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A Bruce Effect in Wild Geladas

Avoiding Infanticide In male dominated hierarchies, newly dominant males will sometimes kill resident infants. In lab studies in mice conducted in the 1950s, Hilda Bruce showed that females introduced to an unfamiliar male will terminate their pregnancies, a process subsequently referred to as a Bruce Effect. Roberts et al. (p. 1222, published online 23 February) followed multiple dominance transitions within wild gelada baboons and showed that live birthrate among females previously identified as pregnant within unstable groups was much lower than within stable groups. Furthermore, females that terminated their pregnancies following transitions had a much shorter interbirth interval than those that did not, suggesting a higher overall reproductive success and fitness. Long-term field studies show female monkeys improve their fitness by terminating their pregnancies when a new male becomes dominant. Female rodents are known to terminate pregnancies after exposure to unfamiliar males (“Bruce effect”). Although laboratory support abounds, direct evidence for a Bruce effect under natural conditions is lacking. Here, we report a strong Bruce effect in a wild primate, the gelada (Theropithecus gelada). Female geladas terminate 80% of pregnancies in the weeks after a dominant male is replaced. Further, data on interbirth intervals suggest that pregnancy termination offers fitness benefits for females whose offspring would otherwise be susceptible to infanticide. Taken together, data support the hypothesis that the Bruce effect can be an adaptive strategy for females.