Taphonomic analysis of skeletal remains from chimpanzee hunts at Ngogo, Kibale National Park, Uganda

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Abstract

This study provides a taphonomic analysis of the largest known sample of bone fragments collected from chimpanzee hunts. The entire sample consists of 455 bone fragments from 57 chimpanzee hunting episodes of 65 prey individuals at Ngogo, Kibale National Park, Uganda. It has low taxonomic diversity, consisting overwhelmingly of primates, especially red colobus monkeys. The age distribution of the prey remains is skewed towards pre-adults. Cranial bones are the dominant element, followed by long bones. Axial postcranial elements have low survivorship, with a complete absence of pre-caudal vertebrae. Bone is damaged in distinct ways, such as: destruction of long bone ends, typically with intact but chewed shafts; fragmentation and compression cracking of crania; and preservation of only the iliac blades of the innominates. Tooth marks are present but uncommon (4.4% of total NISP).

These analyses enable us to: 1) describe and characterize consistent patterns of bone damage inflicted by chimpanzees across a much larger prey sample than has been previously studied; 2) make a preliminary comparison of the generalized chimpanzee taphonomic signature to that of leopard and eagle consumption of primates, as well as modern human consumption of small mammals; and 3) assess the utility of such samples for recognition of early hominin small mammal carnivory. We present a model that may be useful for detecting a pre-technological hominin carnivory and suggest some fossil locales at which close inspection of cercopithecoid remains for the above patterns might reveal traces of hominin hunting, though we caution that a pre-technological hominin hunted “assemblage” is not likely to be archaeologically visible.

Introduction

Studies of chimpanzee hunting behavior in the wild have yielded considerable information about prey selection and hunting participation, frequency, and success (e.g., Uehara, 1997, Stanford, 1998, Mitani and Watts, 1999, Boesch and Boesch-Achermann, 2000, Watts and Mitani, 2002). Chimpanzees hunt vertebrate prey everywhere that they have been studied in any detail (Uehara, 1997; Table 1). While 38 species of nonhuman primates hunt and/or eat vertebrate prey, only chimpanzees and baboons hunt in groups, stalk their prey, and share meat (Strum, 1981, Strum, 1983, Butynski, 1982, Boesch and Boesch, 1989, Stanford, 1998, Mitani and Watts, 1999). The close evolutionary relationship between chimpanzees and humans, as well as anatomical and possible niche similarities, provides the rationale for using chimpanzee predatory behavior to model the hunting ecology and behavior of pre-technological hominins (e.g., Stanford, 1996, Stanford, 1999).

Little is known about prey remains from chimpanzee hunts. Analyses of such remains might provide insights into potential early hominin prey remains in the paleontological record. Observations of chimpanzee predation at the Mahale Mountains made note of bone refuse, but this was not systematically recorded (Nishida et al., 1979, Kawanaka, 1982, Takahata et al., 1984). More recent work documented: 1) bone modification inflicted by captive chimpanzees on bovid and cervid bones (Pickering and Wallis, 1997: hereafter called the “captive” sample); 2) damage to bones found in chimpanzee feces at Kibale National Park (Tappen and Wrangham, 2000: the “fecal” sample); and 3) the bony remains of five red colobus monkeys captured in a single hunting bout at the Gombe National Park (Plummer and Stanford, 2000: the “Gombe” sample).

The most comparable of these studies to the hunting behavior of pre-technological hominins is the Gombe sample. Plummer and Stanford (2000) found that crania and mandibles had high survivorship, followed by scapulae and long bones. They observed a high proportion of crenulation and step fracturing on long bones and ribs, and a tooth puncture in one of the cranial specimens. As we will outline, their descriptions of chimpanzee-hunted faunal assemblages are similar to ours: small prey size, low taxonomic diversity, a focus on immature individuals, and a high frequency of skull bones. Our study supports all of these observations with a much larger sample size.

Chimpanzees of the Ngogo community, Kibale National Park, Uganda, hunt frequently and are unusually successful predators compared with chimpanzees at other sites (Mitani and Watts, 1999, Watts and Mitani, 2002). The frequency and success of hunting by chimpanzees at Ngogo furnish an especially good opportunity to collect bones modified by predation. In this paper, we describe the species composition, age distribution, skeletal element distribution, and bone damage patterns of a very large sample of prey remains from Ngogo. The bone assemblage we analyze here is the largest chimpanzee-hunt refuse collection assembled to date. Our results permit us to make generalizations about what chimpanzees do to prey remains, compare these results to those from other small mammal predators, and speculate about the utility of these remains for recognizing small mammal hunting by early hominins.

Section snippets

Study site and subjects

At an altitude of about 1400 meters above sea level, Ngogo (Kibale National Park, Uganda) lies at an interface between lowland and montane rainforest. Old growth forest, characterized by a continuous canopy 25–30 meters high, covers most of the Ngogo community territory, but human disturbance has created some spots of regenerating forest and grassland. At Ngogo, chimpanzees live sympatrically with six other common diurnal primates who form their primary prey (Table 2). These include two colobines

Chimpanzee prey consumption

Teleki (1973: 141) vividly described chimpanzee prey consumption at Gombe National Park, Tanzania:

“Small bones are thoroughly cleaned by sucking and scraping and are then chewed apart or discarded (and collected by others); large bones such as those of arms and legs are cracked between the molars, and the marrow sucked out while bones themselves are gradually consumed.”

At Ngogo, chimpanzees typically process bones in the same manner. Ngogo chimpanzees often begin by disemboweling adult prey and

Discussion

The Ngogo sample exhibits specific taphonomic patterns that we contend are characteristic of a chimpanzee-modified prey assemblage. These patterns include low taxonomic diversity (in this case, with a predominance of red colobus monkeys) and an age distribution skewed towards pre-adults. The skeletal remains are dominated by cranial bones, followed by long bones, and ribs. Pre-caudal vertebrae are absent. Long bone epiphyses are usually chewed off, leaving fraying, peeling, and crenulated edges

Summary and conclusions

We present a taphonomic analysis of the largest collection of the remains of primates hunted by chimpanzees to date. This study confirms some of the taphonomic patterns described in an earlier study analyzing a much smaller sample (Plummer and Stanford, 2000). The taphonomic signature of chimpanzee kills includes a focus on colobus monkeys, an age profile skewed towards pre-adults, and a skeletal element profile dominated by cranial fragments (but with no complete crania) followed by long

Acknowledgements

Field research was supported by grants to J.C.M. from the Detroit Zoological Institute, L.S.B. Leakey Foundation, National Geographic Society, NSF (SBR-9253590, BCS-0215622, and IOB-0516644), University of Michigan, and Wenner-Gren Foundation. We gratefully acknowledge additional support from the NSF Graduate Research Fellowship program to B.L.P. and J.D., and the Smithsonian Institution to B.L.P. We thank several individuals for help with this paper: J. Lwanga, A. Magoba, G. Mbabazi, L.

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