Preliminary evidence for seasonal deposition patterns from member 2 of the Swartkrans hominid site, South Africa

Journal qf’ifrchaeological

Science 1985, 12,163-115

Preliminary Evidence for Seasonal Deposition Patterns from Member 2 of the Swartkrans Hominid Site, South Africa A. Turner” (Received 8 July 1983, accepted 29 October 1984) Frequencies of specimensin juvenile age classes,based on an analysis of tooth eruption and wear sequences,in specimens of an extinct speciesof springbok, Antidorcas bondi Wells and Cooke, from Member 2 of the Swartkrans hominid site, suggest that the remains of this animal were deposited during summer months. These indications of seasonal activity may offer support for previous suggestions of annual game movements. Such periodic movements have implications for our understanding of the behaviour patterns of the animals, including perhaps the hominids, which are represented in the assemblagesfrom Swartkrans and other sitesin the vicinity. Keywords: SWARTKRANS, IOUR, FOSSIL SPRINGBOK,

SEASONAL DEPOSITION,

HOMINID

BEHAV-

AGE CLASSES.

Introduction

In a recent publication, Brain (1981) has discussed a number of aspects of the taphonomy of fossils found in association with early hominid remains in the South African Sterkfontein Valley cave sites of Kromdraai, Sterkfontein and Swartkrans. In his reconstruction of the palaeoecology of the various species represented in the deposits, Brain suggests that the characteristically cold nights of the Transvaal highveld may have led to primates, including the hominids, seeking shelter in the caves. By taking into account the extent of bone damage and skeletal part frequencies seen in the primate remains, he then argues that sleeping-site raids by leopards, Panthera pardus L., and

other large cats may account for much of the observed patterning in the assemblages. However, Brain also suggests that the primate occupations may have been most frequent during spring and autumn cold spells, since migrations to warmer bushveld areas are likely to have taken place during the full winter periods. He cites a number of lines of evidence (1981: 50) which point to seasonal movements as a likely strategy for the occupants of many southern African caves with archaeological remains, and argues that annual game movements are also likely to have been a major feature of the past ecology of the area. With reference to the hominid sites of the highveld area, Brain’s argument may therefore be seen as essentially a predictive one. Clearly, any evidence from the sites themselves which could demonstrate appropriate seasonal components “Department of Palaeontologyand Palaeoanthropology,Transvaal Museum, P.O.Box413, Pretoria0001,SouthAfrica. 163 03054403/85/030163+13SO3.00/0

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of behaviour or activity would tend to substantiate Brain’s model and thus offer considerable support for any interpretation of the palaeoecology of the species represented. In this paper I should like to present a probable instance of such evidence based on a study of material from Member 2 of the Swartkrans deposit. Inevitably, when discussing material from sites with early hominid remains, attention tends to focus on the interpretation of the material within an archaeological or anthropological framework. Such an emphasis may be inappropriate in given circumstances, particularly if, as Brain (1981) has suggested, the hominid remains in the Sterkfontein Valley sites were largely accumulated by predators. The assemblage in such cases records the behaviour of hominids not as prime movers in the events which lead to their deposition but as victims of predation. In the study presented here, inferences regarding hominid activities are rendered more difficult still by the fact that the analysis is based on the remains of an ungulate species which also appears to have fallen prey to leopards. But hominid behaviour will continue to be a significant issue in any interpretation of these sites and their contents, if only because the material constitutes a large proportion of the total available sample of early hominid fossils. Such indirect approaches to the question of seasonal components in the spectrum of hominid activities must therefore be considered. Material and Methods Evidence which will demonstrate, or support an argument for, seasonal movement may be sought in the remains of animals found in the deposits of a site. Such remains represent the animals whose presence or absence at various times of the year we wish to determine. If we can isolate seasonally correlated features in the skeletal evidence then we may be able to infer the time of the year at which the animals died and the bones became incorporated in the deposits. We should thus seek evidence of characters which we can correlate with known seasons: a suitable such character would be the sequences of antler shedding and regeneration seen in various cervid species of the temperate and arctic zones. An alternative approach is to look at age distributions in remains of animals which are known, or may be reasonably considered, to have bred seasonally. Any such animals which are present at a site or within its catchment area for a portion of the year only will, if they become incorporated in the deposits, be aged at yearly multiples of the time since birth. Thus, for the sake of simplicity, if young of a species are born on 30 December and deaths in the population occur at a site on 30 June of each year then all the dead will be 6 months old, 18 months old, 30 months old, and so on. Any such periodicity seen in the sample of specimens recovered from the site could then be used to infer seasonal deposition. Of course to then infer seasonal presence of the species at the site, and thus seasonal patterns of movement, involves another step in the argument. One could, for instance, argue that seasonal patterns of deposition reflect periodic activities on the part of an accumulating agent, rather than changes in the presence of the material accumulated. The basic approach to the problem remains the same, however, and I shall return to the question further below and endeavour to show that seasonal patterns in movement of the species accumulated offer the most reasonable interpretation in this case in the light of modern ethnological studies. The most widely used method for detailed ageing of fossil specimens is based upon sequences of tooth eruption and wear. Use of such a technique to infer seasonally correlated events was first proposed by Kurten (1953). More recent discussions have been published by Klein et al. (1981). In the cases studied by these authors, species with suitably hypsodont teeth are employed in the search for periodicities in wear decrement

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stages. Such a method has proved useful for the discernment of seasonal movement patterns in remains of horses of Devensian (last) Glaciation age in the British Isles when supported by a detailed statistical assessment of wear stages in cheek teeth (Turner 1981; Turner & Fieller, in prep.). However, methods which depend upon periodicities in wear stages are inapplicable to species which do not breed seasonally. This restriction is likely to rule out any of the Sterkfontein Valley primates including of course the hominids. The carnivore species found in the sites may possibly have bred seasonally, but the numbers of remains in the deposits are inadequate for any investigation of age stages. Both primates and carnivores would tend to be further ruled out by the low height of their tooth crowns which would provide inadequate numbers of measurable wear stages in most cases. Therefore I have approached the matter by looking at the ungulate species present in the sites. These species offer a number of advantages for any study which seeks age-related information. Ungulate teeth tend to be high-crowned, providing a potentially large number of age classes, and many of the species tend to breed seasonally. Ungulate movements will also tend to reflect seasonal changes in vegetation and hence may act as an overall guide to environmental conditions at a site. Such movements may in turn be used to infer seasonally-correlated behaviour patterns in other species: changes in vegetation and food availability may also affect primate feeding patterns, and if ungulate and primate species move then carnivores which prey upon them may move also. The major remaining requirement for the palaeontologist is then sample size. The only ungulate species which is present in the Sterkfontein Valley sites in sufficient quantities for the study of periodicities in tooth wear is the extinct springbok, Antidorcus bondi Wells and Cooke, from Swartkrans Member 2. Brain (198 1) estimated at least 70 individuals in the sample on the basis of dental and cranial remains, although my own examination would suggest a minimum number in excess of that figure (see below). Unfortunately, the majority of the teeth in the collection are still contained within the jaws, making measurement of crown heights impossible. One possible solution to the problem of non-isolated teeth would be to radiograph the specimens and then measure from the plates, but I feel that such a procedure would introduce considerable error into the analysis. Specimen orientation and the difficulties of accurate measurement from the radiograph are two obvious sources of error. A second possible solution would be to section the teeth and to look for evidence of seasonal patterning in growth rings either in the dentine or in the cementum of the roots, as proposed by Spinage (1973). The major problem with either method, as Klein et al. (1981) have pointed out, is that some degree of specimen destruction is involved. Looking for annuli in the root cementum is least destructive of the two methods, but again faces the problem that many of the specimens are contained in jaws. A different approach has therefore been adopted, involving a search for gaps, or discontinuities, in the sequence of juvenile age groups on the basis of tooth eruption and wear up to the point of attainment of the full adult dentition in the mandible. This approach has been made possible by the detailed study conducted by Rautenbach (1971) on the ageing of modern springbok, Antidorcas marsupialis Zimmerman, using precisely the kind of mandibular tooth eruption and wear-stage criteria which are most suited to a palaeontological investigation. Of course one must ask whether such criteria, involving as they do considerations of breeding patterns and rates of tooth eruption and wear, may be legitimately extrapolated back from an extant to an extinct species. On the question of parallels in age structure, I am in accord with Vrba (1973: 317) who used Rautenbach’s stages to analyse adult age groupings in the same sample of A. bondi remains as are being considered here. She expressed her view as follows:

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as all specieshere investigated belong to the genus Antidorcas and are of roughly the same size,it is likely that their life-spans as a whole, and the time taken to progress from one tooth wear stage to another, might have been of similar duration.. . ” “

.

.

Therefore, I believe that it is reasonable to assessthe age structure of A. bondi samples by the application of tooth eruption and wear data derived from A. marsupialis. However, Klein (in lit.) has suggested to me that the breeding pattern of A. bondi may have differed from that of the extant species. His view is based on his own unpublished studies of A. bondi material from both Swartkrans and the last glacial-age site of Equus Cave in the northern Cape Province. According to Klein’s analysis, the mortality structures for the animals at both sites exhibit the attritional profile of a species which regularly bears more than one young per female per year. This conclusion is based on analytical methods discussed in detail elsewhere by Klein (1982), and on considerations of the size of the animal which suggest that it was small enough to have had a gestation period of under 6 months. Such a breeding pattern would, as Klein argues, tend to imply that the species was sedentary and therefore of little or no use as an indicator of seasonal events, quite apart from the cluttering effect of such a pattern on the age distributions in a fossil assemblage. Klein also proposes that the animal may have been solitary and territorial, based on parallels of skull morphology with highly territorial antelopes. I shall discuss these points further below and in the next section of the paper when I consider the results of my own analysis. Suffice it to say for the moment that I believe the bimodal birth pattern inferred from the mortality profile to be open to question in the circumstances, and, as I hope to show, contradicted directly by the evidence for discontinuities in the juvenile age groups. I believe that contradiction may also extend to the morphologically-based inferences for territoriality. I shall therefore proceed by assuming that a single season of birth is correct, and will consider other possibilities in the course of the discussion. The logic behind applying the juvenile age stages derived by Rautenbach to the fossil material is similar to that employed in seeking evidence of periodicities in adult tooth-wear stages. Thus if animals of a seasonally breeding species are present in an area, and being incorporated in deposits, on a year-round basis, then all age groups within that year should be present in any assemblage subsequently recovered. Absence of the species for a period of the year, or failure on the part of an accumulating agent to act during a period, should result in a gap in the age classes. To return to the simplified example given above, if the animals were born on 30 December and either absent from the area or not “collected” between 1 April and 30 September of each year, then there should be gaps in the ages represented between 3 and 9 months, between 15 and 21 months, and so on. However, we should not expect the pattern displayed by a fossil sample to be anything like as simple as this example, and three other points should be considered. First, we should be able to argue that the agent responsible for the incorporation of the remains in the deposit would tend to sample throughout the year if the animals sampled were indeed present for the whole time. Second, we should be able to distinguish between an assemblage patterning likely to have been produced by accumulating agent activities and that produced by the activities of the sampled species. Third, our age classes must provide sufficient resolution while also taking into account the fact that births to a seasonally-breeding species will not take place on a single date but are likely to fall within a range. Taking the first and second points, Brain (198 1) has presented good evidence for the leopard having been the probable accumulating agent for the A. bondi specimens from Member 2 at Swartkrans, based on studies of modern leopard predation behaviour and

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carcase treatment and on the distribution of skeletal parts in the fossil sample. Studies of modern leopard behaviour by Pienaar (1969) and Schaller (1967; 1972) suggest that if both leopard and springbok were present in an area on a year-round basis then springbok would be taken at all times of the year. Certainly, there is no evidence to suggest that any significant and consistent gaps in the killing of springbok would occur. Neither is it necessary to assert that all springbok kills by leopards had to take place in the vicinity of the site in order for a year-round kill pattern to have been established in the assemblage. It would be entirely reasonable to expect an averaging out effect over a number of years, with kills at various times of the year being made in the vicinity of the site and the bones incorporated. The effect of the behaviour of other predators on leopard activities also seems unlikely to have altered the pattern of deposition to any significant extent. Leopards are known to seek refuge in trees in the presence of hyenas (Brain, 198 l), carrying their prey with them, precisely the mechanism suggested by Brain for the incorporation of many of the bones in the Sterkfontein Valley sites. Yearround presence of hyenas would have tended to produce year-round effects upon leopard activities. Were the hyenas active periodically, that would imply some seasonal component in behaviour. Such a seasonal component would most likely arise if prey were moving seasonally, and any seasonal movement pattern would most probably involve springbok which are known to seek warmer bushveld cover in times of winter cold (Haltenorth & Diller, 1980). A similar argument may be presented for the probable effects of the behaviour of other felid predators upon leopard activities. In short, year-round presence of the springbok would most probably result in year-round incorporation in the Swartkrans deposits, averaged out over time. Any periodicities in springbok age groupings in the samples would most probably represent seasonal absence of the species from the vicinity of the site and be less likely to reflect seasonally selective behaviour on the part of the leopard. Turning to the third point, it is clear that a reasonably restricted birth period was required by Rautenbach for the application of his ageing categories to modern springbok. He quotes Bigalke (1970) for evidence that around 95% of a springbok population is born in the October breeding season, although a second and much smaller peak, providing the remaining 5% of the births, may be observed in autumn. Each of these peaks has its own associated range, and these ranges are particularly likely to clutter the picture and reduce resolution in any palaeontological analysis. Moreover, two birth peaks spaced around 6 months apart might appear to suggest that a fossil sample simply cannot answer questions of year-round presence versus seasonal movement since the data may be incapable of exhibiting unambiguous gaps in the age ranges represented. When dealing with extant springbok the problem is minimized: a dead 3 month old calf specimen examined in isolation could, without further information, be interpreted as an autumn or a spring birth. But when found in the field, either alive or freshly dead, it could only come from the latest birth period because the previous period was 6 months before that. In the case of a fossil sample, a 3-month old calf could have been born in autumn or spring also but in this case no resolution is possible. This problem would seem to rule out the possibility of using the A. bondi material as an indicator of seasonally-correlated events from the outset. However, although interpretation is complicated by the fact that not all possible results have equally unambiguous implications, I believe that some resolution is possible. Bigalke’s results for A. marsupialis, quoted by Rautenbach, suggest that most births take place at either autumn or spring calving, and that the most usual time is during the spring season around October. Within each calving season, most births would appear to cluster around the central period. Therefore, as a working assumption, I shall first derive age-group distributions for the Swartkrans springbok material based on a relatively

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restricted spring calving season. I shall then examine the implications of alternative interpretations of the results, including the effect of allowing a wider birth period and a more equal distribution of births in spring and autumn. This approach is, I believe, justified by the nature of the enquiry. Consider the possible results of examining the fossil data for age distributions. If we find evidence for all juvenile age groups, then we will tend to interpret that in turn as evidence for year-round presence. Such an interpretation should not be affected by a bimodal birth peak if one peak is numerically much greater than the other, although if both spring and autumn peaks were equal then a false picture of year-round presence could result from the subsequent distribution of age classes if the species managed to breed twice per year and move seasonally. In other words, such apparent evidence for year-round presence could, while perhaps being unlikely, be misleading. However, a year-round result should not be an artifact of the spread of births about one age peak, because the spread is relatively short in comparison with the whole year and the confinement of the investigation to the first 2 years of life means that sufficient age stages should be observable to prevent such a mistaken interpretation of constant presence. But if we do find gaps in the age classes of springbok present at the site, then we may be in a position to make a more secure interpretation of seasonal deposition. In short, gaps in the age classes may imply a relatively clear-cut pattern of deposition while an absence of gaps may not. Discussion of the results obtained in this study should make these points clear. Results and Discussion

The springbok age classes employed in this study, together with the criteria for their recognition, are given in Table 1. In Table 2, a list is given of the specimens of half mandibles and isolated teeth of A. bondi from Swartkrans Member 2 which were assigned to each age class. Figure 1 displays the dispersal of these age classes for the first 3.5 years of life of a hypothetical cohort, the maximum age for which adequately narrow age classes could be defined using the criteria derived from extant springbok (Rautenbach 1971: 112-l 14, table 3). The numbers of specimens given in Table 2 are taken to be the numbers of individuals in each age class for the construction of Figure 1, except in the case of age class D. In this latter case three of the right specimens could have come from the same individuals as three specimens on the other side of the body, thereby reducing the possible number of Table 1. Description of age classes to which juvenile and young adult specimens of Antidorcas bondi from Swartkrans Member 2 have been allocated. Age class data derivedfrom Rautenbach (1971). See text for discussion

As

Description

Up to 3.5 months 3.5-7.5

months

12-16 months 1619

months

2442

months

From birth until full eruption M, which exhibits full wear on all occlusal surfaces From full eruption of M, until M, is half erupted and exhibiting wear on the anterior cusp only From coming into wear of the anterior two cusps of M,. This stage grades into the next. Wear on anterior two cusps of M, still, with the upper limit given by P, just coming into wear From the time when all three cusps of M, are in wear, P, is in wear and the infundibulum on the anterior cusp of M, has been lost through wear. The upper limit of this class is marked by the lower limit of the next, loss of both infundibula on M,

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Table 2. Specimens qf Antidorcas as described in Table I

bondi from

Age class

Swartkrans

Member

2 in age classes

Specimens

A. Up to 3.5 months

B. 3.5-7.5

169

Left: SK2962, SK1921, SK7698 SK1 1099, SK2096 Right: SK2321, SK12526, SK2338 SK401 1, SK5987 No. of specimens 10 No. of individuals 10 Left: SK14050, SK4497, SK2998 SK6093, SK10622, SK2375 Right: SK5945, SK6076, SKI 1986, SK6045, SK5908, SK14126, SK2409 No. of specimens 13 No. of individuals 13 Left: SK5974, SK12623, SK10417, SK3054, SK14051 Right: SK4074, SK3079, SK3075, SK12628 No. of specimens 9 No. of individuals 9 Left: SK4570, SK2289, SK2977, SK5988, SK2020, SK2367 Right: SK3144, SK2351, SK5956, SK5934, SK4049, SK6038, SK2518, SK2399, SK6101 No. of specimens 15* No. of individuals 12* Left: SK6116, SK6088, SK2306, SK6021, SK2958, SK11389, SK2113 Right: SK3029, SK2030, SK2490, SK2256, SK6052, SK2085, SK3009, SK2970 No. of specimens 15 No. of individuals 15

months

C. 12-16 months

D. 1619

months

E. 2442

months

“1 I:I: LI

*See text. .

6

9

12

1

D

I5

16

Months

Figure 1. Numbers of individuals of Antidorcas bondi from Swartkrans Member 2 falling within juvenile age classes and the first adult age class. A, O-3.5 months; B, 3.5-7.5 months; C, 12-16 months; D, 1619 months; E, 2442 months. -, numbers in these age classes assuming a l-month birth period about the centre of October (A). Divisions on the abcissa mark the centres of subsequent months. ~ -, numbers in age classes assuming a 3-month birth period about the centre of October. See Tables 1 and 2 for details and text for discussion.

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individuals. With these exceptions, left and right specimens in each age class shown in Table 2 are considered to come from separate individuals since I have been unable to find matches between either mandibles or isolated teeth from opposite sides of the body when assessing the material on the basis of both size and morphology. Such an absence of matches implies that the number of dead animals in each age class from which the available sample was drawn is likely to have been considerably higher than the minimum number of individuals count derived directly from the specimen quantities, as shown in detail elsewhere (Fieller & Turner 1982; Turner 1983). It is worth stressing two points in this context. First, such implied increases in the numbers of individuals in each age class heightens the contrast with those unrepresented juvenile age classes in Figure 1. Second, by carefully examining the material under consideration, it is possible in this case to argue that virtually every specimen comes from a different individual. The result is twofold: the total sample size suggested by Brain (198 1) to be a minimum of 70 individuals is actually seen to be considerably larger, and the number of juvenile individuals up to the age of 3.5 years forming the sample shown in Figure 1 alone is of 59 individuals. I would suggest that such a sample size from a fossil deposit is a respectable one, and that the numbers of individuals falling into each age class are adequate for the analysis undertaken. This latter point may be underlined by considering the analogous example of the sample sizes employed by Klein et al. (1981) in their discussion of crown height measurements as indicators of age profiles and seasonal deposition. In many instances, the numbers of specimens within each crown-height class which they display do not reach double figures. This is not a criticism of their analysis: the authors stress sample size as a problem several times. But it is worth using this example to draw attention to the issue of specimen versus individual numbers. For many purposes, such as estimations of average size, the use of both left and right specimens of a given skeletal part is not a serious problem although some pairs of left and right specimens may be expected to come from single individuals. This point is usefully discussed by Fieller (1980). However, in the construction of bar charts of specimen numbers in age groups, whether based on tooth crown height, as employed by Klein et al. (198 1), or based on tooth eruption and wear stages as in the present study, the obvious similarity in size between left and right elements of a single individual, a point stressed by Klein et al. (198 1: 7) with reference to modern material, may pose a serious problem. Briefly, the effect of increasing numbers in each class interval by adding together numbers of left and right specimens may be to lend false support to the appearance of high periodic frequencies. For absolute certainty in instances where matching pairs cannot be ruled out, only teeth from one side of the body should be used. This point, which seems to have been overlooked by Klein et al. (1981) as a general one to be born in mind in such analyses, may of course result in a considerable reduction of the sample size by up to a half. The solid lines in Figure 1 show the age class distributions given by taking a single birth period in October and assuming a.spread of 1 month about the centre. In other words, births are assumed to have occurred at any time within the month. It will be seen that this grouping of the younger dental specimens produces two clear gaps, one between mid June and the beginning of the following October, and a second between the beginning of the following June and the beginning of the October after that. I should stress here that in producing these age groupings I have sorted the entire sample into age classes using the criteria given by Rautenbach (1971). Although only juveniles and the first adult class are shown in Figure 1, most of the rest of the material could also be placed in Rautenbach’s older adult groups. Unassignable specimens, usually isolated teeth or mandible fragments with insufficient detailed characteristics, could nevertheless be clearly seen to be of older animals, and I am therefore convinced that the gaps in Figure 1 represent real discontinuities in the juvenile age groups of the sample.

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The broken lines on Figure 1 show the results of assuming a greater spread in the birth period centred on October, such that births could have occurred at any time between the beginning of September and the end of November, a 3 month period. The effect of this assumption is to reduce the gaps seen with the solid lines. Does this reduction refute the idea of gaps in the age groups? I think we must bear in mind the fact that any spread of the birth period would still produce a central clustering of births, with a “tail” at either end. Because we cannot tell whether an individual fossil specimen is likely to have been born at the beginning, middle or end of the birth period, we must show all of a certain age group as a rectangle on the bar chart. When we then allow for a relatively wide potential birth period, the possible gaps between the age groups are inevitably reduced. But if the gaps in time during which the animals were not deposited were indeed as the solid lines in Figure 1 suggest, then the 7.5 month old ones which appear to extend the presence of that age group forward into the first age gap would not be those born late in the previous birth period but those born early. If we are correct in inferring that gaps in the age classes represented point to seasonal movements on the part of the springbok, then the later-born 7.5 month old individuals would have moved elsewhere with the rest of the herd. Similarly, the 12 month old ones which appear to extend their presence back into the same time gap may be those born late in the previous birth period rather than those born early. The early-born individuals in this case would have reached their first birthday while the herd was elsewhere. In both cases, these early- and late-born individuals indicated by the broken lines in Figure 1 should represent roughly half of a relatively small tail of their main age-group distributions. Precisely the same argument could then be applied to the apparent closing of the gap in the next “year” between the 19 and 24 month old specimens. I therefore feel that the apparent closing of the age-group gaps produced by extending the birth period to 3 months is simply an artifact of allowing for an extended birth period in constructing the diagram rather than a reflection of reality, and that it does not refute the idea of seasonal deposition and hence seasonal movement. More importantly still, taking such a closure of the gaps as evidence of constant springbok presence would also fail to explain why we actually have no remains aged between 7.5 and 12 months and between 19 and 24 months, the major feature of Figure 1. Interpreting the age group distributions as evidence of seasonal movement with a birth period centred on October would make considerable ecological sense. Absence from the Sterkfontein Valley would then, as Brain (1981) argued, most probably have been during the winter months, when better fodder is likely to have been available elsewhere. A main birth period in autumn, centred on 1 April, would make considerably less sense from a palaeoecological viewpoint, because the age groupings in Figure 1 would then imply that the animals were absent from the sites at the best time of the year to be present. For that reason I believe that we can reject the April period as a time of the major birth peak. The question which remains, however, is the extent to which equal birth peaks in both autumn and spring might have occurred and the effect of such a calving distribution on the interpretation so far advanced for the pattern in Figure 1. The problem may first be visualized by simply assuming that the numbers of individuals shown in Figure 1 in age classes resulting from a single October birth period were divided equally between October and April calvings in circumstances where annual movements also occurred. The result would be that the gaps in Figure 1 which have so far been taken to represent absences from the site would go, since individuals born in April would be of appropriate ages during the annual movements to the area to fill the spaces presently based on an assumption of a single October birth peak. In considering this point one should bear in mind that we do not independently know the birth date of any specimen, but must construct Figure 1 using some working assumption. Therefore

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we would not know that the observed distribution reflected a bimodal birth peak in a seasonally migrant species, since the same result, all juvenile age groups represented, would be produced by the year-round presence of a species which bred once or more than once per year. The important point from the perspective of this discussion is that in any of these events the chances of the juvenile individuals presently missing being incorporated in the deposits would be at least as high as for other individuals. In other words, the gaps in the age groups shown in Figure 1 would in themselves again seem to offer strong support for the view that a single main calving period in a seasonally migrant species was the norm for the extinct springbok represented in Swartkrans Member 2. How may we then reconcile that interpretation with the argument proposed by Klein (in lit.) that the mortality pattern of the Swartkrans A. bondi remains exhibits the attritional profile of a species which regularly bears more than one young per year? Very briefly stated, Klein’s argument is based on the analysis of life table and mortality profile data given in his recent paper on assemblages from the early Pliocene site of Langebaanweg and the Pleistocene site of Elandsfontein, both in the Cape Province (Klein, 1982). He shows that, in species producing one young per year, a bar chart of the steady-state age profile of the living population will resemble a “down staircase” with the highest numbers of individuals in the youngest class and the least in the oldest, a commonsense expectation. The death assemblage profile of those individuals who die each year will in contrast be L-shaped or possibly slightly U-shaped, with most deaths occurring in the juvenile and young adult classes and a secondary peak perhaps appearing among the aged (Klein, 1982: figure 1). In species producing more than one young per year, both the age profile of those dying annually and the age profile of the living will be similar, and both will resemble a down staircase (Klein, 1982: figure 2) with a very steep first step. The mortality profile of such a species will show a higher incidence of mortalities continuing beyond the juvenile stage and into the young adult age classes, in contrast with the pattern in a species producing one young per year. The living and mortality profiles in the case of species producing more than one young per year may be difficult to distinguish. As Klein (1982: 58) points out, the fossil assemblage from a site is always in one sense a death profile: the animals are dead. But in some circumstances, such as natural disasters, a census-like assemblage of the living population may be produced. Since we have no reason to assume that the Swartkrans A. bondi assemblages resulted from such a catastrophe, it clearly makes sense to see the mortality profile as attritional. Is it then likely to be that of a species having one young per year or more than one? The differences between the two respective attritional profiles in Klein’s hypothetical example (1982: figures 1, 2) may be adequate for discrimination, but in the case of an actual assemblage the difficulties may loom large. In the first place, the form of any age profile will depend upon how one counts the items in each age class, as discussed above. This may or may not be a problem in specific circumstances, and with a single agent of accumulation one may plausibly argue that the errors are reduced. But the fact that an agent has been identified would seem to raise further problems. Klein’s argument appears soundly based on the fact that attritional profiles arising from different reproductive circumstances may be recognized, but the implicit assumption is that one has the attritional profile of the population. This is strongly underlined in Klein’s discussion of modern-day use of mortality profiles as one of the best methods of estimating the age structure and mortality patterns of game (Klein, 1982: 54-57) where stress is laid on the systematic collection of ageable body parts. The fossil assemblage accumulated by a predator may thus be seen as an attritional profile, and therefore not necessarily one from which inferences about reproductive behaviour may be drawn. We may instead be

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looking at the selection of age classes, or at a death assemblage very heavily affected by such activities, and it is not clear to me at this stage how one should go about unravelling such complexities. For these reasons, I prefer to base my interpretation on the evidence of the juvenile age groupings. Finally, it may be appropriate at this point to return to the questions of sample size and of the validity of the results obtained here. As I stressed above, the sample size available in this case is by no means inconsiderable when set against many which are employed in palaeontological analyses. To say this is not to imply that more would not be better, nor even to make some spurious virtue out of necessity. The gaps in age classes of extinct springbok found at the site have their validity underlined by being visible over two inferred annual periods, occurring approximately 12 months apart and being of approximately equal duration. Conclusions The apparent conflict between the interpretation presented here and that proposed by Klein (in lit.), based on his assessment of the breeding behaviour of A. bondi, is in my view largely procedural. While agreeing with his argument for distinctive attritional profiles in species with varying reproductive rates, I feel that the extent to which a successful application of the approach depends upon having the attritional profile of the species may present problems. In the present case, the more direct evidence of the gaps in juvenile age classes seems in my view to outweigh the inferential reconstructions of a reproductive pattern from the mortality profile and the extent of annual movement and territoriality from morphological parallels. That view does not imply a dismissal of such approaches: it merely reflects my ordering of the evidence. However, it would be fair to point out that inferences are also involved in my construction of juvenile age classes, particularly in the extent to which one may extrapolate from the extant springbok and the precision with which one may legitimately allocate specimens to any given category. I have argued that one may use age categories derived from modern animals referred to the same genus, and cited other opinion to that effect. One could then adopt a conservative approach, choosing to employ only the very broadest of age groupings, but such an approach would ultimately render the entire exercise meaningless. I have explicitly used non-conservative groupings which I believe to be justified by modern comparative work, groupings which are capable of achieving the necessary resolution. If we then accept that the pattern indicated by the solid-line bar chart in Figure 1 is a valid one, we have, in my view, reasonable evidence that the remains of A. bondi from Member 2 of Swartkrans were deposited seasonally during the summer months. Even if we allow a greater spread to the spring birth period, producing the pattern shown by the broken-line bar chart in Figure 1, the suggestion of a seasonal correlation remains quite strong. When that evidence is considered in conjunction with that for leopard activity in the accumulation of the bones and with the results of studies on modern leopard behaviour, then seasonal presence of the springbok at the site is strongly indicated. At the very least, the evidence would support a case for further work to be undertaken on periodic deposition in Member 2 at the site. Clearly, the exercise carried out here refers in the strict sense only to the incorporation of the remains of one species in a portion of one member at a single site. Both Brain (198 1, 1982) and Vrba (1982) have stressed the complexity of events which appear to have led to the formation of Member 2 at Swartkrans. The deposit as a whole appears to be the latest of the major horizons in the three Sterkfontein Valley sites, with the A. bondi fossils forming a late component within the member and probably dating to after 1.0 million years ago, according to Vrba (1982). In the same paper, Vrba suggests that Swartkrans Member 1, with its rich sample of remains of Australopithecus robustus

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(Broom), and Sterkfontein Member 4 with the fossils of Australopithecus africanus Dart, may date close to 1.6 and 2.6 million years ago respectively. These distances in time from the deposition of springbok remains in Swartkrans Member 2 are considerable. As Vrba (1982; in press) has also shown, conditions in the area of the sites appear to have changed somewhat over time, introducing yet another variable. In the light of all these difficulties and confounding variables, any efforts to draw wider conclusions from the results presented here must proceed with caution. Since the inclusion of the A. bondi remains in the Swartkrans deposit would seem to have had little to do with hominid activities, relating the seasonal pattern inferred from those remains to hominid behaviour is particularly difficult. Whether one would be

justified in going beyond the evidence presented and arguing for confirmed patterns of seasonality for other Sterkfontein Valley deposits is in many respects a matter of taste. Nor should one overlook the fact that the interpretation of seasonally deposited springbok remains resulting from leopard activities raises its own questions and problems. If springbok and other ungulates moved away from the sites during the winter periods,

what was the response of the leopards? Did they too move, or did they seek such other prey as may have remained in the vicinity of the caves during the colder months? If the leopards stayed, then what did they exist on and how might that in turn be tested? Dassies or “rock rabbits”, Procavia caper&s Pallas, are a source of food for leopards today (Brain 1981), for instance, and the discussion by Pienaar (1969) would certainly suggest that they could survive on such small prey in the absence of larger items. The problem, as stressed at the outset, is that neither the dassie nor any other species is present in the sites in sufficient numbers to permit a search for a seasonal complement to springbok as a potential component of leopard diet. But, with continuing excavation in progress at the Sterkfontein Valley sites, the search for such components in the light of the results presented here is clearly worthwhile. Acknowledgements

I thank D. H. Gordon and G. Turner for commenting on earlier versions of the manuscript, C. K. Brain and E. S. Vrba for discussion of my results and their implications and I. L. Rautenbach for discussion of the suitability of his ageing methods for application to fossil material. I am particularly grateful to R. G. Klein for critical reading of the text and for his generous sharing of his own unpublished results and opinions. I take full responsibility for the views expressed. References Bigalke, R. C. (1970). Observations on springbok populations. Zoologica Africana 5,59970. Brain, C. K. (1981). The Hunters or the Hunted? Chicago: University of Chicago Press. Brain, C. K. (1982). The Swartkrans site: stratigraphy of the fossil hominids and a reconstruction of the environment of early Homo. Proceedings Congrks International de PalPontologie Humaine ler Congrks, Nice. 676-706.

Fieller, N. R. J. (1980). Appendix: a note on the statistical procedures. In (P. A. Mellars & M. R. Wilkinson, Eds), Fish otoliths as indicators of seasonality in Prehistoric shell middens: the evidence from Oronsay (Inner Hebrides), 42. Proceedings of The Prehistoric Society 46, 1944. Fieller, N. R. J. & Turner, A. (1982). Number estimation in vertebrate samples. Journal of Archaeological

Science 9,49-62.

Haltenorth, T. & Diller, H. (1980). A Field Guide to the Mammals of Africa. London: Collins. Klein, R. G. (1982). Patterns of ungulate mortality and ungulate mortality profiles from Langebaanweg (Early Pliocene) and Elandsfontein (Middle Pleistocene) south-western Cape Province, South Africa. Annals of the South African Museum 90,49-94. Klein, R. G., Wolf, C., Freeman, L. G. & Allwarden, K. (1981). The use of dental crown heights for reconstructing age profiles of red deer and similar speciesin archaeological samples. Journal of Archaeological Science 8, 1-3 1.

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Kurten, B. (1953). Age groups in fossil animals. Commentationes Biologicae 13(13), l-6. Pienaar, U. de V. (1969). Predator-prey relationships amongst the larger mammals of the Kruger National Park. Kodoe 12,108176. Rautenbach, I. L. (1971). Ageing criteria in the springbok, Antidorcas marsupialis (Zimmerman, 1780) (Artiodactyla: Bovidae). Annals ofthe Transvaal Museum 27(6), 83-133. Schaller, G. B. (1967). The Deer and the Tiger. Chicago: University of Chicago Press. Schaller, G. B. (1972). The Serengeti Lion. Chicago: University of Chicago Press. Spinage, C. A. (1973). A review of age determination of mammals by means of teeth, with especial reference to Africa. East African Wild& Journal 11, 165-187. Turner, A. (1981). Aspects ofthepalaeoecology of largepredators, including man, during the British Upper Pleistocene, with particular emphasis on predator-prey relationships. Unpublished Ph.D. thesis, University of Sheffield. Turner, A. (1983). The quantification of relative abundances in fossil and subfossil bone assemblages. Annals of the Transvaal Museum 33,311-321. Vrba, E. S. (1973). Two species of Antidorcas Sundevall at Swartkrans (Mammalia: Bovidae). Annals of the Transvaal Museum 28,287-352. Vrba, E. S. (1982). Biostratigraphy and chronology, based particularly on Bovidae, of southern hominid-associated assemblages. Proceedings Congres International de Paleontologie Humaine ler Congres, Nice, pp. 707-752. Vrba, E. S. (in press). Palaeoecology of early Hominidae, with special reference to Sterkfontein, Swartkrans and Kromdraai. In (Y. Coppens, Ed.) L’Environment des Hominide’s. Proceedings of Colloque International held at Fondation Singer-Polignac, Paris, June 198 1.