Thursday, March 8, 2012

Bruce Effect Reported in Wild Gelada Baboons

a gelada baboon
Roberts et al. report this week in Science (subscription required) that the Bruce effect occurs in wild gelada baboons. The Bruce effect is a phenomenon where females spontaneously abort pregnancies after a new male takes over a group. This has previously been observed under experimental conditions in rodents and horses. The Roberts et al. paper provides convincing evidence of this effect by examining long term demographic data from geladas. In case the demographic data isn't compelling enough for you, they also used fecal hormone data to demonstrate that females were in fact pregnant and that they were in fact terminating pregnancies after male takeovers. Further, they demonstrate that females which terminated pregnancies lowered their inter-birth interval compared to females who brought the fetus to term and later lost the infant to infanticide.

Roberts et al., 2012 Fig 1
The reason all this is important to primatology is that it is highly relevant to debates over whether or not infanticide is an adaptive strategy in primates. This data may suggest that infanticide is common enough and significant enough that female geladas have responded by adaptively evolving the proximate mechanisms to terminate pregnancies in order to avoid infanticide and thereby produce another offspring sooner. Alternatively, the pregnancy terminations could conceivably be due to elevated stress levels associated with male takeovers, in which case they aren't adaptations at all.  The authors argue that their data demonstrates that the Bruce effect can be advantageous for females, but they acknowledge the need to collect more and different types of data to demonstrate the underlying physiological mechanism at work, which is key to understanding what this new data tells us about infanticide. 

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