Geladas live in units where a single dominant male lords over several related females, whom he monopolises as mates. It’s an enviable position, and males often have to fend off takeover bids by eager bachelors. If a newcomer ousts the chief monkey, it’s bad news for the group’s females. A wave of ᴅᴇᴀᴛʜ sweeps through the unit, as the new male ᴋ.ɪ.ʟ.ʟs all the youngsters whom his predecessor fathered. Indeed, babies are 32 times more likely to ᴅ.ɪ.ᴇ after a takeover than at any other time.
In 1959, biologist Hilda Bruce first demonstrated the so-called Bruce efꜰᴇᴄt in mice, where recently pregnant females ᴍ.ɪ.s ᴄ.ᴀ.ʀ ʀy after being exposed to novel males. Since then, researchers have documented the phenomena in other rodent species. But the Bruce efꜰᴇᴄt has always remained a quirk of captivity. No one really knew if wild animals do the same thing. There have been anecdotes involving various monkeys (including geladas), but no solid records or experimental evidence. Two stuᴅ.ɪ.ᴇs with wild rodents failed to find any evidence for the Bruce efꜰᴇᴄt.
To see if the Bruce efꜰᴇᴄt exists in gelada monkeys (Theropithecus gelada), Jacinta Beehner, an anthropologist at the University of Michigan, and her colleagues tracked 110 females across 21 groups of wild geladas living in the Simien Mountains National Park in Ethiopia.
“We saw that as soon as a new male came into a group, there were no births for the next six months,” Beehner says. In fact, the researchers documented only two births in these replacement groups in the five years of the study. “We get this big gap, screaming out that something is going on — it’s statistically almost impossible to get this by chance.”
To be sure what they were seeing was indeed the Bruce efꜰᴇᴄt, the researchers also took hormonal data from the ꜰᴇᴄal samples of females before and after a new male arrived. Out of the 10 cases of pregnancies the researchers looked at, eight of the females ᴍ.ɪ.s ᴄ.ᴀ.ʀ ʀied within two weeks of a new male coming on to the scene. Most surprising to the researchers, the ᴍ.ɪ.s ᴄ.ᴀ.ʀ ʀiages happened the same day the male took over.
Of the two females that didn’t ᴍ.ɪ.s ᴄ.ᴀ.ʀ ʀy, one quickly showed signs of ꜰᴇʀᴛɪʟɪty sᴡᴇʟʟing and eventually mated with the new male while still pregnant. The other didn’t, and probably as a result, the male ᴋ.ɪ.ʟ.ʟed her infant, but didn’t ᴋ.ɪ.ʟ.ʟ the infant of the female with whom he mated. This behavior suggests that the males figure out which babies are theirs simply by knowing which females they mated with, Beehner said.
Females that ᴍ.ɪ.s ᴄ.ᴀ.ʀ ʀied as soon as new males arrived also became pregnant again, and the researchers saw a twofold increase in births during the seven to 12 months after new males took over. They also found that females that experienced such primate ɪɴꜰᴀɴᴛɪᴄɪᴅᴇ took longer to become pregnant again, suggesting these ᴍ.ɪ.s ᴄ.ᴀ.ʀ ʀiages are evolutionarily advantageous to the mama monkeys.
But why would a pregnant female ᴀʙᴏʀᴛ her own foetus? Roberts thinks that it’s an adaptive tactic in the face of a new male’s ᴍᴜʀᴅᴇʀous tendencies. Since the male would probably ᴋ.ɪ.ʟ.ʟ the newborn baby anyway, it’s less costly for the female to ᴀʙᴏʀᴛ than to waste time and energy on bringing a ᴅᴏᴏᴍed 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.
This is one concrete example. It’s still not clear how widespread the Bruce efꜰᴇᴄt is among mammals. As Roberts showed, proving it is very difficult, and few scientists can afford to collect ꜰᴀᴇᴄᴀʟ samples from a large population for many years.
Peter Brennan, a physiologist at the University of Bristol in the United Kingdom who was not involved in the research, said that the study was quite convincing. “It’s a great example of pregnancy block being demonstrated quite convincingly in the wild,” said Brennan, who has stuᴅ.ɪ.ᴇd the Bruce efꜰᴇᴄt in lab mice. “And there’s good evidence that it’s adaptive in evolutionary terms.”
Brennan is curious as to exactly how the females ᴍ.ɪ.s ᴄ.ᴀ.ʀ ʀy. In lab mice, he notes, females ᴍ.ɪ.s ᴄ.ᴀ.ʀ ʀy after picking up on chemical signals put off by the new males. “The actual physiological mechanism (in geladas) may be different,” he said, adding that the ᴍ.ɪ.s ᴄ.ᴀ.ʀ ʀiages might be a response to social sᴛʀᴇss.
Beehner said that the next step is to pinpoint this mechanism, though this research cannot be conducted on a ᴛʜʀᴇᴀᴛened wild primate like the gelada. Domestic horses may be good candidates for further research, as scientists have seen the Bruce efꜰᴇᴄt in the species before, she said.