Mother's age and social integration modulate sex-biased maternal investment in wild spider monkeys (Ateles geoffroyi)

Abstract

In many mammal species, mothers are crucial for the survival and development of young offspring. In primates, maternal investment may ensure immatures' survival and also foster their social integration in the group, providing long-term fitness benefits. In this study, we analysed maternal investment in a wild group (N = 49) of male philopatric spider monkeys (Ateles geoffroyi). We assessed whether maternal investment is biased toward sons (compared with daughters) and which factors modulate this relation. We studied 20 mother-offspring dyads, measuring the time mothers spent in body contact, nursing, carrying, grooming, and playing with their offspring, for a total of 359 focal samples from February to July 2022. We then ran generalized linear mixed models to assess how these responses varied depending on the two-way interactions of offspring sex with offspring age, maternal age, and maternal centrality. Our results showed that mothers were more likely to nurse, carry, and have body contact with younger than older offspring, regardless of their sex. However, we also found that mothers invested more in male than female offspring; differences were mediated by mothers' age and social integration in the group. Older mothers, in particular, were more likely to carry sons than daughters, whereas the contrary was true for younger mothers. Moreover, socially more central mothers were more likely to be in body contact with sons than daughters. Overall, our study shows that some maternal behaviours are sex-biased in male-philopatric species, although maternal experience and social integration may modulate this relation