Abstract
Social group composition can have fitness implications for group members by determining opportunities for affiliative and competitive interactions. Female-female competition may be particularly acute when many groupmates have young infants at the same time, with potential consequences for infant survival. Here, we used decades of data on wild baboons (Papio sp.) in Amboseli, Kenya, to examine the effects of ‘early lactational synchrony’ (here, the proportion of females in a group with an infant <90 days old) on female-female agonistic interactions and infant survival. Because early lactation is an energetically demanding time for mothers and a risky time for infants, we expected early lactational synchrony to produce intensified competition for food and/or male protectors, resulting in more frequent female-female agonistic interactions and high infant mortality. In support of these predictions, we found that the frequency of female-female agonistic interactions increased with increasing early lactational synchrony. Reproductive state affected this relationship: while females in all states (cycling, pregnant, and postpartum amenorrhea) initiated more agonistic interactions when early lactational synchrony was high, only females in postpartum amenorrhea (including, but not limited to, females in early lactation) received more agonistic interactions. Furthermore, while high early lactational synchrony was rare, it strongly predicted infant mortality. This association may result from both aggression among adult females and infanticidal behavior by peripubertal females. These findings provide novel evidence that social dynamics may shape reproductive phenology in a nonseasonal breeder. Specifically, both competition among reproductive females and harassment from nonreproductive females may select against synchronous reproduction.
Significance Statement Competition between females is a powerful force shaping the behavior and fitness of wild animals, but how it is exacerbated (or mitigated) by the overall composition of social groups is understudied. We asked if female-female competition is enhanced when more females in baboon social groups have young infants, a time that is particularly energetically demanding for mothers and particularly dangerous for their offspring. In these settings, we found that both adult female competitive behavior and infant mortality risk increased. Strikingly, almost all recorded cases of infanticide by adolescent, nulliparous females in our study population occurred on days when many mothers had young infants. These results highlight how social interactions may shape the timing of reproduction in wild social mammals.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Competing Interest Statement: The authors declare no competing interest.
