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A novel approach to studying social relationships in captive adult chimpanzees (Pan troglodytes) was taken by using principal components analysis (PCA) to extract three key components of relationship quality from nine behavioural variables. Based on the loadings of the behavioural variables, the components appeared to match previously hypothesized critical aspects of social relationships and were therefore labelled Value, Compatibility and Security. The effects of kinship, sex combination, age difference and time spent together on each of the relationship quality components were analysed. As expected, kin were found to have more valuable, compatible and secure relationships than non‐kin. Female–female dyads were found to be more compatible than male–male or mixed‐sex dyads, whereas the latter were found to be most secure. Partners of a similar age were found to have more secure and more valuable relationships than those with a larger age gap. Individuals that were together in the group for longer were more valuable and more compatible, but their relationships were found to be less secure than individuals that were together in the group for a shorter time. Although some of the results may be unexpected based on chimpanzee socio‐ecology, they fit well overall with the history and social dynamics of the study group. The methods used confer a significant advantage in producing quantitative composite measures of each component of relationship quality, obtained in an objective manner. These findings therefore promote the use of such measures in future studies requiring an assessment of the qualities of dyadic social relationships.

The hypothesis that allogrooming functions as a tension-reduction mechanism was tested. Tension was measured by the frequency of displacement activities by an animal. Two adult male and 11 adult female Java (or long-tailed) monkeys (Macuca fusciculuris) that were relatively unfamiliar with each other were paired 1 hour per day for five consecutive days during the periovulatory portion of the menstrual cycle. Female allogrooming was found to reduce the frequency of male displacement activities both during the course of interaction and outside it, and this decrease was proportional to the amount of allogrooming received. Female allogrooming did not, however, exert long-lasting effects on the frequency of female displacement activities. An increase in the frequency of male displacement activities was recorded during the 10-second interval immediately after the end of the female allogrooming bouts. Neither postinhibitory rebound nor frustration owing to the cessation of a pleasant situation, i.e., the two advanced explanatory hypotheses, accounted for this increase. The results of the present study concur with physiological findings that support the tension-reduction hypothesis. The social function of allogrooming appears quite important and is entirely compatible with the functional hypothesis that emphasizes hygiene.

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Social primates spend a significant proportion of their time exchanging grooming with their group companions. Although grooming is mainly exchanged in kind, given its hygienic and tension-reducing functions, it is still debated whether grooming also provides some social benefits, such as preferential access to resources (e.g., food or mating partners). In this study we analyzed grooming distribution among wild female Japanese macaques living in two groups on Yakushima. We tested the tendency of monkeys to reciprocate the amount of grooming received, and to direct their grooming up the hierarchy. Then we analyzed the relation of grooming to three of its possible benefits: reduced aggression, increased tolerance over food, and agonistic support against a male aggressor. The data were analyzed by means of row-wise matrix correlations. Grooming was highly reciprocated (i.e., exchanged in kind) and directed up the hierarchy in both the study groups. No significant relationship was found between grooming and aggression. Conversely, grooming favored tolerance over food, since it was positively correlated with presence on the same food patch, close proximity, and close approaches (both within 1 m) during feeding. Grooming was also positively related to agonistic support against adult males, although this relationship became nonsignificant when we controlled for kinship. Although these results are not definitive, they suggest that monkeys may derive various social benefits from grooming. This conclusion is supported by the fact that in various primate species animals tend to prefer high-ranking individuals as grooming partners.