For over 40 years, the sexual selection hypothesis of infanticide has sparked primatological interest in the evolutionary basis of this presumptively adaptive behavior. Infanticide by males has been largely accepted as a reproductive strategy in that when a new dominant sire kills unweaned infants, lactational amenorrhea ends, and females conceive more rapidly than if their infants had survived to weaning age. Thus, infanticide has been mainly investigated from the male perspective. Sexual conflict theory, which posits that males and females of the same species typically have different reproductive interests that can reduce fitness in the other sex, in general has focused on taxa with shorter life spans. A detailed study of sexual conflict in primates provides the opportunity to advance knowledge in organisms with long lifespans and complex behavior. I investigate several facets of how infanticide affects white-faced capuchins (Cebus imitator), a medium-sized platyrrhine with relatively slow life histories that experiences alpha male replacements (AMRs; change in dominant sire of group) accompanied by infanticide in the Sector Santa Rosa, Área de Conservación Guanacaste, Costa Rica. In my investigation, I found that AMR type does not predict infant survival but that infant age does, with younger infants having a greater risk of mortality. I also found that infants likely have the physiological ability to survive earlier weaning, but they do not appear to be able to wean early enough to decrease the risk of infanticide. My comparative analyses indicated that white-faced capuchins have longer lactation periods than expected based on body size. Allonursing also has the potential to function as a female counterstrategy to infanticide by enabling a faster resumption of cycling to decrease the risk of infanticide and/or decreasing the energy input that each mother makes into her young infants thereby decreasing the costs of infanticide. Finally, I found that AMRs increase reproductive synchrony and shift birth seasonality but that this shift in seasonality does not affect infant survival. Combined, my findings indicate that infanticide has a broader range of effects in white-faced capuchins than previously known and contributes to a greater understanding of sexual conflict theory.
