Is there evidence for the Bruce effect in white-faced capuchins?

Infanticide by males is an extreme form of sexual conflict that can increase male reproductive success at a cost to female reproductive success. Females have evolved a variety of strategies to reduce the occurrence and the cost of infanticide, including the termination of pregnancy after nontraumatic exposure to non-sire males, known as the Bruce effect. A recent model (Zipple 2020) proposed that the Bruce effect will evolve in populations if the risk of infanticide is high and alpha male replacements occur routinely but infrequently relative to gestation length. We tested this model using 38 years of demographic data from a population of wild white-faced capuchins (Cebus imitator) in Sector Santa Rosa of the Área de Conservación Guanacaste, Costa Rica. We found that this population has a high rate of infanticide associated with alpha male replacements that occur every 2.9 years on average. Applying the Zipple (2020) model to our population leads to the prediction that capuchins should exhibit the Bruce effect, as the associated reproductive costs would be lower than the expected costs of future infanticide. However, we did not find evidence of any type of male-mediated prenatal loss in this species: female birth rates after alpha male replacements were not lower compared to stable periods. Possible explanations include that white-faced capuchins do not respond to extreme events with reproductive inhibition, or that other female strategies such as allonursing better mitigate the costs of infanticide. Finally, the evolution of female reproductive strategies may not be labile enough that it can be predicted from population-specific social and demographic patterns without regard to phylogenetic constraints. More studies are needed to understand the conditions that determine the occurrence and evolution of the Bruce effect in wild mammalian populations.