As any mother knows, having a supportive social group is crucial when a new baby arrives. Duke researchers have recently shown that female chimps also recognize this need for support.
Published Nov. 20 in the "Journal of Human Evolution," the study analyzed records of famous primatologist Jane Goodall to reveal that female chimps who are not surrounded by family and peers wait longer to start having offspring.
“What we’re interested in is what determines differences in reproductive success of individuals in terms of social behavior and social situation,” said Anne Pusey, James B. Duke professor of evolutionary anthropology and an author of the study.
The scientists delved into more than 50 years' worth of data that recorded the activities of 36 female chimps in Gombe National Park in western Tanzania. These records were stored in the Jane Goodall Institute Research Center at Duke, which also contains databases filled with close observations of hundreds of wild chimpanzees.
“Chimpanzees are important as a study subject because they, together with bonobos, are humans’ closest living relative and can be informative in thinking about our own evolution,” Kara Walker, postdoctoral associate in the department of evolutionary anthropology at Duke and another author of the study, wrote in an email.
The researchers’ analysis showed that wild female chimps who leave home or are orphaned wait about three years longer to have babies. Those who stay at home have their first baby when they are about 13, and the ones who migrate wait until around age 16.
Although some female chimps remain with the group they are born in, others move to another circle when they reach adolescence, which Pusey said scientists believe happens so the chimps can avoid inbreeding.
Existing research on chimp reproduction has mostly focused on the chimps who stay at home, but using Goodall’s archived data opened new doors for the scientists. Walker explained in her email that the previous studies were limited by small sample sizes.
“What we were able to do is follow these chimps to where they move and see what age they reproduce in a new group,” Pusey said.
One reason for the delay in reproduction could be that migrants have to establish their position in the new group. Females already in the group often push them around. However, those who stay at home typically have the support of relatives.
“We know that new immigrants receive more aggression from females than older residents, and attacks can continue for several years after entry,” Walker wrote. “Attacks can be unpredictable, so immigrants are probably under considerable social stress.”
Chimps also have to learn the social politics in their new communities and locate feeding spots, also contributing to the delay, she added.
If the females have mothers who remained in their lives while they were growing up, they typically start reproducing earlier as well. This is especially the case if their moms had a high ranking, because this affords them better access to food.
Having babies at an earlier age means that these female chimps have the chance to produce more total offspring during their lives, Pusey explained.
Walker noted that chimp and human reproduction differ in many ways as well.
“They are interesting from a life history perspective because they reproduce very slowly, and they typically give birth every five or so years, a pattern very different from human females,” Walker wrote.
The study’s findings are significant in helping explain the differences in length of development between humans and chimps, Pusey noted. Past research has said that chimps begin reproducing around age 13 or 14, yet this new study indicated that a more realistic figure is 15 or 16, revealing less of a difference between humans and chimps than previously thought.
The larger sample size in this study allowed the scientists to better estimate the age of maturity and first birth for female chimps.
“Because these data cannot be compiled at most field sites, this is the first time that anyone has examined factors that may hasten or delay parenthood,” Walker noted.
In the future, the researchers plan to explore why certain chimps stay home and others move away. If chimps’ mothers are still alive, they might be more likely to stay. But if they have lots of male relatives—and thus a greater chance of inbreeding—they might try out a new group.
Walker explained that it is puzzling why most females leave home when there are so many benefits to staying.
“We are also eager to examine environmental and demographic factors—such as chimpanzee density, chimpanzee body size and rainfall—that may affect maturation timing,” she wrote.
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