Abstract

Mothers provide indispensable care for infants in many mammalian species. In some long-lived species, the maternal-offspring bond persists after infancy with mothers continuing to provide resources and social support to their adult progeny. Maternal presence is associated with fitness benefits through adolescence for male chimpanzees despite the fact that mature males dominate females and form their strongest bonds with other males. How mothers support grown sons is unknown, because few studies have examined developmental shifts in mother-son relationships during adolescence and adulthood. We investigated social interactions between 29 adolescent (9–15 years) and young adult male (16–20 years) chimpanzees and their mothers at Ngogo in Kibale National Park, Uganda, over 3 years. All males under 12 years old had their mother as their top grooming and proximity partner, as did one-third of the young adult males. As males grew older, the amount of time they associated with, maintained proximity to, groomed with, and kept track of their mothers while traveling decreased. When males were together in the same party as their mothers, however, young adult males affiliated with their mothers as frequently as did adolescent males, with sons initiating the majority of these interactions. In contrast to adult sons, however, adolescent sons became distressed when separated from mothers and relied on their mothers for agonistic support and reassurance after conflicts. These findings indicate that the chimpanzee maternal-offspring bond continues but changes through adolescence and adulthood, with mothers remaining occasional social companions for most adult sons and frequent companions for some. Mammalian mothers protect and provision their infants, and in some species, mothers provide social support for their adult offspring. The importance of mothers in the lives of adult sons is clear in humans and in one of our closest relatives, bonobos. Here we show mothers are also important social partners for sons throughout adolescence and into young adulthood in chimpanzees. Despite prevailing knowledge that adult males are key alliance partners for male chimpanzees, we demonstrate that male chimpanzees maintain social bonds with their mothers into adulthood. We also suggest that social bonds reflect not only the total time spent together, but what animals do during those times, as many young adults rarely groomed their mothers, but at times when they were in the same subgroup with her, they did so frequently. Future research will assess the physiological, psychological, and reproductive benefits of such mother-son bonds.

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