Abstract

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 1m)
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.