Abortions and contraception have been all over the news lately, so this news from the primate world can serve to remind us that our biology and social arrangements are not the only ones in the animal world, even with our relatively close relatives.
Pregnant gelada monkeys abort when new male enters group
The "Bruce effect" – in which pregnant females spontaneously miscarry after being exposed to an unfamiliar male – has been found repeatedly in laboratory rodents. However, no conclusive evidence for this effect had ever been demonstrated in a wild population prior to this study. Geladas are Old World monkeys that are close relatives of baboons. The findings appear in the new issue of Science Express. The U-M research examined five years of data collected from a wild population living in the Simien Mountains National Park in Ethiopia.
Geladas live in family groups with one dominant male and up to a dozen related females. The dominant male gets to mate with his females as long as he remains dominant. However, there are many "bachelor" males that are just waiting for their chance to oust him. If a bachelor is successful, he gains reproductive access to all of the group's females and often kills any dependent offspring sired by his predecessor. Predecessor males are allowed to stay on in the group as subordinates.
By killing infants, the new male hastens the return to fertility for these otherwise lactating females. Older juveniles that are already weaned are safe, since their mothers can mate with the new male. "But the unweaned infants don't stand a chance against an adult male – even with their mother's protection," Beehner said.
So what is a female to do about this? Clearly, this represents an enormous reproductive loss for her, she said. Although somewhat counterintuitive, the leading adaptive hypothesis behind the Bruce effect is that it evolved as a "loss-cutting" strategy for pregnant females when new males arrive. Females that miscarry for whatever reason are able to quickly return to fertility, mate with the new male, and suffer only a minor delay in reproduction.
Females who miscarried were able to reproduce much sooner than females that suffered infanticide, supporting the idea that terminating a pregnancy is an adaptive strategy for females after the dominant male is ousted.
Dominant males are replaced by a bachelor after about 3 years. In the months after a male replacement, nearly 44 percent of the infants in the group are lost to infanticide by the new male.
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