Gelada monkeys offer evidence that abortion is part of evolutionary fitness
Liz Bugarin stashed this in She blinded me with SCIENCE!
"According to ethologist Neville Pillay — who has studied the Bruce effect in captive rats, but was not involved in the study — the argument presented by Beehner and her colleagues is sound. From a cost/benefit standpoint, he explains "termination is better than carrying [the fetus] to term [if it will] eventually be killed by the new male." Even though the female gelada would suffer an immediate loss, explains Pillay, on a long enough timeline, the termination is actually advantageous to her ability to successfully give birth to offspring that will survive.
That being said, such an evolutionary adaptation would only make sense for females in species where male infanticide is a significant risk. By that reasoning, explains Beehner, a Bruce effect almost certainly does not exist in humans; although the mechanisms behind pregnancy termination in geladas (be they regulated by pheromones or by hormonal shifts brought on by stress), could very well hold the clues to understanding pregnancy success across a variety of species — humans included."