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Male dominance hierarchies occur in many group-living primates and some non-primate mammals. Variation in aspects of agonistic relationships such as how many dyads show bidirectionality in aggression leads to variation in dominance hierarchies along a continuum from egalitarian (relatively small agonistic power differences between adjacently-ranked individuals, shallow hierarchies) to despotic (relatively large differences, steep hierarchies). Ranks usually depend mostly or entirely on individual characteristics that influence fighting ability (e.g., body size) and show inverse-U shaped relationships to age. However, coalitionary support sometimes also influences ranks. Chimpanzees (Pan troglodytes) form multi-male, multi-female communities within which males compete for status. Males typically form dominance hierarchies, and data from multiple study show that rank is positively related to paternity success. Males also often form coalitions and some dyads form long-term alliances. Effective coalitionary support can help individuals improve and maintain their ranks, and some evidence supports the hypothesis that coalitionary aggression generally, and the positions that males hold in coalitions networks specifically, influences paternity success. Hierarchy steepness varies among communities and within communities over time; variation in the number of prime-aged males per community is a likely source of this variation. Long-term data from an extremely large chimpanzee community with unusually many males, at Ngogo, Kibale National Park, Uganda, are largely consistent with previous analyses of male chimpanzee dominance hierarchies, but show several notable contrasts. Males at Ngogo formed significantly linear hierarchies and hierarchy steepness was greater than expected if the outcomes of agonistic interactions were random. However, variation in steepness did not show the significant inverse relationship to the number of "prime-aged" males documented for other chimpanzee communities and average steepness was high given the large number of males. Ranks showed an inverse-U shaped relationship to age, although individual rank trajectories varied considerably, but males attained their highest lifetime ranks at later ages and maintained relatively high ranks to later ages than those at other chimpanzee research sites. Two measures of coalition networks, strength and Bonacich power, showed significant positive relationships with male ranks. Strength is the rate at which males joined coalitions. Bonacich power is a measure of network centrality that assesses a male's relational power, or influence (Bonacich, 1987): a male with high Bonacich power formed coalitions with relatively many other males who were also central in the coalition network, i.e., he was strongly connected to powerful others. On average, males also attained maximum values for these and other network measures relatively late and maintained relatively high values to relatively late ages. High coalition network strength, Bonacich power, and eigenvector centrality early in adulthood were associated with high peak ranks at later ages. However, the direction of causality between participation in coalition networks and ranks is not yet clear, and the effects of body size on dominance ranks and individual rank trajectories remains to be explored. Ngogo is a favourable habitat for chimpanzees and survivorship there is unusually high; this presumably facilitates the ability of males to maintain high competitive ability longer than at other sites and shifts rank trajectories toward older ages and leads to relatively steep hierarchies despite the fact that many male dyads have similar competitive ability. Future work will assess the impact of coalitions on dominance relationships in more detail and the relationship of coalitionary aggression to paternity success. [ABSTRACT FROM AUTHOR]

Reconciliation, or peaceful postconflict interaction, can restore the usual pattern of interaction between social partners after open conflict has disrupted it—i.e., it can resolve conflicts. Researchers have documented reconciliation in >20 primate species, but the tendency to reconcile typically varies among dyads and dyad classes. The valuable-relationships hypothesis proposes that differences in the value of social relationships account for much of this variation. Value depends on how likely partners are to act in ways that benefit each other, where the benefits are ultimately direct or indirect increases in fitness. Researchers have responded to studies that have tested predictions of the hypothesis with extensive, if not universal, support. For example, kin show higher conciliatory tendencies than nonkin in many species, and conciliatory tendencies between unrelated females are high in several cercopithecines in which nonkin agonistic support is important for rank acquisition and maintenance. However, most of the support is indirect, because we lack direct evidence on the link between assays of relationship value and fitness. Also, some studies have methodological weaknesses, e.g., analyses based on pooled data and insufficient sample sizes. I review evidence in favor of the hypothesis with special attention to studies that come closest to providing evidence for predicted fitness effects. I also present new data on postconflict interactions between adult male chimpanzees at Ngogo that show how often pairs of males formed coalitions and how much time they spent grooming influenced the likelihood that they would reconcile after conflicts, and that allies were particularly likely to reconcile and to do so by grooming each other. The most important future research direction is to integrate detailed data on conflict management, analyzed at the level of dyads, with long-term data on reproductive success, such as that now available from several study sites, on the same populations. [ABSTRACT FROM AUTHOR]

Chimpanzees (Pan troglodytes) have hostile intergroup relations throughout most or all of their geographic range. Hostilities include aggressive encounters between members of neighboring communities during foraging and during patrols in which members of one community search for neighbors near territory boundaries. Attacks on neighbors involve coalitions of adult males, and are sometimes fatal. Targets include members of all age/sex classes, but the risk of lethal intergroup coalitionary aggression is highest for adult males and infants, and lowest for sexually swollen females. The best-supported adaptive explanation for such behavior is that fission-fusion sociality allows opportunities for low-cost attacks that, when successful, enhance the food supply for members of the attackers' community, improve survivorship, and increase female fertility. We add to the database on intergroup coalitionary aggression in chimpanzees by describing three fatal attacks on adult males, plus a fourth attack on an adult male and an attack on a juvenile that were almost certainly fatal. Observers saw four of these attacks and inferred the fifth from forensic and behavioral evidence. The attackers were males in two habituated, unprovisioned communities (Ngogo and Kanyawara) in Kibale National Park, Uganda. We also summarize data on other intercommunity attacks at Ngogo. Our observations are consistent with the “imbalance of power” hypothesis [Manson & Wrangham, Current Anthropology 32:369–390, 1991] and support the argument that lethal coalitionary intergroup aggression by male chimpanzees is part of an evolved behavioral strategy. Am. J. Primatol. 68:161-180, 2006. © 2006 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Social relationships in nonhuman primates result from investments that individuals make while pursuing fitness-maximizing strategies. These strategies sometimes include social exchange, either reciprocity (exchange of the same acts) or interchange (exchange of different acts). Individuals in many species may negotiate for services in biological markets, particularly grooming and agonistic support. They also may compete for access to valuable social partners. Abundant evidence for reciprocity in grooming and in support and for competition over partners exists, notably for females in some cercopithecines. However, evidence for interchange of grooming and support is scarcer, and apparent interchange may be a byproduct of correlations between grooming or support and some third variable (e.g. dominance rank). Chimpanzees have been prominent in discussions of social exchange, especially because male chimpanzees cooperate in many ways. Most analyses of interchange have used data on captive chimpanzees; these provide good evidence for reciprocity, but ambiguity with regard to interchange. Data on an unusually large chimpanzee community at Ngogo, Kibale National Park, Uganda, strongly support the argument that social exchange is prominent in social relationships among males. Males at Ngogo show reciprocity in grooming and support. They also interchange grooming given and support received, as well as grooming received and support given, independently of reciprocity in grooming and support and of correlations of support and grooming with dominance rank. However, most cooperation in contests with third parties took low risk forms (e.g. both participants outranked their opponent). In this, males at Ngogo resemble captive chimpanzees and female cercopithecines. Reciprocity and interchange in this context may be important in the maintenance of social bonds between males, and in attainment and maintenance of high dominance rank, but probably represent mutualism, not reciprocal altruism. [ABSTRACT FROM AUTHOR]

Wild chimpanzees form temporary parties that vary in size and composition. Previous studies have revealed considerable intraspecific variation in party compositions. We examined patterns of association among age, sex, and reproductive classes of chimpanzees at Ngogo in the Kibale National Park, Uganda. We employed a class-based association index and a randomization procedure to control for confounding factors and to test for differences between classes. Results indicate that males associated with other males significantly more than expected if all classes behaved equivalently, while females generally associated with individuals of the same sex less than expected. To interpret these patterns we used two additional indices that separate associations into two components: general gregariousness and preference for particular classes of associates. Males and estrous females were more gregarious than other classes, while anestrous females were less so. After controlling for general gregariousness, adult males as a class showed no specific preference for associating with each other. Anestrous females preferred each other as party members, and estrous females avoided each other. These results are consistent with previous findings that adult males are more gregarious than females. They diverge from the standard picture of chimpanzee society, however, by suggesting a mutual affinity among anestrous females, but not among adult males as a class. [ABSTRACT FROM AUTHOR]

Female transfer is common in mountain gorillas, but most adult females reside with female relatives for at least some of their lives. In four mountain gorilla social groups, co-resident relatives had higher rates of affiliative interaction and lower rates of aggressive interaction, and were more tolerant of each other, than non-relatives. These differences were greater for maternal than for paternal relatives. Non-relatives typically had few affiliative interactions, but some (mostly but not entirely long-term co-residents) had friendly relationships. Females showed reciprocity in affiliative behavior. Long-term residents in two groups maintained less tolerant relationships with more recent immigrants than with each other and than did most pairs of immigrants. The effects of relatedness and residence status on relationships may sometimes be complementary because both factors can lead to similar reproductive interests. A potential exists for some resemblance between female-female social relationships in mountain gorillas and those in female-bonded primate species. However, even female relatives usually spend little time interacting socially with each other, and the benefits of relationships with male outweigh those of relationships with other females. [ABSTRACT FROM AUTHOR]

Data on the time budgets of mountain gorillas (Gorilla gorilla beringei) were collected during field studies in the Virunga Volcanoes region of Rwanda and Zaire. Focal sampling was used to determine the proportion of time that individuals of different age/sex classes spent in several mutually exclusive activity states. The gorillas spent the majority of daylight hours feeding; most of the rest of the day was devoted to resting, with little time spent moving or engaged in social activity. The time budget varied among the different subhabitats used by the gorillas, and the gorillas satisfied subsistence needs more quickly when in areas where food was more abundant and/or of better nutritional quality. Silverbacks spent more time feeding than all other age/sex classes, but age/sex class differences were not great. All age/sex classes responded to variability in habitat quality in similar fashion. Unlike the case for many other primates, there was no significant seasonal variation in time budgets: There was a direct relationship between group size and time spent feeding, although variation in relation to group size was lower than that in relation to variation in habitat quality. These results are consistent with the relationship of feeding time to body size in primates. They are also consistent with other evidence that social foraging entails a cost to gorilla females, but that this cost is low in comparison to those faced by many other primates. Permanent association with males apparently offers litle ecological disadvantage to females, who are likely to be more than compensated by mutualistic benefits. [ABSTRACT FROM AUTHOR]

This report describes observations on the ontogeny of food choice in mountain gorillas (Gorilla gorilla beringei), made during a 17-month field study of mountain gorilla feeding ecology in the Pare National des Volcans, Rwanda. Data are presented on the feeding behavior of two infants observed from birth to the age of 8 months and on older infants and juveniles. This information is compared with data on the composition and diversity of the diets of young adults and adults in the same social group. Initial feeding by infants is usually synchronized with the mother's behavior: infants ingest the same food, or a different part of the same plant species, currently being eaten by the mother or just eaten by her. This suggests that observational learning is largely responsible for the transmission of food preferences. Most feeding by young infants, whether or not synchronized with the mother's, is on those foods eaten most frequently by adults. Infants also independently sample potential foods, some of which are apparently not consumed by adults. The frequency of sampling declines with age, although even adults occasionally ingest foods not observed to be eaten by other adults. By the age of 3 years, young mountain gorillas have developed the basic dietary patterns of adults, in terms of the number of foods eaten, the proportions in which specific foods are consumed, and diet diversity and equitability. There is a strong possibility that chemical cues influence food choice, but their role remains unclear. [ABSTRACT FROM AUTHOR]

Data are presented here on the feeding ecology of wild mountain gorillas (Gorilla gorilla beringei) which were observed for approximately 2,400 hours over a 17-month period in the Parc National des Volcans, Rwanda. Aspects of the relationship between the composition, diversity, and temporal consistency of the gorillas' diet and spatial and temporal variability in food distribution patterns are described. Mountain gorillas are folivores that show considerable specialization on plant parts, species and families. This pattern is facilitated by the general richness of their habitat. Their environment is heterogeneous, and spatial variability in food distribution is more pronounced than temporal variability. The gorillas rely almost completely on perennially available foliage of herbs and vines. Their diet varies little in association with seasonal factors but varies markedly in space in association with variability in the vegetational composition of the habitat. Individuals in the main study group shared basically similar patterns of food choice. Different groups also shared a similar general pattern, although there were differences in detail that apparently resulted largely from vegetational contrasts between home ranges. The gorillas' behavioral responses to environmental complexity lend general support to recent ideas concerning the evolution of their social system. [ABSTRACT FROM AUTHOR]