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RICE (US)—Passive protection of their mates may be one reason male and female lemurs are the same size, contends evolutionary biologist Amy Dunham.

In most primate species, larger size is an advantage for males that guard females to keep other males from mating with them. Why have lemurs evolved differently? Some theories suggest that environment played a role or that lemur social development was altered due to the extinction of predatory birds.

“Scientifically, this is quite a big question that researchers have debated for over 20 years,” says Dunham, assistant professor of ecology and evolutionary biology at Rice University, who began studying lemurs as an undergraduate, working in Ranomafana National Park in Madgascar. “The question about size monomorphism has bugged me since then.”

Rather than guarding mates in an aggressive way, Dunham says, male lemurs deposit  a solid plug inside the female’s reproductive tract just as they finish mating. The plug is deposited as a liquid protein but quickly hardens and stays in place for a day or two.

Since many female lemurs are sexually responsive to males for only one day out of the entire year, the plug serves the purpose of preventing other males from mating with the female, while also freeing the male to mate with other females during the brief time they are available.

“If the female has a short receptivity period, as most lemurs do, then we hypothesize that this is likely to be an advantageous strategy,” says Dunham, who coauthored the paper with Rice evolutionary biologist Volker Rudolf. Findings appear in the Journal of Evolutionary Biology.

Dunham and Rudolf examined 62 primate species and found that copulatory plugs were most likely to occur in species where female sexual receptivity was very brief and where males and females were the same size. This was true both for lemur species and for a few other species, like South American squirrel monkeys.

“Our idea needs further testing because it’s new, but it’s more parsimonious than some of the old theories, and we’re very excited about looking into it further,” Dunham says. “We’ve made some explicit predictions about the conditions where this strategy should be favored, so there are plenty of ways it can be tested.”

Dunham has examined why, unlike other primates, female lemurs tend to dominate males. She suggests it is because females do all the work in rearing the young and therefore have more will to fight—and win.

“Game theory predicts that when the fighting abilities of two contestants are comparable, the outcome will depend upon the value that each contestant places on the resources they are fighting over,” she says.

“In this case, the females clearly have more at stake, but the only reason the females are in a position to compete for dominance is because they’re roughly the same size and strength as the males.”

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