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

Area of Science:

• Primate Behavioral Ecology
• Sexual Conflict and Reproductive Strategies
• Mammalian Conservation Biology

Background:

• Infanticide by males is a significant sexual conflict impacting female reproductive success.
• The Bruce effect, a form of reproductive inhibition, is a female strategy to avoid infanticide by terminating pregnancy upon exposure to unfamiliar males.
• A model by Zipple (2020) predicts the Bruce effect evolves when infanticide risk is high and male takeovers are infrequent.

Purpose of the Study:

• To test the Zipple (2020) model's prediction of the Bruce effect in wild white-faced capuchins (Cebus imitator).
• To investigate female reproductive responses to alpha male replacements in a wild primate population.
• To understand the evolutionary constraints on female reproductive strategies in response to sexual conflict.

Main Methods:

• Utilized 38 years of demographic data from a wild white-faced capuchin population in Costa Rica.
• Analyzed infanticide rates and female birth rates in relation to alpha male replacements.
• Applied the Zipple (2020) model parameters to the capuchin population data.

Main Results:

• The white-faced capuchin population experiences frequent alpha male replacements (average 2.9 years) and high rates of infanticide.
• Despite the model predicting the Bruce effect, no evidence of male-mediated prenatal loss was found.
• Female birth rates did not decrease following alpha male replacements compared to stable periods.

Conclusions:

• White-faced capuchins may not employ the Bruce effect as a strategy against infanticide.
• Alternative female strategies, such as allonursing, might mitigate infanticide costs.
• The evolution of female reproductive strategies may be influenced by phylogenetic constraints, not solely by current social and demographic patterns.