A Story of Sex, Betrayal, and Murder

Imagine that you are a female gelada monkey, living in a large group of females under the protection of one alpha male. On one particular lackluster day in between your usual routine of naps, walks, grooming sessions, and meals of grass and grubs, an unfamiliar male arrives. He is intriguing, he is fascinating, he is… sexy. He sneaks in surreptitiously along your patch of grass and proceeds to challenge your alpha male. A gigantic duel unfolds between the two males, with canines flashing and flesh tearing. When it finally subsides, your former alpha male slinks into the mountains beyond and your group of females is left with the new triumphant male. It quickly becomes clear that this will not be a simple adjustment to life with a different dominant male. The new fellow initiates a wave of infant murders, asserting his status and ensuring that all the females can now carry his own offspring. But that’s not all…studies reveal that the introduction of a new, novel male can have monumental impacts on the females, as well.

a gelada male fight
Photo credit: Andreas Schürkmann
Photo credit: Andreas Schürkmann

The mere presence of an unfamiliar male can induce spontaneous abortion. How can a male possibly terminate a pregnancy without physical infanticide (or intentional killing of a fetus or infant)? This peculiar outcome is termed “the Bruce effect,” after the British zoologist Hilda M. Bruce. Described in 1959, Bruce observed this effect in rodents that would frequently terminate their pregnancies in response to exposure to an unknown male. The main hypothesis follows that this effect is regulated by pheromones, or hormones secreted in bodily fluids that are believed to alter the behavior and reproductive state of the opposite sex. This phenomenon was then observed in domestic horses, where females aborted their fetuses almost a third of the time when exposed to males that were not the father of their offspring.

Photo credit: Andreas Schürkmann
Photo credit: Andreas Schürkmann

Remarkably, the Bruce effect has now been documented in wild populations of gelada monkeys, the first studies of any species outside of the laboratory. Geladas live in multi-female units where a single dominant male monopolizes all matings with his small harem of females. Bachelor males without a harem seek any opportunity to viciously fight for access to females. When an alpha male is de-throned by a bachelor, infants become highly susceptible to death: offspring are 32 times more likely to die during a takeover than any other time. For a male, these murderous tendencies are actually adaptive: a male can father more offspring for himself by killing infants. This is because when a female loses her baby, she becomes fertile again.

Photo credit: Andreas Schürkmann
Photo credit: Andreas Schürkmann

Dr. Eila Roberts, a postdoctoral scholar at Arizona State University, examined this effect in geladas for her Ph.D. dissertation at the University of Michigan with Dr. Jacinta Beehner. They discovered that the base pregnancy failure rate for females is set at 2%. When a new male arrives however, this failure rate escalates to a whooping 80%. Yes, that means that 80% of pregnant females abort their fetuses when an unfamiliar male takes over a harem.

A male gelada with his harem of females

Why would a pregnant female so rapidly abort her healthy fetus? Roberts and Beehner speculate that this effect actually serves an important evolutionary adaptation. It is less costly for a female to abort her unborn infant than to waste the time and energy on birthing a baby that would inevitably be killed by the new male anyways. This permits a female to invest more in a future offspring with this new male that has a higher chance at survival.

While the Bruce effect has been observed in several species, it remains unclear how prevalent this phenomenon is among mammals.



Bruce, H. M. (1959). An exteroceptive block to pregnancy in the mouse. Nature. 184, 105.

Bartoš, L., Bartošová, J., Pluháček, J., & Šindelářová, J. (2011). Promiscuous behaviour disrupts pregnancy block in domestic horse mares. Behavioral Ecology and Sociobiology, 65(8), 1567-1572.

Roberts, E. K., Lu, A., Bergman, T. J., & Beehner, J. C. (2012). A Bruce effect in wild geladas. Science, 335(6073), 1222-1225. http://dx.doi.org/10.1126/science.1213600

Yong, Ed. 2012. The Bruce effect- why some pregnant monkeys abort when new males arrive. http://blogs.discovermagazine.com/notrocketscience/2012/02/23/the-bruce-effect-why-some-pregnant-monkeys-abort-when-new-males-arrive/#.VPfnA8YqaSV

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