To sleep, perchance to dream--and maybe not get devoured? When it comes to how much shuteye animals get in the wild, fear and food matter most.
Researchers have spent decades trying to understand the confusing array of sleep patterns found in mammals. A donkey typically snoozes for just 3 hours a day, for instance, and armadillos and bats can be dead to the world for 20 hours a day. To explain the differences, scientists have offered a slew of theories, ranging from the idea that smaller animals need more sleep to conserve energy and maintain body temperature to the need to avoid predators.
It's been hard to sort out which of these ideas have legs, however. In part, that's because many sleep studies take place in artificial laboratory settings, coloring the results. And there are relatively few studies of sleep habits in the wild; wiring up sleep monitors to free-ranging animals can be difficult (ScienceNOW, 14 May).
In one of the most comprehensive efforts yet to tackle the question, a team led by evolutionary biologist Isabella Capellini of Durham University in the U.K. spent 2 years combing through the scientific literature and stockpiling information on the sleep habits of more than 150 mammals. Then they used statistical tools to see what factors best explained the sleep habits of about 60 of the best studied creatures.
Two factors stood out, the researchers report in the current issue of Evolution. One is the threat of predation: Animals sleep longer if they bed down in protected places, such as burrows or treetops. So burrowing moles, for instance, may sleep longer than rodents that seek less cover. The other was food. Animals sleep less if they need to graze extensively--as with herds of horses. "Sleep has costs and benefits," says Capellini, and understanding how an animal's environment shapes tradeoffs could aid everyone from evolutionary biologists to conservationists keen to reduce stress on endangered species.
One surprising result was that social animals that sleep in groups, such as herds of grazing animals, get less sleep than species that live alone. That's unexpected, the researchers say, because group living typically provides protection and presumably more time to rest assured. They speculate that social mammals may sleep less because they are often plant eaters and need more time to collect food. But there's not enough evidence to nail down that idea, Capellini says.
"We really lack a lot of needed information," agrees Jerry Siegel, a sleep researcher at the University of California, Los Angeles. For instance, he says it would be nice to have better studies on how many sleeping animals are actually killed by predators before scientists embrace that explanation. And his own work suggests that some mammals sleep to conserve energy--an idea that got little support in Capellini's study. But he applauds the researchers "for taking on something that's really difficult to study and trying do it from a broad perspective."