Hibernation is a simple concept: In response to cold weather, an animal enters suspended animation to conserve resources until warm days return. Somebody forgot to explain this to the fat-tailed dwarf lemur, a foot-long primate in Madagascar that hibernates even though winter temperatures there range into the 70s and 80s.
“Natives suspected that these animals might be undergoing some kind of dormancy response because they suddenly disappear during winter,” says Kathrin Dausmann, a biologist at Philipps University in Marburg, Germany. Curious, she and her colleagues outfitted the elusive primates with radio transmitters and followed them over several seasons. The lemurs were definitely hibernating, the only tropical mammal—and the only primates—known to do so.
Since cold weather isn’t a problem, why do fat-tailed lemurs drop out? Dausmann suggests that they save energy during food-scarce winter months by minimizing their investment in body-temperature regulation. “Winter temperatures on Madagascar fluctuate widely, sometimes more than 36 degrees in a 24-hour period. Maintaining a constant body temperature is energetically expensive,” she says. So the lemurs grow plump during the summer, when food is abundant, then hole up in trees and let their bodies fluctuate with the weather. “If they stored fat all over, they’d overheat, so it all goes to their tails,” Dausmann says. “They get a bit odd looking.”