STONY BROOK / U. ARIZONA (US) — Both male and female lemurs stray from the safety of the group, but the females eventually settle down—perhaps a clue to why they tend to outlive the males, researchers say.
Sex differences in aggression, hormones, or appearance drive males of many species to an earlier grave. But in the Milne-Edwards’ sifaka, males and females have similar levels of testosterone, and are equally likely to pick fights.
Both sexes grow at similar rates and reach roughly the same size, have similar coloration, and are equally likely to be spotted by predators.
In the Milne-Edwards’ sifaka, males and females have similar levels of testosterone and are equally likely to pick fights. (Credit: Stony Brook University)
The study, published in Behavioral Ecology, analyzed detailed records of births, deaths, and dispersal behavior for more than 70 individual lemurs living in Ranomafana National Park in southeastern Madagascar—a data set spanning 23 years from 1986 to 2009.
According to the data, most males died by their late teens. But females lived, on average, into their early 30s. What could explain the gender gap? By taking a closer look at dispersal behavior across the lifespan, the researchers think they have a clue.
In Milne-Edwards’ sifaka society, both sexes are known to leave the groups where they were born in search of a new group to call their own—sometimes dispersing repeatedly throughout their lives.
The data suggest that on average, males and females disperse equally frequently, and wander just as far. But when the researchers broke down dispersal across their lifespan, from infancy to old age, they found that males and females differed in their timing.
The differences do not start to emerge until later in life. Females generally stopped dispersing after a certain age, typically when they reached 11 years old. But males switched groups three times in an average lifespan.
“Female lemurs are leaders,” says study co-author Patricia Wright, professor of biological anthropology at Stony Brook University. “It’s exciting to know that even when females lead they are still living longer.”
Researchers don’t know why females eventually settle down, whereas males continue to strike off on their own. But dispersing at older ages could carry greater costs, especially if older animals are not as agile or quick to heal from injury.
“When you’re a social animal and you go off on your own into unfamiliar territory, finding food can be more of a challenge. Plus you don’t have the extra protection of other group members who can help look out for predators,” says co-author Jennifer Verdolin of the National Evolutionary Synthesis Center.
“Even when you find a new group to join, you may have to fight your way in and there’s a chance of getting injured in a fight,” adds Verdolin.
The study does not help explain why women tend to outlive men in humans, the authors caution. But it does suggest that fine-scale studies of risk-taking behavior at different ages could reveal age-specific mortality risk factors that researchers have not considered.
Researchers at the University of Arizona, Colorado State University, and the University of Massachusetts, Amherst, collaborated on the study, which was funded by the National Science Foundation.
Source: Stony Brook University