High early lactational synchrony within baboon groups predicts increased infant mortality

In species where multiple breeding females co-reside in a social group, female-female competition may be particularly acute when many females have dependent young at the same time, with potential negative consequences for offspring survival. Here, we used more than four decades of data on wild baboons (Papio sp.) in Amboseli, Kenya, to examine the effects of early lactational synchrony (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 energetically demanding for mothers and high-risk for infants, we expected both female-female aggression and protection by males to intensify during this period. In support, when early lactational synchrony was high, rates of female-female agonism increased. High-ranking females increased their time associating with adult males, while the time that low-ranking females associated with adult males decreased. Furthermore, high early lactational synchrony strongly predicted infant mortality, even though periods of high synchrony tended to be brief in duration in this nonseasonally breeding population. This association may result from both aggression among adult females and infanticidal behavior by peripubertal females. These findings provide novel evidence that synchronous reproduction alters competitive regimes and compromises reproductive outcomes even in nonseasonal breeders.