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The adult female rhesus macaque, Macaca mulatta, shows an ability for vocal recognition of both individuals and kin. Playback experiments show that single-trial playbacks of a matrilineal relative lead to significantly faster and longer responses as compared to calls of non-kin. Females habituated to successive presentations of various exemplars of one matrilineal relative's contact calls show a significant rebound in response to contact calls from a second matrilineal relative. The response to playbacks suggests a categorical recognition of matrilineal kinship.

Encounters and associations between adult male, adult female and subadult male orang-utans, Pongo pygmaeus, were examined at the Kutai National Park and Gunung Palung Nature Reserve, Indonesia. Observed rates of encounters, durations of associations and proportions of time spent in associations of two members of each age-sex class from each population were compared with expected values generated from null models of associations. Encounter rates, durations of associations and proportions of time spent in associations were greater than expected by chance. Two factors appear to generate these non-random patterns: subjects at both sites met conspecifics at food sources more often than expected, and associations frequently involved social behaviour. Three differences in association patterns existed between populations. Orang-utans at Kutai encountered each other more frequently, engaged in longer associations, and spent a greater proportion of time together than conspecifics at Gunung Palung. Variations in association patterns between populations reflected age-sex class differences in behaviour. In contrast to adult and subadult males at Kutai, males at Gunung Palung avoided conspecifics. Similarly, adult females and subadult males at Gunung Palung spent less time with others than did their Kutai counterparts. An analysis of the nature of associations revealed two additional behavioural differences between populations. Mating did not occur at Gunung Palung, but was observed frequently at Kutai, and subadult males at Kutai received aggression, while those at Gunung Palung did not. These data support the hypothesis that variations in the reproductive states of adult females account for differences in the patterns of association between orang-utans. Sociality among orang-utans appears primarily the result of aggregation at common resources, either food or mates.