Age and sex differences in juvenile bonobos in party associations with their mothers at Wamba.

Mothers are the most fundamental individuals for the survival and development of their immature offspring. Sex-biased dispersal has an effect on strong and long-lasting social bonding between mothers and their philopatric offspring. Nepotistic relationships are often seen in species with male-biased dispersal but less frequently in species with female-biased dispersal. Bonobos are unique among female-biased dispersal primates in that the females are codominant with males, exert influence on the mating opportunities of their mature sons, and tolerate each other regardless of relatedness. Few studies on bonobos have examined sex differences in social relationships between juveniles and their mothers, and how this relates to mother-son and female-female cooperative relationships amongst adults. We investigated whether the association index of juveniles with their mothers changed with age depending on the sex of the juveniles in parties of bonobos at Wamba, the Democratic Republic of the Congo. We found that female bonobos had decreased party associations with their mothers prior to their emigration from the natal group, while male bonobos maintained relatively high party associations with their mothers. These results indicate that sex differences that emerge during the juvenile period in bonobos in their spatial relationships with their mothers differ from previous findings on chimpanzees, spider monkeys, and muriquis, in which philopatric sons tend to spend less time in proximity to their mothers and disperse earlier than, or at a similar age to, daughters. The contrasts between bonobos and other primate species suggest that the high social status of female bonobos relative to that of males may facilitate the extension of maternal support for their philopatric sons, and that high tolerance among unrelated females may enable the spatial independence of daughters from their mothers at an earlier stage.