Chimpanzee tool technology in the Goualougo Triangle, Republic of Congo

With the exception of humans, chimpanzees show the most diverse and complex tool-using repertoires of all extant species. Specific tool repertoires differ between wild chimpanzee populations, but no apparent genetic or environmental factors have emerged as definitive forces shaping variation between populations. However, identification of such patterns has likely been hindered by a lack of information from chimpanzee taxa residing in central Africa. We report our observations of the technological system of chimpanzees in the Goualougo Triangle, located in the Republic of Congo, which is the first study to compile a complete tool repertoire from the Lower Guinean subspecies of chimpanzee (Pan troglodytes troglodytes). Between 1999 and 2006, we documented the tool use of chimpanzees by direct observations, remote video monitoring, and collections of tool assemblages. We observed 22 different types of tool behavior, almost half of which were habitual (shown repeatedly by several individuals) or customary (shown by most members of at least one age-sex class). Several behaviors considered universals among chimpanzees were confirmed in this population, but we also report the first observations of known individuals using tools to perforate termite nests, puncture termite nests, pound for honey, and use leafy twigs for rain cover. Tool behavior in this chimpanzee population ranged from simple tasks to hierarchical sequences. We report three different tool sets and a high degree of tool-material selectivity for particular tasks, which are otherwise rare in wild chimpanzees. Chimpanzees in the Goualougo Triangle are shown to have one of the largest and most complex tool repertoires reported in wild chimpanzee populations. We highlight new insights from this chimpanzee population to our understanding of ape technological systems and evolutionary models of tool-using behavior.     

Chaînes opératoires and resource-exploitation strategies in chimpanzee (Pan troglodytes) nut cracking

We apply archaeological methods to extend our knowledge of chimpanzee material culture. The cha?ne opératoire conceptual framework, as introduced by ethnography, established technology as a phased process. Prehistoric archaeology adopted this concept to elucidate technological variability in tool-making procedures, based on knowledge of tool functions or subsistence patterns. We focused on the detection of operational sequences by wild chimpanzees (Pan troglodytes verus) when nut cracking with lithic implements at the sites of Bossou and Diecké, Guinea, West Africa. Thus, while it has recently been claimed that chimpanzees leave behind recognizable assemblages of stone hammers that can be morphologically distinguished from Oldowan hammers, this is the first study to focus specifically on the existence of operational sequences during the utilization of stone tools by wild chimpanzees. By combining primatological and archaeological methods and examining ecological areas inhabited by different chimpanzee groups, we sought technological variability and identified variables influencing regional diversity in tool typology and technology. We compared three case studies: (1) Bossou-direct recording of experimental nut-cracking sessions; (2) Bossou- direct and indirect monitoring of nut-cracking sites in the wild; (3) Diecké-indirect monitoring of nut-cracking sites in the wild. Results suggest that chimpanzees perform sequences of repeated tool transport and nut cracking. Data show discrimination of tool functions based on tool features. We identified the most technologically complex tool for nut cracking, which was composed of four stones. We found regional diversity in chimpanzee stone assemblages. Raw-material type and tool mobility constrain technological development in human and nonhuman primates. Spatial analysis of tool distribution indicates a pattern of resource-exploitation strategy, revealing affinities with Oldowan.     

Survey of savanna chimpanzees (Pan troglodytes verus) in Southeastern Sénégal

A survey of the western subspecies of chimpanzee (Pan troglodytes verus) was conducted from 1 February to 9 April 2000 in Sénégal, West Africa, by the Miami Assirik Pan Project (MAPP). In addition to the Assirik area of the Parc National du Niokolo Koba (PNNK), areas south and east of the park were surveyed. Nests made by chimpanzees were used to estimate chimpanzee distribution and densities. Within the PNNK, chimpanzees were estimated to occur at an average of 0.13 individuals/km(2). Chimpanzee nests were recorded in nine of 10 locales surveyed outside of the PNNK, as well as within the park. Data on 994 nests made by chimpanzees were recorded outside the PNNK, while 736 nests were recorded inside the park. Nest density in areas surveyed outside the PNNK, such as Bandafassi, Tomboronkoto, and Segou, was comparable to that of Assirik in habitats where nests were concentrated (i.e., evergreen gallery forest). The purpose of MAPP was to initiate long-term research of chimpanzees in southeastern Sénégal, as a follow-up to the Stirling African Primate Project (SAPP) of the 1970s. We sought to replicate the standards set by the SAPP project, except when technological innovations allowed improvement in data collection procedures (e.g., the global positioning system (GPS)).     

Establishment,maintenance, and behavior of free-ranging chimpanzees on Ossabaw Island,Georgia, U.S.A.

A small number of adult chimpanzees were released on Ossabaw Island, Georgia, in order to evaluate the feasibility of establishing a free-ranging reproductive colony of chimpanzees in a semitropical North American climate. First, three females and one male were released in June 1972. Following the unexpected deaths of two of these females, four more females were added. Except for one newborn that was removed from the colony, the chimpanzee colony remained intact, and was still in existence as of February 1975. Preparation for and maintenance of the chimpanzees is described. Behavioral adaptations to the environment, and some aspects of chimpanzee social behavior are reported. The potential importance of colonies such as this, for reducing over-exploitation of naturally occurring primate populations is discussed. The financial support of Yerkes Regional Primate Research Center, Emory University, Atlanta, Georgia, and The Pennsylvania State University, Department of Poultry Science, University Park, Pennsylvania, permitted us to make these observations. Funding for planning and construction of the island facilities, and relocation and maintenance of the chimpanzees was provided by a National Institutes of Health grant toGeoffery Bourne, Director of the Center.     

Use of leaves as cushions to sit on wet ground by wild chimpanzees

A new type of tool use, leaf cushion, by wild chimpanzees (Pan troglodytes verus) at Bossou, Guinea, was found. We report two cases: one is indirect evidence; the other is direct observation of a chimpanzee who used the tool. Both cases indicate that chimpanzees used a set of leaves as a cushion while sitting on wet ground. Chimpanzees at Bossou show various kinds of tool use, some of which are unique to the community. Most of these behavioral patterns are subsistence tool use for obtaining food, as at other study sites. The use of leaves as a cushion adds to the few instances of nonsubsistence, elementary technology seen used by wild chimpanzees. Am. J. Primatol. 44:215–220, 1998. © 1998 Wiley-Liss, Inc.     

Hominization in the rainforest: The chimpanzee's piece of the puzzle

Many models of human evolution propose that key behavioral innovations were involved in the divergence of the human line from the ape line. An ecological reason, the shift from dense forests to a more open habitat, is suggested as the basis for these innovations. Primate models can be useful to our understanding of how environmental factors can affect such key behaviors. New results on forest chimpanzees demonstrate that for most patterns of behavior considered to favor a savanna model, the environmental influences are in opposition to the expectations. A new evaluation of the environmental influences on human evolution is required. We propose that either patterns of key human behaviors were inaccurately distinguished or that the hominization process started while the common ancestor of the chimpanzee and man was alive.     

Menstrual cycles continue into advanced old age in the common chimpanzee (Pan troglodytes)

A long postreproductive lifespan may distinguish women from all other female primates. A long-held consensus among reproductive scientists has been that our closest living relative, the chimpanzee (Pan troglodytes), experiences menstrual cycles until death. However, a recent study of biannual assessments of gonadotropins, but lacking observations of menstruation, concluded that menopause occurs in chimpanzees between 35 and 40 yr of age. A separate report, but on wild chimpanzees, documented fertility through the 40-44 age range in all populations studied. These contradictory reports pose questions about differences between wild and captive populations and about assessments of menopause. The present study revisits this controversy by analyzing longitudinal records of anogenital swelling and menstruation in 89 female chimpanzees aged 6 to 59 yr (n = 2386 records on cycle length), monitored for most of their adult lives at the Yerkes National Primate Research Center. Twenty of these chimpanzees were observed past 39 yr of age; all 20 displayed menstrual cycles beyond this age, as confirmed by at least two observations of menses about 35 days apart. Three of these were older than 50 yr and still displayed menstrual cycles. Only the oldest female appeared menopausal, with cycles of anogenital swelling ceasing 2 yr prior to her death at age 59. Random-effects statistical modeling reveals a slight decrease in cycle length until 20 yr of age and a slight lengthening thereafter. Mean cycle length across the lifespan is 35.4 days. Our findings, based upon actual observations of menstrual cycles, suggest that menopause in the chimpanzee is rare, occurring near the end of the lifespan.     

Sleeping behavior and associations in a group of captive chimpanzees.

The present study investigated the sleeping behavior and preferences of a group of six adolescent chimpanzees at the Delta Regional Primate Research Center in Louisiana, USA. The study sought to relate sleeping partner preferences to other aspects of social relationships. Comparative observations between those chimpanzee behaviors seen in the wild and in this group are noted.     

Successful reproduction in wild-released orphan chimpanzees (<Emphasis Type="Italic">Pan troglodytes troglodytes</Emphasis>)

We report a case of successful reproduction in wild-released orphan chimpanzees. Using non-invasive genetic analysis, we determined the paternity of an infant born to a female chimpanzee released by HELP (Habitat Ecologique et Liberté des Primates) Congo into the Conkouati-Douli National Park, Republic of Congo. The sire was a released male, thus demonstrating successful reproduction in both male and female released chimpanzees. These results provide evidence that release into the wild may be a viable response to the plight of orphan chimpanzees in Africa, and we discuss further applications of non-invasive genetic tagging to release programs. Electronic Publication     

Comparative locomotor behavior of chimpanzees and bonobos: The influence of morphology on locomotion

Results from a 10 month study of adult male and female bonobos (Pan paniscus) in the Lomako Forest, Zaire, and those from a 7 month study of adult male and female chimpanzees in the Tai Forest, Ivory Coast (Pan troglodytes verus), were compared in order to determine whether there are species differences in locomotor behavior and substrate use and, if so, whether these differences support predictions made on the basis of interspecific morphological differences. Results indicate that bonobos are more arboreal than chimpanzees and that male bonobos are more suspensory than their chimpanzee counterpart. This would be predicted on the basis of male bonobo's longer and more narrow scapula. This particular finding is contrary to the prediction that the bonobo is a “scaled reduced version of a chimpanzee” with little or no positional behavior difference as had been suggested. This study provides the behavioral data necessary to untangle contradictory interpretations of the morphological differences between chimpanzees and bonobos, and raises a previously discussed (Fleagle: Size and Scaling in Primate Biology, pp. 1–19, 1985) but frequently overlooked point–that isometry in allometric studies does not necessarily equate with behavioral equivalence. Several researchers have demonstrated that bonobos and chimpanzees follow the same scaling trends for many features, and are in some sense functionally equivalent, since they manage to feed and reproduce. However, as reflected in their morphologies, they do so through different types and frequencies of locomotor behaviors. © 1993 Wiley-Liss, Inc.     

Fatal Chimpanzee Attack in Loango National Park,Gabon

In some populations, chimpanzees engage in lethal aggression within and between social units. We report a fatal attack on an adult male chimpanzee at a new research site in Loango National Park, Gabon. We found a fresh corpse of an adult male chimpanzee only a few hundred meters from the research camp, after noting numerous vocalizations and chimpanzee movements the previous evening. Previous contacts with chimpanzees and fresh tracks in the area around the corpse suggest that 2 communities of chimpanzees range where the attack occurred and that members of the neighboring community killed the chimpanzee. To support the conclusion, we conducted genetic analysis for 13 Y-chromosome loci and 9 microsatellite loci of fecal samples from the dead individual, 5 possible attackers, and 2 members of the other community Though we cannot exclude the possibility of an intracommunity killing, the combined observational and genetic evidence suggest an intercommunity attack. The case study adds to the growing evidence that intercommunity killings are a rare but widespread phenomenon among chimpanzees and not an artifact of human provisioning or habituation.     

AVPR1A Variation in Chimpanzees (Pan troglodytes): Population Differences and Association with Behavioral Style

Primates and other mammals show measurable, heritable variation in behavioral traits such as gregariousness, timidity, and aggression. Connections among behavior, environment, neuroanatomy, and genetics are complex, but small genetic differences can have large effects on behavioral phenotypes. One of the best examples of a single gene with large effects on natural variation in social behavior is AVPR1A, which codes for a receptor of the peptide hormone arginine vasopressin. Work on rodents shows a likely causal association between AVPR1A regulatory polymorphisms and social behavior. Chimpanzees also show variation in the AVPR1A regulatory region, with some individuals lacking a ca. 350-bp segment corresponding to a putative functional element. Thus, chimpanzees have a “short” allele (segment deletion) and a “long” allele (no deletion) at this locus. Here we compare AVPR1A variation in two chimpanzee populations, and we examine behavioral and hormonal data in relation to AVPR1A genotypes. We genotyped AVPR1A in a captive population of western chimpanzees (Pan troglodytes verus, New Iberia Research Center; N = 64) for which we had quantitative measures of personality (based on 15 behavioral style indices, calculated from 3 yr of observational data), dominance rank, and baseline testosterone levels. We also provide the first assessment of AVPR1A genotype frequencies in a wild eastern chimpanzee population (Pan troglodytes schweinfurthii, Ngogo community, Kibale National Park, Uganda; N = 26). Our results indicated that the AVPR1A long allele was associated with a “smart” social personality in captive western chimpanzees, independent of testosterone levels. Although the frequency of the long allele was relatively low in captive western chimpanzees (0.23), it was the major allele in wild eastern chimpanzees (0.62). Our finding that allele and genotype frequencies for the AVPR1A polymorphism differ among chimpanzee populations also highlights the need for comparative studies —across subspecies and research sites— in primate behavioral genetics.     

Primate subsistence patterns: Collector-predators and gatherer-hunters

Hominization via predation has become a pervasive anthropological theme in recent years. Indeed, the assumption that hunting behavior originated within the primate phylogenetic sequence as a “human” subsistence pattern has generated numerous subsidiary hypotheses about how secondary traits were initiated, propagated or enhanced when a terrestrial, savanna-dwelling, meat-eating hominid line emerged from an arboreal, forest-dwelling, plant-eating ancestral stock. New field evidence on the behavioral and organizational features of subsistence in nonhuman and human primates now provides the basis for reconsidering these views.Many monkey, ape and human populations no longer seem to fit the stereotyped images sketched in past decades, when little or no comparative information was available to anthropologists. The discrepancy between the old concepts and new facts is particularly evident in Sub-Saharan Africa, where numerous primate taxa have been studied in climatically and biotically similar zones. In this region alone, more than 364 cases of predation, involving 22 different species of mammalian prey, have been recorded among at least 10 supposedly “vegetarian” baboon and chimpanzee populations dispersed between Ethiopia and South Africa. Furthermore, many of the human populations living within this same region—such as the Mbuti pygmies, the Hadza and the Kalahari bushmen—have been characterized as “hunters” but actually subsist for the most part on foods other than meat. These basic facts about collector-predator and Gatherer-hunter subsistence patterns are a mere beginning, however, for popular conceptions of primate lifestyles are eroding swiftly along many axes of investigation. It is becoming clear, for instance, that many primates—from prosimians to humans—are actually omnivores even though anthropologists have persistently miscast them as frugivores or carnivores. This false dichotomization of nonhuman versus human diets has led to a series of equally erroneous dichotomies in nonhuman versus human behavior. Thus, the possession of culture, technology, language and other similarly amorphous traits, many of which were in fact derived from this presumed shift in subsistence, have become entrenched as concepts of human uniqueness. In recent years, however, many new discoveries in primatology, and in ethnography and archeology, have weakened the theoretical structure to which “man-the-hunter” has been pinned. It is probable that savanna-dwelling, tool-using, seed-eating, scavenging and other independent schemes can now be replaced by a single, much simpler model wherein subsistence shifts among both nonhuman and human primates are perceived as smooth transitions within a graded continuum of evolution. Thus, the central objective of this report is to show that the subsistence activities of several extant cercopithecid, pongid and hominid populations in Africa can be arranged along an integrated spectrum which reflects gradual processes in the evolution of primate behavior and organization. This spectrum serves as the crux for a unifying model of behavioral evolution, and can in turn be broken down into a linked series of subsidiary models which elucidate specific aspects of primate prehistory.     

Interaction sequences between chimpanzees and human visitors at the Zoo

Data were collected on the behavior and physical characteristics of 259 human visitors and 24 chimpanzees at Chester Zoo. The successive responses of humans and chimpanzees to each other's behavior were recorded, the resulting long sequence being referred to as an interaction sequence. There was no particular set of characteristics that distinguished interactors from noninteractors in either humans or chimpanzees, although there was some evidence that chimpanzees were particularly likely to respond to men carrying objects. Chimpanzee responses were random with respect to the previous human behavior, but human responses were significantly associated with the preceding chimpanzee behavior. In particular, chimpanzee sounds were likely to be followed by human sounds, and begging was likely to be followed by the offer of food. Interaction sequences varied in length, but 9% of chimpanzee-initiated sequences went as far as a ninth interaction. Sequences resulted in the chimpanzees being given food in 25% of human-initiated, but only 8% of chimpanzee-initiated sequences. The results are consistent with the interpretation that humans and chimpanzees are motivated to interact with one another and that the chimpanzees do this primarily to obtain food. © 1995 Wiley-Liss, Inc.     

Scavenging by chimpanzees at Ngogo and the relevance of chimpanzee scavenging to early hominin behavioral ecology

Chimpanzees regularly hunt a variety of prey species. However, they rarely scavenge, which distinguishes chimpanzee carnivory from that of some modern hunter-gatherers and, presumably, at least some Plio-Pleistocene hominins. I use observations made over an 11-year period to document all known opportunities for scavenging encountered by chimpanzees at Ngogo, Kibale National Park, Uganda, and describe all cases of scavenging. I also review data on scavenging from other chimpanzee research sites. Chimpanzees at Ngogo encountered scavenging opportunities only about once per 100 days and ate meat from scavenged carcasses only four times. Scavenging opportunities are also rare at other sites, even where leopards are present (Mahale, Ta?, Gombe), and scavenging of leopard kills is known only from Mahale. Feeding on prey that chimpanzees had hunted but then abandoned is the most common form of scavenging reported across study sites. For example, several individuals at Ngogo ate meat from a partially consumed red colobus carcass abandoned after a hunt the previous day. Such behavior probably was not common among Oldowan hominins. Ngogo data and those from other sites also show that chimpanzees sometimes eat meat from carcasses of prey that they did not see killed and that were not killed by chimpanzees, and that scavenging allows access to carcasses larger than those of any prey items. However, chimpanzees ignore relatively many opportunities to obtain meat from such carcasses. Scavenging may be rare because fresh carcasses are rare, because the risk of bacterial infections and zoonoses is high, and because chimpanzees may not recognize certain species as potential prey or certain size classes of prey species as food sources. Its minimal nutritional importance, along with the absence of technology to facilitate confrontational scavenging and rapid carcass processing, apparently distinguishes chimpanzee foraging strategies from those of at least some Oldowan hominins.     

Behavioral Adaptation of Pan troglodytes troglodytes

As wild primate populations decline, numbers of orphaned primates, sanctuaries, and attempts to release primates back to the natural environment increase. Release projects frequently are poorly documented despite IUCN guidelines recommending post-release monitoring and systematic data collection as central to the process. Since 1996, Habitat Ecologique et Liberté des Primates (HELP) has been releasing wild-born orphaned chimpanzees into natural habitat in the Conkouati-Douli National Park, Republic of Congo. HELP developed a post-release monitoring system as an integral component. We present activity budgets and diet of released chimpanzees, and compared them to those of wild chimpanzee, as primary indicators of successful release. Feeding, moving, and resting dominated activity budgets, reflecting the overall patterns in wild populations. Diet was diverse and dominated by fruit, and the released chimpanzees showed specialization on a smaller number of species, as in many wild communities. The high survival rates of the chimpanzees and overall success of the release program are attributed to careful planning and post-release support facilitated via the monitoring process. Systematic post-release data collection monitoring has confirmed that wild-born chimpanzees can adjust behaviorally and nutritionally to the wild. Survival statistics of the reintroduced chimpanzees—confirmed 56%, possible 88%— reflect the behavioral adaptability.2nd revision March 11, 2005An erratum to this article can be found at     

The evolutionary history of human and chimpanzee Y-chromosome gene loss

Recent studies have suggested that gene gain and loss may contribute significantly to the divergence between humans and chimpanzees. Initial comparisons of the human and chimpanzee Y-chromosomes indicate that chimpanzees have a disproportionate loss of Y-chromosome genes, which may have implications for the adaptive evolution of sex-specific as well as reproductive traits, especially because one of the genes lost in chimpanzees is critically involved in spermatogenesis in humans. Here we have characterized Y-chromosome sequences in gorilla, bonobo, and several chimpanzee subspecies for 7 chimpanzee gene-disruptive mutations. Our analyses show that 6 of these gene-disruptive mutations predate chimpanzee-bonobo divergence at approximately 1.8 MYA, which indicates significant Y-chromosome change in the chimpanzee lineage relatively early in the evolutionary divergence of humans and chimpanzees.     

Hunting and Occasional Consumption of Prey Items by Chimpanzees at the Primate Foundation of Arizona

Researchers have documented hunting by wild chimpanzees (Pan troglodytes) for >30 yr. However, no one had reported the behavior in captive chimpanzees. The Primate Foundation of Arizona (PFA) is a biomedical research facility, and though every effort is made to ensure that rodents and other indigenous animals do not enter cages, an occasional small mammal or bird enters the outside cage area. We document 18 known successful hunts over 2.5 yr by ≥8 captive chimpanzees housed at PFA. The most common prey were rock squirrels (Cittallus variegatus; n = 8) and birds (n = 7). The most common killing method was a bite to the head (n = 9), indicating that the deaths were deliberate and not accidental. Two adult males, 1 wild- and 1 captive-born, together accounted for 11 of the 18 hunting episodes. The chimpanzees generally captured and killed the prey in their outdoor enclosures. The results have implications for the surveillance of outdoor areas in other facilities and the potential for disease or introduction of parasitic vectors, and are also an interesting comparison to wild chimpanzee behavior.     

Census and distribution of chimpanzees in Côte D’Ivoire

Most methods of estimating chimpanzee population densities rely on nest counts. We tested the most frequently used techniques on a known chimpanzee community living in the rainforest of the Taï National Park, Côte d’Ivoire. The best density estimates are given by counts that assume groups of nests to be distributed randomly and that use the mean group size for homogenous habitat but the median for heterogenous habitats. Correction for real forest cover within the region should be made because chimpanzees make nests only in forested regions. This method gave the exact chimpanzee density for the Taï population, i.e. 1.7 nest builders/km². For the nationwide survey, we first estimated the chimpanzee density for different types of habitat (e.g. intact primary forest: 1.64 chimpanzees/km²; degraded forests: 0.4 chimpanzees/km²; human encroached forests and mosaic habitats: 0.09 chimpanzees/km²). Second, we estimated the total forest cover of the country with satellite pictures. This gave an estimated chimpanzee population in Côte d’Ivoire of about 11,676 ± 1,168 individuals, which equals the number of spectators at a soccer game in an average European town. Sadly, only three National Parks may have chimpanzee populations large enough to be viable, whereas the rest are scattered and isolated small populations that are already threatened in their survival.     

Gene flow in wild chimpanzee populations: what genetic data tell us about chimpanzee movement over space and time

The isolation of phylogenetically distinct primate immunodeficiency viruses from at least seven wild-born, captive chimpanzees indicates that viruses closely related to HIV-1 may be endemic in some wild chimpanzee populations. The search for the chimpanzee population or populations harbouring these viruses is therefore on. This paper attempts to answer the question of whether or not such populations of chimpanzees are likely to exist at all, and, if so, where they are likely to be found. We summarize what is known about gene flow in wild populations of chimpanzees, both between major phylogeographical subdivisions of the species, and within these subdivisions. Our analysis indicates that hitherto undocumented reproductively isolated chimpanzee populations may in fact exist. This conclusion is based on the observation that, despite limited geographical sampling and limited numbers of genetic loci, conventional notions of the nature and extent of chimpanzee gene flow have recently been substantially revised. Molecular genetic studies using mitochondrial DNA sequences and hypervariable nuclear microsatellite markers have indicated the existence of heretofore undocumented barriers to chimpanzee gene flow. These studies have identified at least one population of chimpanzees genetically distinct enough to be classified into a new subspecies (Pan troglodytes vellerosus). At the same time, they have called into question the long-accepted genetic distinction between eastern chimpanzees (Pan troglodytes schweinfurthii) and western equatorial chimpanzees (Pan troglodytes troglodytes). The same studies have further indicated that gene flow between local populations is more extensive than was previously thought, and follows patterns sometimes inconsistent with those documented through direct behavioural observation. Given the apparently incomplete nature of the current understanding of chimpanzee gene flow in equatorial Africa, it seems reasonable to speculate that a chimpanzee population or populations may exist which both harbour the putative HIV-1 ancestor, and which have remained reproductively isolated from other chimpanzee populations over the time-scale relevant to the evolution of the SIVcpz-HIV-1 complex of viruses. Continued extensive sampling of wild chimpanzee populations, both for their genes and their viruses, should be performed quickly considering the high probability of extinction that many wild chimpanzee populations face today. The history of human-chimpanzee contacts is discussed.