Mother-infant vocal interactions play a crucial role in the development of human language. However, comparatively little is known about the maternal role during vocal development in nonhuman primates. Here, we report the first evidence of mother-daughter vocal interactions contributing to vocal development in gibbons, a singing and monogamous ape species. Gibbons are well known for their species-specific duets sung between mates, yet little is known about the role of intergenerational duets in gibbon song development. We observed singing interactions between free-ranging mothers and their sub-adult daughters prior to emigration. Daughters sang simultaneously with their mothers at different rates. First, we observed significant acoustic variation between daughters. Co-singing rates between mother and daughter were negatively correlated with the temporal precision of the song’s synchronization. In addition, songs of daughters who co-sang less with their mothers were acoustically more similar to the maternal song than any other adult female’s song. All variables have been reported to be influenced by social relationships of pairs. Therefore those correlations would be mediated by mother-daughter social relationship, which would be modifiable in daughter’s development. Here we hypothesized that daughters who co-sing less often, well-synchronize, and converge acoustically with the maternal acoustic pattern would be at a more advanced stage of social independence in sub-adult females prior to emigration. Second, we observed acoustic matching between mothers and daughters when co-singing, suggesting short-term vocal flexibility. Third, we found that mothers adjusted songs to a more stereotyped pattern when co-singing than when singing alone. This vocal adjustment was stronger for mothers with daughters who co-sang less. These results indicate the presence of socially mediated vocal flexibility in gibbon sub-adults and adults, and that mother-daughter co-singing interactions may enhance vocal development. More comparative work, notably longitudinal and experimental, is now needed to clarify maternal roles during song development.

The question of the flexibility of nonhuman primate vocal communication remains open today, especially due to early evidence of innately guided vocal production. However, socially determined flexibility can be found when the debate is moved from vocal structure to vocal usage. While increasing evidence shows that the audience quality influences the vocal behaviour of nonhuman primates, the impact of the caller's characteristics has been far less studied. Here, we tested the influence of an individual's sex and age on the usage style of contact calls. We recorded contact calls of male and female Japanese macaques and compared the vocal usage styles of approximately 1-year-old juveniles with those of adults at various ages. We found, first, important differences in call usage style between juveniles and adults, the latter forming temporally ruled vocal exchanges respecting an interindividual turntaking principle. Moreover, sex differences were substantial in adults but nonexistent in juveniles. Finally, age continued to influence female vocal behaviour during adulthood, whereas dominance rank explained differences between adult males. Two nonexclusive mechanisms can explain this phenomenon, that is, a socially guided development of the appropriate form of calling versus an emotional maturation to control call emission, opening new lines of research on nonhuman primate vocal development of appropriate usages.

We studied long-tailed macaque (Macaca fascicularis) populations in Padang, West Sumatra, Indonesia, focusing on the effect of human provisioning on their demography and dietary composition. We conducted a field survey at three sites in the city: Gunung Meru, Gunung Padang, and Gunung Panggilun. Mean troop size (range 28-68) and infant ratio (range 0.38-1.00) were greater in Gunung Meru, where the macaques have been highly provisioned, than at the other two study sites (troop size 10-15; infant ratio 0.00-0.33). The macaques at all sites consumed both natural and human foods, but dependence on the latter differed among sites: three-quarters of the diet of macaques in Gunung Meru consisted of human foods, while human foods comprised less than 5% of the macaque diet at the other sites. The ability of macaques to modify the proportion of human food is a behavioral flexibility that facilitates the survival of the long-tailed macaque in urban habitats. Without restrictions on provisioning, the degree of dependence of macaques on human foods and population size could increase, especially in Gunung Meru, and human-macaque conflict could escalate. In order to create an effective management policy for urbanized monkeys, long-term quantitative data on macaque behavior and monitoring of population parameters are required.