Mothers letting go: postnatal maternal investment shapes sex-specific social development in wild vervet monkeys

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Abstract
Summary: Sex differences in behaviour and life-history trajectories are widespread across species, yet the mechanisms through which mothers shape these differences remain poorly understood. Classic theories emphasize sex-biased allocation at birth or differential energetic investment, but how maternal effects might operate instead through postnatal investment remains understudied. Using long-term demographic and behavioural data from female-philopatric vervet monkeys ( Chlorocebus pygerythrus ), we examined how maternal age and dominance rank influence offspring sex ratios at birth, survival to adulthood, maternal investment, and offspring social integration in both sons and daughters. Maternal age, but not rank, influenced offspring sex ratios, with older females producing more daughters. Maternal rank was positively associated with daughters’ survival and social engagement, with estimated effects consistently stronger in daughters than in sons. While both sexes were highly vulnerable to maternal loss, post-hoc trends suggested a potentially steeper effect on sons. Sons received more maternal proximity (under some maternal conditions) and maternal grooming, whereas daughters seemed to gain earlier and greater engagement with other group members and appeared to derive indirect advantages from maternal rank through social exposure. Together, these findings indicate that maternal investment in this species differs in form rather than in magnitude, primarily through postnatal developmental pathways rather than biased allocation at birth. By demonstrating how maternal age and social status shape divergent early-life trajectories, our study highlights the role of early social environments in generating sex-specific life histories. Highlights: Sex differences in life-history trajectories can arise through postnatal social development, not only sex allocation at birth; In wild female-philopatric vervets, maternal age (not rank) predicts offspring sex ratios, with older females producing more daughters; Maternal rank shows stronger associations with daughters’ survival and social engagement, while sons were potentially more dependent on maternal presence; Maternal investment differs in form rather than magnitude, consistent with role-specific developmental preparation.

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