Young chimpanzees show sex differences in learning

04/12/04

MINNEAPOLIS / ST. PAUL--Female and male chimps both learn from their mothers how to insert sticks into termite mounds and pull out a tasty meal of insects, but females learn earlier, spend more time at it and tend to catch more termites with each try, according to a University of Minnesota study to be published in the journal Nature April 15. The distinct sex differences in learning this skill are akin to differences between young girls and boys as they learn fine motor skills like writing, the researchers said. The authors said the study, which was done at Gombe National Park in Tanzania, has yielded the first systematic evidence of a difference between the sexes in the learning or imitation of a tool-use technique in wild chimpanzees, and it implies that sex-based learning differences have an ancient origin.

"This finding is a heads up to researchers studying the learning of relatively complex skills that they should take sex into account," said Elizabeth Lonsdorf, who headed the study as a graduate student at the university and is now director of field conservation at the Lincoln Park Zoo in Chicago. While young females perfected the art of termite fishing, young males spent more time playing and swinging around--behaviors that may help them in typically male adult activities like hunting and struggling for dominance, even though they see little of these activities while they are with their mothers. Mothers showed no preference for allowing either male or female offspring to observe them, yet sons and daughters did different things. And it makes perfect sense for chimps.

"The availability of animal protein is limited for chimpanzees. They can fish for termites or hunt colobus monkeys," explained Lonsdorf. "Mature males often hunt monkeys up in the trees, but females are almost always either pregnant or burdened with a clinging infant. This makes hunting difficult. But termites are a rich source of protein and fat. Females can fish for termites and watch their offspring at the same time. Adult females spend more time fishing for termites than males do. The young of both sexes seem to pursue activities related to their adult sex roles at a very young age."

Because this learning difference is similar to published sex differences in human learning, a sex-based learning difference may date back at least to the last common ancestor of chimpanzees and humans, the researchers said. According to current estimates, the evolutionary lines of humans and chimps split about five to six million years ago, said coauthor Anne Pusey, Distinguished McKnight University Professor in the department of ecology, evolution and behavior in the university's College of Biological Sciences. Lynn Eberly, a biostatistician in the university's School of Public Health, was the third author of the study.

Lonsdorf spent four years videotaping 14 young chimps under the age of 11 and their mothers. She knew the ages of six youngsters--three females and three males--when they first fished for termites successfully. On average, females began extracting termites on their tools at 31 months of age, males at 58 months. In young animals that had already acquired the skill, females fished more often and caught more termites each time they dipped a stick into a termite mound. Females also paid closer attention to their mothers' fishing techniques and copied them almost exactly.

Each mother fashioned fishing tools from sticks, palm fronds or grass and inserted the first several inches into tunnels in termite mounds. The length of the inserted section varied from a short distance (up to three chimp fist widths) to a medium distance (three to five fist widths) and a long distance (more than five fist widths). Daughters copied their mothers' patterns exactly. For example, if a mother inserted her tools a short distance 80 percent of the time and medium or long distances each 10 percent of the time, her daughter did the same. Sons spent less time watching their mothers, and their fishing patterns were completely unrelated to their mothers'.

Termite fishing qualifies as an aspect of chimpanzee culture, Lonsdorf said.

"Culture has been defined as passing on information by social learning from other members of a community," she said. "Female chimpanzees usually move to a new community when they reach maturity. Chimp communities are now isolated, but when individuals had room to move, the females dispersed, so they could have dispersed culture to a new community. It's less likely to happen today."

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