Abstract: A large body of theoretical and empirical research suggests that kinship influences the development and maintenance of social bonds among group-living female mammals, and that human females may be unusual in the extent to which individuals form differentiated social relationships with nonrelatives. Here we combine behavioral observations of party association, spatial proximity, grooming, and space use with extensive molecular genetic analyses to determine whether female chimpanzees form strong social bonds with unrelated individuals of the same sex. We compare our results with those obtained from male chimpanzees who live in the same community and have been shown to form strong social bonds with each other. We demonstrate that party association is as good a predictor of spatial proximity and grooming in females as it is in males, that the highest party association indices are consistently found between female dyads, that the sexes do not differ in the long-term stability of their party association patterns, and that these results cannot be explained as a by-product of the tendency of females to selectively range in particular areas of the territory. We also show that close kin (i.e. mother-daughter and sibling dyads) are very rare, indicating that the vast majority of female dyads that form strong social bonds are not closely related. Additional analyses reveal that “subgroups” of females, consisting of individuals who frequently associate with one another in similar areas of the territory, do not consist of relatives. This suggests that a passive form of kin-biased dispersal, involving the differential migration of females from neighboring communities into subgroups, was also unlikely to be occurring. These results show that, as in males, kinship plays a limited role in structuring the intrasexual social relationships of female chimpanzees.

Abstract: The use of noninvasive measures of hypothalamic-pituitary-adrenal (HPA) axis function is of growing interest among preclinical and clinical investigators. This report describes a method for the repeated assessment of salivary free cortisol in awake, unrestrained squirrel monkeys (Saimiri sciureus) based on a saliva sampling technique previously developed for rhesus monkeys. Individually housed adult male squirrel monkeys were trained to chew on dental rope attached to a pole, from which saliva was extracted by centrifugation and analyzed for cortisol by radioimmunoassay (RIA). Eight of nine monkeys readily acquired the task, reliably providing adequate saliva samples for the assay. Salivary free cortisol levels were examined in these subjects under basal conditions and in response to two types of neuroendocrine challenge. Levels of salivary free cortisol showed relatively low intra- and interindividual variability, with mean individual morning levels ranging between 17.1 and 37.9 µg/dl. Squirrel monkeys demonstrated a consistent daily rhythm in salivary free cortisol ranging from a high of 27.4 ± 5.2 µg/dl (mean ± SEM) at 12 P.M. to a low of 7.5 ± 1.6 µg/dl at 6 P.M.. Intravenous (IV) challenges with 1 µg/kg ACTH, or 10 and 50 µg/kg CRF resulted in significant increases in salivary free cortisol. The described sampling technique provides a reliable and sensitive means for repeated measurement of HPA activity in unrestrained, awake squirrel monkeys. In addition, our findings illustrate several features of HPA system rhythmicity and reactivity using salivary cortisol instead of blood plasma or serum. Am. J. Primatol. 60:69–75, 2003. © 2003 Wiley-Liss, Inc.

Abstract: It was recently demonstrated that capuchin monkeys notice and respond to distributional inequity, a trait that has been proposed to support the evolution of cooperation in the human species. However, it is unknown how capuchins react to inequitable rewards in an unrestricted cooperative paradigm in which they may freely choose both whether to participate and, within the bounds of their partner's behavior, which reward they will receive for their participation. We tested capuchin monkeys with such a design, using a cooperative barpull, which has been used with great success in the past. Contrary to our expectations, the equity of the reward distribution did not affect success or pulling behavior. However, the behavior of the partner in an unequal situation did affect overall success rates: pairs that had a tendency to alternate which individual received the higher-value food in unequal reward situations were more than twice as successful in obtaining rewards than pairs in which one individual dominated the higher-value food. This ability to equitably distribute rewards in inherently biased cooperative situations has profound implications for activities such as group hunts, in which multiple individuals work together for a single, monopolizable reward.

Abstract: Captive studies can make a unique contribution to primate socioecology by documenting species-typical social dispositions under controlled conditions. Recent theories seek to connect the dominance relationships, group cohesiveness, and feeding ecology of primates. The present study explores the first two aspects by comparing the social organization of rhesus (Macaca mulatta) and stumptail monkeys (M. arctoides). Data were collected over a period of eight years, with five different methods, on three well-established captive groups in identical environments. The groups were found to share one characteristic: a clear-cut, linear formal dominance hierarchy as expressed in teeth-baring displays. The two main study groups (one of each species) differed significantly, however, with respect to nine of eleven behavioral measures. In addition to a previously reported higher frequency of reconciliation in the stumptail group, this group showed (1) more frequent but less severe aggressive behavior, (2) greater symmetry of contests, (3) greater social tolerance, (4) more nonagonistic approaches, and (5) more allogrooming. The differences can be summarized as a contrast in dominance style, with the stumptails having a more relaxed style and placing greater emphasis on social cohesion than the rhesus monkeys. An egalitarian attitude was also reflected in approach behavior: contacts in the rhesus group were mostly initiated by dominants, whereas contacts in the stumptail group were initiated independent of rank. Comparisons with a second rhesus group, and with published reports, suggest that while some of the observed differences are probably representative of the two species, considerable intraspecific variation does exist, and a more comprehensive program of comparative studies is needed.

Abstract: Meeting the psychological needs of chimpanzees (Pan troglodytes) can be a challenge given their aggressiveness on the one hand and the complexity of their social lives on the other. It is unclear how to balance the need to provide opportunities for species-appropriate behavior against potential risks of injury chimpanzees may inflict on each other. This study evaluates the suggestion that simpler social environments protect chimpanzees from wounding. Over a two-year period all visible injuries to 46 adult males, 64 adult females, and 25 immature chimpanzees were recorded at the Yerkes Regional Primate Research Center. Approximately half of the subjects were mother-reared, and the rest were nursery-reared. Housing included compounds containing about 20 chimpanzees, interconnected indoor-outdoor runs for groups of up to 12 individuals, and smaller indoor-outdoor runs for pairs and trios. Annual wounding rates were calculated for serious wounds (extensive injuries and all those requiring veterinary intervention) as well as for minor wounds. Compound-housed chimpanzees incurred the highest level of minor wounding, but serious wounding levels were not affected by housing condition. Even with a period of dominance instability and elevated levels of wounding in one compound, compound chimpanzees were not injured more than those in smaller social groups over the long term. Nursery-reared females in moderate-sized groups were wounded more than mother-reared females. Also, nursery-reared males and females were wounded less often when paired with mother-reared companions. Overall, this study indicates that maintaining chimpanzees in pairs and trios would not be an effective means for reducing injuries. The management of wounding in chimpanzee colonies is influenced more by the sex and rearing composition of a colony.