of tissue turnover vis a vis isotopic values, we are currently reconstructing the presence of rapid, seasonal excursions in !13C within single osteons using surface ionization mass spectrometry (SIMS) to determine total !13C (collagen+bone mineral carbonate). This will allow us to see progressive changes in the isotopic composition of bone through the course of a year that would track changes in diet. This may allow us to determine to what extent other populations made use of seasonally available agricultural commodities as they appeared rather than pooling them in long-term storage. In the meantime, the analysis of hair as a rapid turnover tissue has hopefully contributed some new insights to the use of stored foods in the history of agricultural economics. Acknowledgements We thank Prof. George Armelagos for providing access to the samples analysed in this and our previous studies at Wadi Halfa. Martin Knyf provided invaluable assistance in the isotopic analyses. The research was supported by grants to HPS and CDW from the Social Sciences and Humanities Research Council and the Natural Sciences and Engineering Research Council of Canada. References [1] S.H. Ambrose, L. 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