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This photo shows a young gelada grooming the dominant breeding male of her group. Credit: Rachel Perlman
Most mammals, including humans and other primates, reach sexual maturity early or late depending on many different factors, such as how much food there is to eat. Now, researchers studying the close primate relatives of baboons known as geladas have shown for the first time that females of this species suddenly rush in and mature when a new male enters the scene. Their findings are published in the journal. Current biology today (November 5, 2020).
When there is a new dominant male in a gelada group, even females up to 3.5 years old (pictured) can mature. Credit: Jacinta Beehner
“We found that prepubertal females are more likely to mature just after a new breeding male arrives in the group, even if that means maturing earlier than expected,” said lead author Jacinta Beehner, professor of psychology and anthropology at the University of Michigan. “We also noticed that some of these females matured much later than expected.”
Many of those late were daughters of the primary reproductive male before the arrival of the new male, the researchers report. Their observations suggest that females can speed up and slow down their maturity to avoid inbreeding with their parents.
“Once their father is expelled by the new male, they seem to remove this suppression and mature immediately,” Beehner said. “Taken together, we see that a new male causes a really obvious increase in the amount of maturation in a group, be it early, early or late.
Findings have been a long time coming, says the research team. About a decade ago, they began to notice that just after a new male arrived, some females were suddenly maturing, all at the same time. That was surprising because researchers generally see only half a dozen females mature in any given year. But getting enough data to show that the timing of maturation was tied to the arrival of a new male took some time.
At Current biology Beehner along with first author Amy Lu (@sbululab) of Stony Brook University and colleagues tracked the age of maturity of 80 women during 14 years of research in the Ethiopian highlands, the only place where they are found. gelada in nature. It’s easy to tell when a gelada matures because they have very noticeable “sexual swellings” surrounding a patch of skin on their chest and neck.
In this photo, a dominant gelada male and his group of females sit on the edge of a cliff in the Simien Mountains National Park, Ethiopia. Credit: Jacinta Beehner
To better understand how this was happening, the researchers looked at the women’s estrogen levels, which they can measure in their stools. They knew that estrogen levels rise just before females visibly mature. Surprisingly, however, their data showed that estrogen levels rose in immature women of all ages right after a new male took over. In fact, that increase in estrogen occurred even in women too young to mature.
“Females usually mature around 4.5 years, but we found that even females as young as one year showed a temporary increase in estrogen,” Beehner said. “We suspect that this increase in estrogen causes women to mature, but that some are too young for this boost to work.”
The findings suggest that maturation in many primates is much more sensitive to social settings than scientists had previously thought. The discovery may even have implications for humans, according to the researchers.
“Many New World monkeys, such as marmosets and marmosets, have long been known to be very sensitive to social variables, with one dominant female suppressing reproduction of all other females in the group,” he says. “But, until now, we had no evidence that Old World monkeys or apes were equally sensitive to the presence or absence of particular individuals. If an Old World monkey, such as geladas, can suppress maturation in response to the presence of its biological parents and remove this suppression in response to the arrival of a new male, it is possible that such a process may be present in apes, and possibly even Inhumans. “
However, they caution against taking gelada results too far in terms of what they might mean for humans, as there are many additional factors at play. In future studies, they hope to identify the costs and benefits associated with early, late, or late maturation for their gelated population.
“Once again,” says Beehner, “this means that we have to be patient and wait until these now mature females live their reproductive lives. So stay tuned and we’ll get back to you in another 14 years. “
Reference: “Male-Mediated Ripening in Wild Jellies” by Amy Lu, Jacob A. Feder, Noah Snyder-Mackler, Thore J. Bergman, and Jacinta C. Beehner, November 5, 2020, Current biology.
DOI: 10.1016 / j.cub.2020.10.003
This work was supported by the National Science Foundation, the Leakey Foundation, the National Geographic Society, the Fulbright Scholars Program, the University of Michigan, Stony Brook University, and Arizona State University.
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