Sex differences in maternal sibling-infant interactions in wild chimpanzees

Abstract

Abstract: Apart from the crucial bonds between mothers and offspring, siblings are an important first social partner in most primate species. Socioecological theory predicts that sibling-infant interactions may differ depending on whether the older immature sibling is male or female. Here, we used 24 years of long-term data from wild chimpanzees in Gombe National Park, Tanzania, to characterize maternal sibling-infant relationships and investigate sex differences therein. Since young female primates typically exhibit higher rates of interest in infants, we hypothesized that older sisters would be more likely than older brothers to groom, play with, and carry infant siblings. Alternatively, due to male-biased philopatry in this species, older brothers may be equally or more likely to interact with these potential future allies. We also examined whether sibling-infant interactions differed according to sibling age and sex of the infant. For both play and grooming, we found a significant interaction between sibling age and sibling sex. Older brothers had increased likelihood of playing and grooming with infant siblings as they themselves aged. Additionally, male-male siblings played significantly more than any other dyad type. Older sisters had decreased likelihood of playing with age and maintained relatively consistent likelihood of grooming; they also spent more time with their infant siblings at later ages. Instances of carrying young infants were exceedingly rare and did not differ by sibling sex. Thus, the sex combination of sibling-infant dyads may have substantial consequences for the social development of both individuals, which we argue is an important focus of future research. Significance statement: The role of siblings in nonhuman primate social development is relatively understudied, and socioecological theory predicts that male and female siblings may interact differently with infants. We conducted the most detailed investigation to date of sibling relationships in wild chimpanzees and found distinct differences in how immature brothers and sisters interact with infant siblings. Older brothers have an increased likelihood of playing and grooming with infant siblings as they themselves age. We also found that male-male siblings played more than all other dyad types, which may have consequences for fostering alliances in adulthood. Older sisters show decreased likelihood of playing with age and maintain relatively consistent likelihood of grooming; they also spend more time with their infant siblings at later ages. We argue that a renewed focus on understanding the importance of siblings on primate social development is warranted.