After observing these animals for more than two decades in the wild in Madagascar, co-author Patricia Wright of Stony Brook University had a hunch that females were living longer than their male counterparts.
Females tend to outlive males in many animals, including humans. But in the Milne-Edwards' sifaka — a rainforest-dweller with orange-red eyes, a black face and woolly dark brown fur — the sexes didn't seem to differ in any of the ways thought to give females a survival advantage in other animals.
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 occasionally stray from the safety of their social groups, explained lead author Stacey Tecot of the University of Arizona. They also grow at similar rates and reach roughly the same size, have similar coloration, and are equally likely to be spotted by predators.
For the study, Tecot, Wright and colleagues 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.