Lemurs are primates, like humans, but they're an odd bunch. Found only on Madagascar, an island off eastern Africa, this group includes some of the only primates known to hibernate—and some of the few that feast primarily on leaves instead of fruits.

By itself, lemurs' preference for greens might not seem all that notable. But Madagascar's birds and bats also consume less fruit than their counterparts in Asia, continental Africa and the Americas. Because the pattern involves so many types of animals, primatologist Giuseppe Donati of Oxford Brookes University in England suspected there might be something different about the island's fruit itself.

Along with other nutrients, fruits often provide animals with the protein they need for processes from building muscle tissue to moving oxygen through the bloodstream. By measuring the amount of nitrogen in fruits from 62 tropical areas across the globe, Donati and his team estimated how much protein those fruits offered.

They found that fruits in the Americas, Asia and much of Africa have similar levels of nitrogen, whereas Madagascar's fruit contains a quarter to a third less. But lemurs still manage to consume the same amount of nitrogen as other primates, suggesting these peculiar animals have found ways to adapt to the lower-quality fruit. “Fruits are not really something that can, in Madagascar, be enough to meet the [lemurs'] nitrogen requirements,” Donati says. And the island's trees fruit at unpredictable times because of the high frequency of cyclones and low soil fertility, so lemurs might have adopted their leafy diets to compensate. Donati and his colleagues published their results in October 2017 in Scientific Reports.

“It's still a puzzle why lemurs are strange in many biological ways, and diet is a really important way to look at that,” says Arizona State University anthropologist Caley Johnson, who was not involved in the study. Indeed, the results could explain why the few lemur species that do consume a lot of fruit lead a cathemeral way of life—that is, they are active during both the day and night, perhaps because they need the extra feeding time to get enough nutrition. The struggle for nutrients might even explain why some lemurs hibernate. Better to sleep through the winter than risk being unable to find enough to eat.

Lemurs face an existential challenge as one of the most endangered groups of primates on the planet. If conservation efforts are to succeed, the forests these primates rely on for food must be conserved, too.