Female monkeys found to terminate own pregnancies to increase survival rates

Sam Durley

Scientists have discovered that the introduction of a new male to a group of female monkeys will cause them to terminate their existing pregnancies, according to a report in the journal Science.

Until now scientists have only observed female mice and horses terminating their pregnancies after introduction of a new male to the group when the animals are exclusively kept in captive conditions. This condition is termed “The Bruce Effect” after the biologist Hilda Bruce who first observed this extraordinary behaviour.

The study produced by a group of American scientists has shown that groups of female gelada monkeys in the Simien Mountains National Park in Ethiopia actively terminate pregnancies after the introduction of a new dominant male, dispelling the theory that the Bruce Effect is only a laboratory artefact.

When a new male joins a female pack of gelada monkeys the infanticide rate is approximately 50%, as those newborns would have been fathered by the males’ predecessor. The researchers have now shown that after the replacement of the dominant male in the pack the female gelada monkeys terminated 80% of existing pregnancies.

The scientists suggest that the female monkeys therefore terminate their existing pregnancy upon the introduction of the new male as it is highly likely that the new male will kill the newborn. This then allows the new male monkey to mate with the female monkeys and the offspring will have a much higher survival rate.

This incredible behaviour is mediated by changes in the hormone levels in the female monkey, specifically by a huge reduction in the amount of oestrogens. This ability to self terminate has evolved over many years and provides an advantage to the female monkeys as it gives them a much higher success rate of reproduction, a process that is highly energy consuming.

Whether the Bruce Effect is more widespread amongst other mammals is an interesting question that scientists will now focus more attention on when observing pack animals in the wild.

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