Mother chimps are more social with sons than with daughters

A study finds that mothers chimpanzees with sons are more social than those with daughters. In this photo shot in 2012 in Gombe National Park, Gremlin holds her male infant Gizmo Carson Murray

It seems young male chimps have more fun growing up than their sisters do.

A Proceedings of the National Academy of Sciences study released Monday found mothers of male Tanzanian chimpanzees in Gombe National Park were more social than their female-rearing counterparts.

"We know from prior work coming out of Gombe that males have more social interaction than females," lead author Carson Murray, an assistant professor at George Washington University who has studied the Tanzania chimps for the past 14 years, told CBS News. "The questions we are asking here is whether moms facilitate those social interactions. Do they create more social opportunities for their sons than their daughters? The short answer is they do. Not only are mothers with son in groups more often but that is true even early in life."

Chimpanzees have a male-dominated society in which rank is a constant struggle for the males. Females with infants might face physical violence and even infanticide if they get too close to these conflicts. For them, it is safer to avoid groups where aggressive males are present.

"Mothers with sons seem willing to incur those costs for the benefit of having their sons socialized," Murray said.

The researchers believe that mothers are giving youngsters the opportunity to observe males in social situations, offering them a head start on developing the social skills they'll need to thrive in the competitive adult world.

The findings are based on an analysis of 37 years of daily observations of East African chimpanzees from the Gombe National Park. Duke University houses all of the data from the famous Kasekela chimpanzee community in the Jane Goodall Institute Research Center, which contains more than 50 years of observational data all the way back to Goodall's first hand-written observations from the early 1960s.

"Drawing from the long-term datasets, we were able to investigate patterns within the same mother, examining how she behaved with her sons versus with her daughters," Murray said. "These results are even more compelling than a general pattern, demonstrating that the same female behaves very differently depending on the sex of her offspring."

In this Feb. 1965 photo from the Jane Goodall Institute Research Center at Duke, Goliath the alpha male gently interacts with an infant (age and identity unknown) while a female and her male infant sit very nearby at right. The Jane Goodall Institute / Hugo van Lawick

In their study, researchers measured gregariousness based on three parameters. They looked at how much time a mother spent with other adults who were not immediate family members; the average size of the mother's party and its composition; and the proportion of time a mother spent in mixed-sex and female-only parties.

They found that mothers with sons were 25 percent more social than those who had daughters and would spend upwards of two hours more per day with other chimpanzees.

Murray said the researchers don't yet understand the mechanism at work here and are careful not imply this behavior was a "conscious" decision by the mothers. Rather, she said it could have something to do with her hormones -- mothers with male fetuses produce more testosterone in others species, which may prime them to be more social with sons.

Another theory is that adult chimpanzees are more attracted to male infants.

"The mom has a choice to be in a group or not," she said. "It's possible that other adults in the community are attracted to male infants and not as much to female infants. We know Spider monkeys have that pattern."

Could this help us understand human parenting strategies and how they differ when raising boys and girls?

Murray said there has been very little research in this area and that it would be a stretch at this point to make the link given ancient humans diverged from chimps about 6 million years ago. Still, they are our closest living relatives.

"We know (human) parents are very different with their daughter than with their sons. We know parents encourage rough and tumble play and risk taking with their sons versus more protective behavior with their daughters," Murray said. "The question of how parents facilitate social opportunities, play dates and things like that hasn't been investigated as much you would think."