Primates can alter the rate at which they reach s.ᴇ.xual maturity. Previous studies have shown a variety of factors, including food availability, can determine whether primates mature earlier or later. Now, researchers studying close primate relatives of baboons known as geladas have shown for the first time that females of this species suddenly hurry up and mature when a new male enters the picture. Their findings are reported in the journal Current Biology on November 5th.
“We found that ᴘʀᴇᴘᴜʙᴇʀᴛal females are more likely to mature right after a new ʙʀᴇᴇᴅing male arrives in the group — even if it means maturing earlier than expected,” senior author Jacinta Beehner, a professor of psychology and anthropology at the University of Michigan.
“We also noticed that some of these maturing females were maturing much later than expected,” said Beehner
Many of those late bloomers were the daughters of the primary ʙʀᴇᴇᴅing male prior to the new male’s arrival, the researchers report. Their observations suggest that females can both speed up and slow down their maturity to avoid inʙʀᴇᴇᴅing with their fathers.
“Once their father is ᴏᴜsᴛᴇᴅ by the new male, they appear to lift this suppression and immediately mature,” Beehner said. “Taken together, we see that a new male causes a really obvious increase in the number of maturations in a group — whether early, on-time, or late.”
Normally, during any given year, the group of geladas being monitored by scientists featured only a handful of s.ᴇ.xually mature females. But researchers began noticing that the arrival of a new male leader was often followed by the sudden s.ᴇ.xual development of several females.
Researchers started monitoring the ᴇsᴛʀᴏɢᴇɴ levels of females, which can be measured in feces, before and after changes in male leadership. Prior to s.ᴇ.xual maturity, female primates start producing higher levels of ᴇsᴛʀᴏɢᴇɴ.
The findings have been a long time coming, the research team says. About a decade ago, they started to notice that — right after a new male arrived — a few females would suddenly mature, all at the same time. That was striking because the researchers typically see only half a dozen or so females mature in any given year. But getting enough data to prove that the timing of maturation was tied to the arrival of a new male took some time.
In the Current Biology study, Beehner along with first author Amy Lu of Stony Brook University and their colleagues kept track of the age at maturity for 80 females over 14 years of research in the highlands of Ethiopia, the only place geladas are found in nature. It’s easy to tell when a gelada matures because they have very conspicuous “s.ᴇ.xual sᴡᴇʟʟɪɴɢs” surrounding a patch of skin on their ᴄʜᴇsᴛ and neck.
The findings suggest the s.ᴇ.xual maturity process in primates is both flexible and sᴇɴsɪᴛɪᴠᴇ to social factors. While scientists have previously observed such characteristics among New World monkeys, the latest research marks the first time researchers have identified the phenomenon among Old World monkeys.
“If an Old World monkey, like geladas, can su.pp.ress maturation in response to the presence of their biological fathers and lift this suppression in response to the arrival of a novel male, it’s possible that such a process could be present in apes, and possibly even in humans,” Beehner said.
However, they ᴄᴀᴜᴛɪᴏɴ against taking the results in geladas too far in terms of what they might mean for humans since there are so many additional factors at play. In future studies, they hope to identify the costs and benefits associated with maturing early, on-time, or late for their gelada population.
“Once again,” Beehner says, “this means we have to be patient and wait until these females, now matured, live out their reproductive lives. So, stay tuned, and we’ll get back to you on that in another 14 years.”