An ethological study of pygmy chimpanzees in Wamba,Zaïre: A comparison with chimpanzees

An ethological study was conducted on a provisioned pygmy chimpanzee group in Wamba, Zaïre and was compared with the author's previous study on common chimpanzees, in Mahale Mountains with special reference to the evolution of behavior systems. The relationships between behavior patterns were investigated by analyses of the intra- or inter-individual sequential behaviors. Analysis of behavior pathways showed that “bipedal” was not found (i.e., observed frequency lower than expected value) in the course of the charging display in pygmy chimps, though it was an important element of the charging display in common chimps. Additionally, “bipedal” was found to be an initating behavior pattern in sexual behavior or dominant-subordinate behavior in pygmy chimps. These differences were related to the decrease of the role of charging display and to the increase of the roles of sexual and dominant-subordinate behaviors in pygmy chimps. Sexual behavior, “female genito-genital rubbing,” “mutual present” and frequent mounting and presenting between males were suggested to have evolved together as a system. These characteristics of behaviors correspond to the fact that pygmy chimps form more stable and larger temporary groups than common chimps.     

Ape imagination? A sentimentalist critique of Frans de Waal’s gradualist theory of human morality

This essay draws on Adam Smith’s moral sentimentalism to critique primatologist Frans de Waal’s gradualist theory of human morality. De Waal has spent his career arguing for continuity between primate behavior and human morality, proposing that empathy is a primary moral building block evident in primate behavior. Smith’s moral sentimentalism—with its emphasis on the role of sympathy in moral virtue—provides the philosophical framework for de Waal’s understanding of morality. Smith’s notion of sympathy and the imagination involved in sympathy is qualitatively different from animal sympathy. I argue that Smithian sympathy includes the ability to represent propositional attitudes and take into account multiple perspectives which are then synthesized into a singular impartial perspective. Furthermore, Smithian moral judgment requires the capacity for emotion regulation and moral self-cultivation, or the ability to shape and control one’s reactive attitudes. Taken together, these capacities far outstrip the capacities of animals, disrupting de Waal’s gradualism.     

Reexamination of the African hominoid trichotomy with additional sequences from the primate beta-globin gene cluster.

Additional DNA sequence information from a range of primates, including 13.7 kb from pygmy chimpanzee (Pan paniscus), was added to data sets of beta-globin gene cluster sequence alignments that span the gamma 1, gamma 2, and psi eta loci and their flanking and intergenic regions. This enlarged body of data was used to address the issue of whether the ancestral separations of gorilla, chimpanzee, and human lineages resulted from only one trichotomous branching or from two dichotomous branching events. The degree of divergence, corrected for superimposed substitutions, seen in the beta-globin gene cluster between human alleles is about a third to a half that observed between two species of chimpanzee and about a fourth that between human and chimpanzee. The divergence either between chimpanzee and gorilla or between human and gorilla is slightly greater than that between human and chimpanzee, suggesting that the ancestral separations resulted from two closely spaced dichotomous branchings. Maximum parsimony analysis further strengthened the evidence that humans and chimpanzees share the longest common ancestry. Support for this human-chimpanzee clade is statistically significant at P = 0.002 over a human-gorilla clade or a chimpanzee-gorilla clade. An analysis of expected and observed homoplasy revealed that the number of sequence changes uniquely shared by human and chimpanzee lineages is too large to be attributed to homoplasy. Molecular clock calculations that accommodated lineage variations in rates of molecular evolution yielded hominoid branching times that ranged from 17-19 million years ago (MYA) for the separation of gibbon from the other hominoids to 5-7 MYA for the separation of chimpanzees from humans. Based on the relatively late dates and mounting corroborative evidence from unlinked nuclear genes and mitochondrial DNA for the close sister grouping of humans and chimpanzees, a cladistic classification would place all apes and humans in the same family. Within this family, gibbons would be placed in one subfamily and all other extant hominoids in another subfamily. The later subfamily would be divided into a tribe for orangutans and another tribe for gorillas, chimpanzees, and humans. Finally, gorillas would be placed in one subtribe with chimpanzees and humans in another, although this last division is not as strongly supported as the other divisions.     

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.     

Comparative Feeding Ecology of Two Communities of Chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>) in Kibale National Park,Uganda

Several recent studies have documented considerable intraspecific and intrapopulation ecological variation in primates. However, we generally lack an understanding of how such variability may be linked to concomitant demographic variation among groups or populations of the same species, particularly in regard to large-bodied and wide-ranging species with high ecological flexibility, such as chimpanzees (Pan troglodytes). We compared the feeding ecology of chimpanzees inhabiting 2 sites in Kibale National Park, Uganda that differ 3-fold in chimpanzee density and support notably different plant communities. Chimpanzees at Ngogo, a site with the largest known chimpanzee community and unusually high chimpanzee density, spent a significantly lower percentage of time resting (and pregnant and lactating females spent more time feeding), incorporated higher percentages of ripe fruit in their diet, had lower dietary diversity values, and had shorter and less variable average patch residency times than did their counterparts at the nearby Kanyawara site, which supports a relatively low density of chimpanzees. In addition, feeding party size was significantly and positively related to feeding patch size at Ngogo, but not at Kanyawara. Together these findings aid in explaining the noted disparity in chimpanzee community size and density between Ngogo and Kanyawara by suggesting that the diet of Ngogo chimpanzees is of higher overall quality than that of Kanyawara chimpanzees. They also highlight the potentially profound influence of even small-scale habitat heterogeneity on the ecology of primates. Researchers must take such influences into account when attempting to draw conclusions about species- or population-level characteristics.     

Influence of chimpanzee predation on the red colobus population at Ngogo, Kibale National Park, Uganda

Frequent hunting of red colobus monkeys (Procolobus rufomitratus) takes place at all long-term chimpanzee (Pan troglodytes) study sites where both species are present. Red colobus are the most commonly selected prey of chimpanzees even when other monkey species are more abundant. In particular, the chimpanzee community at Ngogo, Kibale National Park, Uganda, preys heavily on red colobus monkeys: the chimpanzee hunting success rate is extremely high, and chimpanzees kill many individuals per successful hunt. Census data had suggested that the red colobus population is declining and that predation by chimpanzees may be contributing to this decline. In this paper, I address the impact of hunting on the red colobus population at Ngogo. To test the hypothesis that chimpanzee hunting is sustainable, I am using demographic data collected on red colobus monkeys over a period of 3 years, as well as fecundity and mortality data from previous studies of this species. I apply matrix models and vortex analyses using a sensitivity analysis approach to project future population development. Results show that current rates of hunting are not sustainable, but that chimpanzees are neither more “noble”, nor more “savage” than humans are, but that they also hunt to ensure maximum benefit without regard for the consequences for the prey population.     

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.     

Able-Bodied Wild Chimpanzees Imitate a Motor Procedure Used by a Disabled Individual to Overcome Handicap

Chimpanzee culture has generated intense recent interest, fueled by the technical complexity of chimpanzee tool-using traditions; yet it is seriously doubted whether chimpanzees are able to learn motor procedures by imitation under natural conditions. Here we take advantage of an unusual chimpanzee population as a ‘natural experiment’ to identify evidence for imitative learning of this kind in wild chimpanzees. The Sonso chimpanzee community has suffered from high levels of snare injury and now has several manually disabled members. Adult male Tinka, with near-total paralysis of both hands, compensates inability to scratch his back manually by employing a distinctive technique of holding a growing liana taut while making side-to-side body movements against it. We found that seven able-bodied young chimpanzees also used this ‘liana-scratch’ technique, although they had no need to. The distribution of the liana-scratch technique was statistically associated with individuals'' range overlap with Tinka and the extent of time they spent in parties with him, confirming that the technique is acquired by social learning. The motivation for able-bodied chimpanzees copying his variant is unknown, but the fact that they do is evidence that the imitative learning of motor procedures from others is a natural trait of wild chimpanzees.     

A simple null model predicts fruit-frugivore interactions in a temperate rainforest

Studies of seed dispersal mutualisms typically test for adaptive relationships between fruits and frugivores. Here, I take the opposite approach, and ask what relationships would be expected based on chance encounters between mutualists. I derived a simple null model to predict pair-wise relationships between fruit and frugivore species. I assumed that all frugivores had identical fruit preferences, but allowed for interspecific variation in plant abundances, frugivore abundances and fruit phenologies. Under these conditions, the number of visits each frugivore species makes to each plant species results from the product of plant abundances and frugivore abundances when each plant species produces fruit. I then tested null model predictions with observations in a temperate rainforest in British Columbia, Canada. I measured the abundance and phenology of seven bird-dispersed plant species, seasonal changes in the abundances of six avian frugivore species and 212 interactions between them. Empirical results were generally consistent with predictions, suggesting that relationships between birds and fruits are structured randomly. However, some variation in relationships between observations and predictions was observed, suggesting deterministic processes may also be important. Overall results illustrate that predictable relationships between fruit and frugivore species can result from random encounters between mutualists.     

EMG of chimpanzee shoulder muscles during knuckle-walking: problems of terrestrial locomotion in a suspensory adapted primate

By most accounts, the upper limb of the chimpanzee is primarily adapted to suspensory postures and locomotion. In order to determine how the derived morphology of the chimpanzee forelimb has affected the form of quadrupedal locomotion displayed by these animals, electromyographic activity patterns of 10 shoulder muscles during knuckle-walking in two chimpanzee subjects were analysed and compared to data on the opossum and cat taken from the literature. Telemetered electromyography coupled with simultaneous video recording was employed in order to study unfettered locomotion in the chimpanzee subjects.
Chimpanzees are characterized by a quadrupedal gait in which the hind limb overstrides the ipsilateral forelimb. Forelimb position in the plane of abduction/adduction is significantly affected by whether the hind limb passes inside or outside its ipsilateral forelimb. The degree of abduction adduction of the forelimb, in turn, influences many of the muscle activity patterns. That is, some muscles would be more frequently or less frequently active, depending on whether the arm was relatively abducted or adducted during a stride. Thus, there can be no single motor programme that generates the step cycle in chimpanzees.
While there are some parallels between muscle recruitment patterns for chimpanzee, opossum and cat quadrupedalism, the results of this study also indicate that many aspects of muscle use in chimpanzees have been significantly influenced by factors related to increased mobility of the upper limb. Finally, this study has revealed that moving the arm forward during swing phase of knuckle-walking is not a simple product of muscular elTort. and that other mechanisms must be involved. However, it is unclear at present exactly what these mechanisms may be.     

Epidermal patterns of the hands and feet of the pygmy chimpanzee (Pan paniscus)

The characteristics of the epidermal ridge system were studied in a series of eighteen lesser or pygmy chimpanzees (Pan paniscus). The general ridge alignments are very similar to those of the chimpanzee (Pan troglodytes); Biegert ('61). On the average the pattern intensity (P.I.) of the palm configurations is considerably higher in the pygmy chimpanzee than in the chimpanzee, thus representing the highest total palm pattern intensity of all species of the Hominoidea. The sole configurations show parallel main results to those of the palm; however, the decreased sole pattern frequency of the pygmy chimpanzee is of a smaller predominance only as compared to the values of the other species of this superfamily. The preliminary data on the finger tip patterns, translated into P.I. values, are much higher than in chimpanzees and within the range of the mean values of gorillas (Brehme, '73), while those of the toes of pygmy chimpanzees seem to possess the lowest P.I. values of the African apes.     

Suitable Habitats for Endangered Frugivorous Mammals: Small-Scale Comparison,Regeneration Forest and Chimpanzee Density in Kibale National Park,Uganda

Landscape patterns and chimpanzee (Pan troglodytes schweinfurthii) densities in Kibale National Park show important variation among communities that are geographically close to one another (from 1.5 to 5.1 chimpanzees/km²). Anthropogenic activities inside the park (past logging activities, current encroachment) and outside its limits (food and cash crops) may impact the amount and distribution of food resources for chimpanzees (frugivorous species) and their spatial distribution within the park. Spatial and temporal patterns of fruit availability were recorded over 18 months at Sebitoli (a site of intermediate chimpanzee density and higher anthropic pressure) with the aim of understanding the factors explaining chimpanzee density there, in comparison to results from two other sites, also in Kibale: Kanyawara (low chimpanzee density) and Ngogo (high density, and furthest from Sebitoli). Because of the post-logging regenerating status of the forest in Sebitoli and Kanyawara, smaller basal area (BA) of fruiting trees most widely consumed by the chimpanzees in Kanyawara and Sebitoli was expected compared to Ngogo (not logged commercially). Due to the distance between sites, spatial and temporal fruit abundance in Sebitoli was expected to be more similar to Kanyawara than to Ngogo. While species functional classes consumed by Sebitoli chimpanzees (foods eaten during periods of high or low fruit abundance) differ from the two other sites, Sebitoli is very similar to Kanyawara in terms of land-cover and consumed species. Among feeding trees, Ficus species are particularly important resources for chimpanzees at Sebitoli, where their basal area is higher than at Kanywara or Ngogo. Ficus species provided a relatively consistent supply of food for chimpanzees throughout the year, and we suggest that this could help to explain the unusually high density of chimpanzees in such a disturbed site.     

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.     

Chimpanzee (Pan troglodytes) responses to caregiver use of chimpanzee behaviors

The relationships between captive primates and their caregivers are critical ones and can affect animal welfare. Friendly relationships can improve quality of life; adversely, agonistic relationships can decrease quality of life. Caregivers in and of themselves should not be stressful to their charges, instead the caregivers' behaviors and the nature of their interactions with captive primates is likely the basis for the stress. One method to promote positive relationships in the captive environment is for caregivers to employ species‐specific behaviors in their interactions with their charges. This study tested the effect of caregivers' use of these behaviors with chimpanzees at The Zoo Northwest Florida in Gulf Breeze. The chimpanzee participants were three males. Data collection occurred during typical interactions between the human participants and the chimpanzees. Some days the caregiver presented chimpanzee behaviors and vocalizations (CB—Chimpanzee Behavior Condition) in the data collection interactions with the chimpanzees. On other days the caregiver presented human behaviors and used speech (HB—Human Behavior Condition) in the interactions with the chimpanzees. The interactions were videotaped. Data coders recorded the behavioral contexts for each chimpanzee as they occurred on the videotape and the time that each context began. Overall they engaged in significantly more friendly behaviors such as play in CB than in HB. They were significantly less interactive in HB than CB. Caregivers should understand and employ species‐specific interactions with chimpanzees to promote friendly interactions and animal welfare. Zoo Biol 27:345–359, 2008. © 2008 Wiley‐Liss, Inc.     

Troublesome neighbours: Changing attitudes towards chimpanzees (Pan troglodytes) in a human-dominated landscape in Uganda

Long-term human–wildlife sympatry depends on the willingness and capacity of local people to coexist with wild animals. With human population growth and deforestation for agriculture, farmers increasingly live in proximity to wildlife, including large mammals of conservation concern. Understanding local perspectives and concerns regarding wildlife is essential for informing appropriate management strategies that reduce conflicts and promote sustainable coexistence. Social science approaches therefore have a critical role in integrated conservation programmes. We undertook an attitude survey to understand residents’ perspectives about sharing a landscape with chimpanzees (Pan troglodytes) in an unprotected forest–agriculture mosaic in Uganda. Interviews (n = 134) in 12 villages demonstrate residents’ ambivalence towards living alongside these protected yet potentially troublesome mammals. Chimpanzee behaviour is reported to have undergone recent changes. Residents claim apes increasingly enter villages for food, threaten people, and pose a particular threat to children's safety. Chimpanzee numbers are believed to have increased locally. Most interviewees fear chimpanzees, considering them dangerous. Crop losses to chimpanzees were widely reported. Farmers tolerate raiding of domestic fruits, but not cash-crops. Results demonstrate that attitudes towards wildlife are not fixed. Reported changes to chimpanzee behaviour are challenging villagers’ traditionally benign attitude towards them. Even so, residents acknowledge benefits to chimpanzees because they reportedly displace other crop-raiding wildlife which, unlike chimpanzees, damage important staple food crops. Survey findings are contextualised with respect to recent, major land-use changes in Uganda (clearance of unprotected forest for timber and agriculture) that have precipitated a sharp rise in farmer–chimpanzee interactions. We discuss the study's broader implications for protected mammal management and conflict mitigation in human-dominated landscapes, and ask whether it is appropriate to expect impoverished rural farmers to accommodate large-bodied mammals that pose a potential threat to their safety and livelihoods.     

Chimpanzees coordinate in a negotiation game

A crucially important aspect of human cooperation is the ability to negotiate to cooperative outcomes when interests over resources conflict. Although chimpanzees and other social species may negotiate conflicting interests regarding travel direction or activity timing, very little is known about their ability to negotiate conflicting preferences over food. In the current study, we presented pairs of chimpanzees with a choice between two cooperative tasks—one with equal payoffs (e.g., 5-5) and one with unequal payoffs (higher and lower than in the equal option, e.g., 10-1). This created a conflict of interests between partners with failure to work together on the same cooperative task resulting in no payoff for either partner. The chimpanzee pairs cooperated successfully in as many as 78–94% of the trials across experiments. Even though dominant chimpanzees preferred the unequal option (as they would obtain the largest payoff), subordinate chimpanzees were able to get their way (the equal option) in 22–56% of trials across conditions. Various analyses showed that subjects were both strategic and also cognizant of the strategies used by their partners. These results demonstrate that one of our two closest primate relatives, the chimpanzee, can settle conflicts of interest over resources in mutually satisfying ways—even without the social norms of equity, planned strategies of reciprocity, and the complex communication characteristic of human negotiation.     

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.     

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.     

Dimensions and moment arms of the hind- and forelimb muscles of common chimpanzees (Pan troglodytes).

This paper supplies quantitative data on the hind- and forelimb musculature of common chimpanzees (Pan troglodytes) and calculates maximum joint moments of force as a contribution to a better understanding of the differences between chimpanzee and human locomotion. We dissected three chimpanzees, and recorded muscle mass, fascicle length, and physiological cross-sectional area (PCSA). We also obtained flexion/extension moment arms of the major muscles about the limb joints. We find that in the hindlimb, chimpanzees possess longer fascicles in most muscles but smaller PCSAs than are predicted for humans of equal body mass, suggesting that the adaptive emphasis in chimpanzees is on joint mobility at the expense of tension production. In common chimpanzee bipedalism, both hips and knees are significantly more flexed than in humans, necessitating muscles capable of exerting larger moments at the joints for the same ground force. However, we find that when subject to the same stresses, chimpanzee hindlimb muscles provide far smaller moments at the joints than humans, particularly the quadriceps and plantar flexors. In contrast, all forelimb muscle masses, fascicle lengths, and PCSAs are smaller in humans than in chimpanzees, reflecting the use of the forelimbs in chimpanzee, but not human, locomotion. When subject to the same stresses, chimpanzee forelimb muscles provide larger moments at the joints than humans, presumably because of the demands on the forelimbs during locomotion. These differences in muscle architecture and function help to explain why chimpanzees are restricted in their ability to walk, and particularly to run bipedally.