Craig B. Stanford

Intergroup aggression in chimpanzees and humans.

The occurrence of fatal attacks during intergroup encounters among chimpanzees suggests that certain aspects of chimpanzee and human intergroup aggression may be explicable in similar ways. This paper addresses three questions: What conditions favor the evolution of lethal raiding in intergroup aggression? Why is intergroup aggression in these two species predominantly the domain of males? Under what circumstances do groups compete over access to females as opposed to material resources? Examination of comparative data on nonhuman primates and crosscultural study of foraging societies suggests that attacks are lethal because where there is sufficient imbalance of power their cost is trivial, that these attacks are a male and not a female activity because males are the philopatric sex, and that it is resources of reproductive interest to males that determine the causes of intergroup aggression.

The social behavior of chimpanzees and bonobos : empirical evidence and shifting assumptions

As our closest living relatives, chimpanzees and bonobos have been widely used as models of the behavior of early hominids. In recent years, as information on the social behavior and ecology of bonobos has come to light, many interspecific comparisons have been made. Chimpanzees have been characterized in terms of their intercommunity warfare, meat eating, infanticide, cannibalism, male status‐striving, and dominance over females. Bonobos, meanwhile, have been portrayed as the “Make love, not war” ape, characterized by female power‐sharing, a lack of aggression between either individuals or groups, richly elaborated sexual behavior that occurs without the constraint of a narrow window of fertility, and the use of sex for communicative purposes. This paper evaluates the evidence for this dichotomy and considers the reasons that contrasting portrayals of the two great apes have developed. While there are marked differences in social behavior between these two species, I argue that they are more similar behaviorally than most accounts have suggested. I discuss several reasons that current views of bonobo and chimpanzee societies may not accord well with field data. Among these are a bias toward captive data on bonobos, the tendency to see bonobos as derived because their behavior has been described more recently than that of chimpanzees, and the possibility that interpretations of bonobo‐chimpanzee differences are reflections of human male‐female differences.

Meat‐Adaptive Genes and the Evolution of Slower Aging in Humans

The chimpanzee life span is shorter than that of humans, which is consistent with a faster schedule of aging. We consider aspects of diet that may have selected for genes that allowed the evolution of longer human life spans with slower aging. Diet has changed remarkably during human evolution. All direct human ancestors are believed to have been largely herbivorous. Chimpanzees eat more meat than other great apes, but in captivity are sensitive to hypercholesterolemia and vascular disease. We argue that this dietary shift to increased regular consumption of fatty animal tissues in the course of hominid evolution was mediated by selection for “meat‐adaptive” genes. This selection conferred resistance to disease risks associated with meat eating also increased life expectancy. One candidate gene is apolipoprotein E (apoE), with the E3 allele evolved in the genus Homo that reduces the risks for Alzheimer’s and vascular disease, as well as influencing inflammation, infection, and neuronal growth. Other evolved genes mediate lipid metabolism and host defense. The timing of the evolution of apoE and other candidates for meat‐adaptive genes is discussed in relation to key events in human evolution.

HUNTING DECISIONS IN WILD CHIMPANZEES

While field studies of wild chimpanzees have investigated the proximate determinants of hunting success, little attention has been paid to the decision to hunt. We present evidence from Gombe National Park, Tanzania, showing that the social factors that most strongly influence the decision to hunt red colobus monkeys are the presence of female chimpanzees with anogenital sexual swellings in the foraging party, the number of adult and adolescent males in the party, and the total size of the foraging party. Of these, the presence of one or more swollen females was the best predictor of a decision to undertake hunts of red colobus groups at all but the smalles chimpanzee foraging party size. Two likely explanations for this pattern are discussed. First, swollen females may be a primary influence on male grouping patterns, which in turn promotes hunting. Second, this finding, together with previous research showing that male chimpanzees preferentially give meat to estrous females, suggests that male chimpanzee hunting performance may be under sexual selection. While nutritional and ecological factors may influence chimpanzee hunting patterns at times, chimpanzees appear to hunt red colobus at least partially to obtain meat for use as a social and reproductive tool.

Hunting and nuclear families: some lessons from the Hadza about men's work

Hadza hunter-gatherers display economic and social features usually assumed to indicate the dependence of wives and children on provisioning husbands and fathers. The wives and children of better Hadza hunters have been found to be better-nourished, consistent with the assumption that men hunt to provision their families. Yet, as is common among foragers, the Hadza share meat widely. Analyses of meat-sharing data confirm that little of the meat from large prey went to the hunters own household. These analyses also show that neither a mans hunting success nor the time he spent hunting made any difference in how much meat his family got from the kills of others. Here we address questions posed by this set of observations. What explains the better nutrition of the children of better hunters if they did not get differential rations of meat? If better hunters got no more meat for their effort and poorer hunters were not punished with less, what incentive could account for the continuing disproportionate contribution that some men made to the groups nutrition? If women were not dependent on their husbands hunting success for meat, an obvious incentive for women to marry hunters disappears. We briefly consider the implications of these patterns for the evolution of marriage and nuclear families.

Behavioral Ecology of Sympatric Chimpanzees and Gorillas in Bwindi Impenetrable National Park, Uganda: Diet

Via a field study of chimpanzees (Pan troglodytes schweinfurthii) and gorillas (Gorilla gorilla beringei) in Bwindi Impenetrable National Park, Uganda, we found that their diets are seasonally similar, but diverge during lean seasons. Bwindi chimpanzees fed heavily on fruits of Ficus sp., which were largely ignored by the gorillas. Bwindi gorilla diet was overall more folivorous than chimpanzee diet, but was markedly more frugivorous than that of gorillas in the nearby Virunga Volcanoes. During 4 mo of the year Bwindi gorilla diet included more food species than that of the chimpanzees. Three factors in particular—seasonal consumption of fibrous foods by gorillas, interspecific differences in preferred fruit species, and meat consumption by chimpanzees—contributed to dietary divergence between the two species. When feeding on fruits, gorillas ate Myrianthus holstii more frequently than chimpanzees did, while chimpanzees included more figs in their annual diet. Chimpanzee diet included meat of duikers and monkeys; gorilla frequently consumed decaying wood.

Avoiding Predators: Expectations and Evidence in Primate Antipredator Behavior

Predation and antipredator behavior are important but poorly studied influences on the evolution of primate societies. I review recent evidence of predation and antipredator strategies among primates. I describe patterns of antipredator behavior and attempt to explain the variation among primate taxa and among antipredator strategies. I use predation by chimpanzees on red colobus and antipredator strategies by the colobus as a case study of how a primate prey species may respond to the threat of predation.

Costs and benefits of allomothering in wild capped langurs (Presbytis pileata)

SummaryData from a 15-month field study of the capped langur monkey, Presbytis pileata, in Bangladesh indicate that allomothering behavior is restricted to particular female-infant dyads. Primary allomothers were all parous adult females; nulliparous females rarely allo-mothered, Newborn infants were transferred from the mother to other females within a short time of birth, as reported for some other colobine species, but over the first 3 months of life each neonates contact with nonmothers was largely restricted to a single allomother in each of five study groups. Capped langur mothers with newborn infants spent more time feeding when the infant was being allomothered than when it was in the mothers care. The model of allomothering as a selfish behavior by nulliparous or pregnant females used to enhance maternal skills at the expense of mothers is not supported by this study. Rather, allomothering may have adaptive significance as altruistic behavior among group females, in that it enables lactating females to increase feeding time. Capped langur allomothering is best interpreted as a low-cost behavior that can benefit recipients that may or may not be related.

The diet of the capped langur (Presbytis pileata) in a moist deciduous forest in Bangladesh

Capped langurs (Presbytis pileata) in Madhupur National Park in north-central Bangladesh have an annual diet that is comprised largely of mature leaves (42% of 20,460 total feeding records). Among colobine monkeys, only some populations of red colobus (Procolobus badius) have a diet richer in mature leaves. New leaves (11%) and fruit (24%) are the other annually important dietary items. Seasonal breakdown of this diet, however, revealed that during the monsoon months of May through September theP. pileata diet is approximately 50% fruit, including pulpy ripe fruit. This is also the period of maximum fruit availability. Data on diet and food availability indicate that while capped langurs subsist on mature leaves during the dry season (80% of diet from November to March), they select fruit and new leaves and switch to these foods whenever they are available. These data support the hypothesis that the colobine feeding strategy is adapted to cope with seasonal food scarcity.

Predation and Male Bonds in Primate Societies

[In this paper I consider the effects of predation in relation to other evolutionary influences on the social systems of nonhuman primates, in particular on the evolution of male bonds in multimale primate groups. Because of the difficulty of documenting its infrequent occurrence, predation on wild nonhuman primates has rarely been studied and its effects on behaviour are poorly understood. Male bonds have also been neglected compared to female bonds in studies of primate societies. Of the accepted evolutionary influences on male bonds — the distribution of females, enhanced individual reproductive success that comes from the control of females, predator avoidance, and protection from intergroup aggression by conspecifics — predation is the only one of these that typically ends all future individual lifetime fitness. I review the widely held hypotheses for the formation of male bonds and consider the extent to which predation has been an evolutionary influence on their formation. It appears that male-bonding for the purpose of repelling predators is not necessarily based on the kin-selected benefits often attributed to cooperative behavior among primates. I also discuss predation risk and predation mortality as agents of natural selection on social behavior. Data are presented from a field study on the effect of predation on the behavioural ecology and demography of one male-bonded primate species, the red colobus monkey (Colobus badius). Field data show that the influence of predation on the red colobus social system appears to be mitigated by behavioral tradeoffs. Predation is a demonstrably strong influence on the structure of some primate societies, but its effects vary widely between species. Male bonding is one response to the risk of predation, but does not appear to be based on kin-selected benefits to the participants in most species., In this paper I consider the effects of predation in relation to other evolutionary influences on the social systems of nonhuman primates, in particular on the evolution of male bonds in multimale primate groups. Because of the difficulty of documenting its infrequent occurrence, predation on wild nonhuman primates has rarely been studied and its effects on behaviour are poorly understood. Male bonds have also been neglected compared to female bonds in studies of primate societies. Of the accepted evolutionary influences on male bonds — the distribution of females, enhanced individual reproductive success that comes from the control of females, predator avoidance, and protection from intergroup aggression by conspecifics — predation is the only one of these that typically ends all future individual lifetime fitness. I review the widely held hypotheses for the formation of male bonds and consider the extent to which predation has been an evolutionary influence on their formation. It appears that male-bonding for the purpose of repelling predators is not necessarily based on the kin-selected benefits often attributed to cooperative behavior among primates. I also discuss predation risk and predation mortality as agents of natural selection on social behavior. Data are presented from a field study on the effect of predation on the behavioural ecology and demography of one male-bonded primate species, the red colobus monkey (Colobus badius). Field data show that the influence of predation on the red colobus social system appears to be mitigated by behavioral tradeoffs. Predation is a demonstrably strong influence on the structure of some primate societies, but its effects vary widely between species. Male bonding is one response to the risk of predation, but does not appear to be based on kin-selected benefits to the participants in most species.]