"Adaptive" spontaneous abortions

Named after its discoverer, the "Bruce effect" is a phenomenon in which pregnant female geladas (relatives of baboons) abort their fetuses when a new male becomes dominant:

If a newcomer ousts the chief monkey, it’s bad news for the group’s females. A wave of death sweeps through the unit, as the new male kills all the youngsters whom his predecessor fathered... But that’s not all. Eila Roberts from the University of Michigan has found that the new male’s arrival triggers a wave of spontaneous abortions. Within weeks, the vast majority of the local females terminate their pregnancies. It’s the first time that this strategy has been observed in the wild...

It’s obvious why the incoming males kill any existing infants. Female geladas don’t become fertile until they stop raising their existing children. Assuming no abortions, they go for three years between pregnancies. That’s longer than the typical reign of a dominant male. So, a newcomer, having finally won the right to mate, has few opportunities to actually do so. To make things worse, his females are busy raising someone else’s children. His solution: kill the babies. The quicker he does this, the sooner the females become fertile again, and the sooner he can father his own children.

But why would a pregnant female abort her own foetus? Roberts thinks that it’s an adaptive tactic in the face of a new male’s murderous tendencies. Since the male would probably kill the newborn baby anyway, it’s less costly for the female to abort than to waste time and energy on bringing a doomed infant to term. Her future offspring, conceived more quickly and fathered by the incumbent king of the hill, will stand a better chance of survival.

Further details are published in the Science article, nicely summarized at Not Exactly Rocket Science.  The mechanism of the abortion is not readily apparent; one supposes that it occurs because of alterations in hormonal balances.