Simple SummaryIn primates, maternal investment is crucial for offspring’s social development, but the benefits it provides may differ depending on their sex. Here, we studied whether female and male immatures in male-philopatric Geoffroy’s spider monkeys (Ateles geoffroyi) differ in their social behaviour, and whether they receive different benefits from maternal investment. Our results showed no sex differences in the social development of offspring with regards to body contact and grooming, as both behaviours followed a similar pattern in both sexes during their first six years of life. However, we found sex differences in patterns of social play: the probability of playing with other group members was rather constant throughout age for females, whereas, for males, it became higher than females around two years of age, peaking between three and four years of age. Moreover, there were differences in how mothers fostered social interactions with other group members in female and male offspring: in sons, higher maternal investment was linked to a higher probability of playing with other group members, but this link was not found in daughters. Overall, mothers appear to play a critical role in the social development of immature spider monkeys by fostering the abilities that their offspring will need as adults.AbstractIn several species, individuals form long-lasting social relationships with other group members, which provide them with important fitness benefits. In primates, patterns of social relationships are known to differ between sexes, but little is known about how these differences emerge through development or the role that mothers might have in this process. Here, we investigated how sex differences in social behaviour emerge during the first six years of primate life and how sex-biased maternal investment can foster immatures’ social development and social interaction with other group members. For this purpose, we observed 20 males and females aged between zero and six years in a wild group of spider monkeys (Ateles geoffroyi) that was male-philopatric and, therefore, expected to show sex-biased maternal investment. Our results showed no sex difference in the social development of offspring with regards to body contact and grooming, but the probability of play was rather constant throughout age for females, whereas, for males, it became higher than females around two years of age, peaking between three and four years of age. Moreover, we found differences between female and male immatures in the importance of maternal investment (which included the time mothers spent nursing, carrying, grooming, touching and playing with their offspring) for their social integration in the natal group. In particular, maternal investment increased the probability of playing with other group members for sons, but not for daughters. Our findings suggest that mothers, through sex-biased maternal investment, might have a crucial function in the social development of spider monkeys, fostering the abilities that young offspring need to thrive as adults. By shedding light on maternal investment and social development in a still understudied primate species, these findings contribute to understanding the evolutionary roots of human maternal care and social development.