John C. Mitani

Why do chimpanzees hunt and share meat

Wild chimpanzees, Pan troglodytes, frequently hunt and share meat. Despite widespread interest and considerable study, continued controversy exists regarding the factors that influence chimpanzee hunting decisions and meat sharing. Three hypotheses invoke the importance of ecological, reproductive and social factors. A nutritional shortfall hypothesis suggests that chimpanzees hunt to compensate for seasonal shortages in food availability. A second hypothesis argues that male chimpanzees hunt to obtain meat that they swap for matings. A third hypothesis proposes that males use meat as a social tool to develop and maintain alliances with other males. We tested these hypotheses using observations of an unusually large community of chimpanzees at Ngogo in Kibale National Park, Uganda. Results did not support the nutritional shortfall or meat-for-sex hypotheses. The Ngogo chimpanzees hunted primarily during times of food abundance rather than scarcity. The presence of oestrous females did not predict the tendency of chimpanzees to hunt. Furthermore, meat-for-sex exchanges occurred infrequently, and males did not gain a mating advantage through sharing meat. Additional observations were consistent with the male social bonding hypothesis. At Ngogo, male chimpanzees were likely to hunt when accompanied by other males. Males shared meat nonrandomly and reciprocally among themselves, and males exchanged meat for agonistic support. Although several factors are likely to affect chimpanzee hunting decisions and meat sharing, these results indicate that primary causes will not be found through invoking simple energetic or reproductive considerations.

Generation times in wild chimpanzees and gorillas suggest earlier divergence times in great ape and human evolution

Fossils and molecular data are two independent sources of information that should in principle provide consistent inferences of when evolutionary lineages diverged. Here we use an alternative approach to genetic inference of species split times in recent human and ape evolution that is independent of the fossil record. We first use genetic parentage information on a large number of wild chimpanzees and mountain gorillas to directly infer their average generation times. We then compare these generation time estimates with those of humans and apply recent estimates of the human mutation rate per generation to derive estimates of split times of great apes and humans that are independent of fossil calibration. We date the human–chimpanzee split to at least 7–8 million years and the population split between Neanderthals and modern humans to 400,000–800,000 y ago. This suggests that molecular divergence dates may not be in conflict with the attribution of 6- to 7-million-y-old fossils to the human lineage and 400,000-y-old fossils to the Neanderthal lineage.

The limited impact of kinship on cooperation in wild chimpanzees

The complex cooperative behavior exhibited by wild chimpanzees generates considerable theoretical and empirical interest, yet we know very little about the mechanisms responsible for its evolution. Here, we investigate the influence of kinship on the cooperative behavior of male chimpanzees living in an unusually large community at Ngogo in Kibale National Park, Uganda. Using long-term field observations and molecular genetic techniques to identify kin relations between individuals, we show that male chimpanzees clearly prefer to affiliate and cooperate with their maternal brothers in several behavioral contexts. Despite these results, additional analyses reveal that the impact of kinship is limited; paternal brothers do not selectively affiliate and cooperate, probably because they cannot be reliably recognized, and the majority of highly affiliative and cooperative dyads are actually unrelated or distantly related. These findings add to a growing body of research that indicates that animals cooperate with each other to obtain both direct and indirect fitness benefits and that complex cooperation can occur between kin and nonkin alike.

BOUNDARY PATROLS AND INTERGROUP ENCOUNTERS IN WILD CHIMPANZEES

Summary Chimpanzees are among the few mammals that engage in lethal coalitionary aggression between groups. Most attacks on neighbors occur when parties made up mostly of adult males patrol boundaries of their community’ s range. Patrols have time, energy, and opportunity costs, and entail some risks despite the tendency of males to attack only when they greatly outnumber their targets. These factors may lead to a collective action problem. Potential benee ts include protection of community members, particularly infants; range expansion and increases in the amount and quality of food available; and incorporation of more females into the community. Males may not share these equally; for example, those able to obtain

Territoriality: The relation of ranging pattern and home range size to defendability, with an analysis of territoriality among primate species

Summary1.Existing theory suggests that territoriality will evolve when resources are limited and defendable, but defendability has seldom been analyzed quantitatively.2.Here we argue that defendability depends on the ability of an animal to monitor the boundaries of its range in order to detect potential intruders and introduce an index of defendability (D) which is the ratio of observed daily path length (d) to an area equal to the diameter (d′) of a circle with area equal to home range area of the animal. This index is sensitive only to extreme deviation from circular shape.3.Review of the literature on primate ranging reveals that all territorial groups for which data are available have an index of 1.0 or greater, and that few nonterritorial species have an index of 1.0 or greater.4.Regression analysis of the relationship of daily path length to feeding group weight and foliage in the diet reveals that both feeding group weight and foliage in the diet account for a large proportion of the variance in daily path length, and that territorial and nonterritorial groups do not differ in day range for a given group weight and diet.

Conflict and Cooperation in Wild Chimpanzees

Publisher Summary This chapter discusses the current knowledge of competition and cooperation in wild chimpanzees. It explicitly focuses on recent field studies that shed new light on how chimpanzees compete, cooperate, and cooperate to compete. It outlines the social, demographic, and ecological contexts within which wild chimpanzees compete and cooperate. Within groups, males compete over status and access to fecundable females. High-ranking males gain clear reproductive benefits as they monopolize mating with females when they are most likely to conceive. Rank striving also incurs significant physiological costs, and the extent to which these are mitigated by survival benefits, such as increased access to resources, is not clear. Males direct frequent aggression against females, much of which appears to function as sexual coercion, decreasing the chance that a female will mate with other males. Females are aggressive primarily in the context of feeding competition. Despite evidence that female rank has important effects on reproduction, aggression by parous females against other parous females is rare, and female dominance ranks are stable over long periods of time. Intergroup relations among chimpanzees are predictably hostile. The evolutionary mechanisms that account for chimpanzee cooperation require further study. Current data suggest little role for kin selection. Some patterns of exchange are suggestive of reciprocal altruism, but better data are required to rule out the alternative hypothesis of mutualism.

Male affiliation, cooperation and kinship in wild chimpanzees.

Long-term field research has revealed that male chimpanzees, Pan troglodytes, affiliate and cooperate in several contexts. Assuming close genetic relationship among males, affiliative and cooperative behaviour have been hypothesized to evolve through the indirect effects of kin selection. We tested the hypothesis that matrilineal genetic relatedness affects patterns of male social affiliation and cooperation in an unusually large community of chimpanzees at the Ngogo study site, Kibale National Park, Uganda. Field observations indicated that six behavioural measures of affiliation and cooperation among 23 adult males were significantly correlated with each other. Sequences of the first hypervariable portion of the mtDNA genome revealed that three pairs of males and one quintet shared mtDNA haplotypes. Matrix permutation tests using behavioural and genetic data showed that males that affiliated and cooperated with each other were not closely related through the maternal line. These findings add to a growing body of empirical evidence that suggest kinship plays an ancillary role in structuring patterns of wild chimpanzee behaviour within social groups. Copyright 2000 The Association for the Study of Animal Behaviour.

Hunting Behavior of Chimpanzees at Ngogo, Kibale National Park, Uganda

Chimpanzees (Pan troglodytes) prey on a variety of vertebrates, mostly on red colobus (Procolobus spp.) where the two species are sympatric. Variation across population occurs in hunting frequency and success, in whether hunting is cooperative, i.e., payoffs to individual hunters increase with group size, and in the extent to which hunters coordinate their actions in space and time, and in the impact of hunting on red colobus populations. Also, hunting frequency varies over time within populations, for reasons that are unclear. We present new data on hunting by chimpanzees at Ngogo, Kibale National Park, Uganda, and combine them with earlier data (Mitani and Watts, 1999, Am. J. Phys. Anthropol. 109: 439–454) to examine hunting frequency and success, seasonality, and cooperation. The Ngogo community is the largest and has the most males of any known community. Chimpanzees there mostly hunt red colobus and are much more successful and make many more kills per hunt than at other sites; they kill 6–12% of the red colobus population annually. The number of kills and the offtake of meat per hunt increase with the number of hunters, but per capita meat intake is independent of hunting party size; this suggests that cheating occurs in large parties. Some behavioral cooperation occurs. Hunting success and estimated meat intake vary greatly among males, partly due to dominance rank effects. The high overall success rate leads to relatively high average per capita meat intake despite the large number of consumers. The frequency of hunts and of hunting patrols varies positively with the availability of ripe fruit; this is the first quantitative demonstration of a relationship between hunting frequency and the availability of other food, and implies that the chimpanzees hunt most when they can easily meet energy needs from other sources. We provide the first quantitative support for the argument that variation in canopy structure influences decisions to hunt red colobus because hunts are easier where the canopy is broken.

Lethal intergroup aggression leads to territorial expansion in wild chimpanzees

Summary Chimpanzees make lethal coalitionary attacks on members of other groups [1]. This behavior generates considerable attention because it resembles lethal intergroup raiding in humans [2]. Similarities are nevertheless difficult to evaluate because the function of lethal intergroup aggression by chimpanzees remains unclear. One prominent hypothesis suggests that chimpanzees attack neighbors to expand their territories and to gain access to more food [2]. Two cases apparently support this hypothesis, but neither furnishes definitive evidence. Chimpanzees in the Kasekela community at Gombe National Park took over the territory of the neighboring Kahama community after a series of lethal attacks [3]. Understanding these events is complicated because the Kahama community had recently formed by fissioning from the Kasekela group and members of both communities had been provisioned with food. In a second example from the Mahale Mountains, the M group chimpanzees acquired part of the territory of the adjacent K group after all of the adult males in the latter disappeared [4]. Although fatal attacks were suspected from observations of intergroup aggression, they were not witnessed, and as a consequence, this case also fails to furnish conclusive evidence. Here we present data collected over 10 years from an unusually large chimpanzee community at Ngogo, Kibale National Park, Uganda. During this time, we observed the Ngogo chimpanzees kill or fatally wound 18 individuals from other groups; we inferred three additional cases of lethal intergroup aggression based on circumstantial evidence (see Supplemental Information). Most victims were caught in the same region and likely belonged to the same neighboring group. A causal link between lethal intergroup aggression and territorial expansion can be made now that the Ngogo chimpanzees use the area once occupied by some of their victims.

Demographic influences on the hunting behavior of chimpanzees.

We investigated hunting in an unusually large community of wild chimpanzees at Ngogo in the Kibale National Park, Uganda. Aspects of predation were recorded with respect to the prey, the predators, and hunting episodes. During 23 months of observation, the Ngogo chimpanzees caught 128 prey items from four primate and three ungulate species. Chimpanzees preyed selectively on immature red colobus primarily during group hunts, with adult males making the majority of kills. Party size and composition were significant predictors of the probability that chimpanzees would hunt and of their success during attempts. Chimpanzees were more likely to hunt red colobus if party size and the number of male hunters were large; party size and the number of male hunters were also significantly larger in successful compared with unsuccessful hunts. The Ngogo chimpanzees did not appear to hunt cooperatively, but reciprocal meat-sharing typically took place after kills. Hunts occurred throughout the year, though there was some seasonality as displayed by periodic hunting binges. The extremely high success rate and large number of kills made per successful hunt are the two most striking aspects of predation by the Ngogo chimpanzees. We compare currently available observations of chimpanzee hunting behavior across study sites and conclude that the large size of the Ngogo community contributes to their extraordinary hunting success. Demographic differences between groups are likely to contribute to other patterns of interpopulation variation in chimpanzee predation.