STUDY PARTICIPANTThe brown-throated three-toed sloth has become the first free-ranging animal to get brain wave monitoring for sleep studies. Read more... Max Planck Institute

Sloths aren’t as slothful when tested for sleep in their rainforest trees instead of in the lab.

Brown-throated three-toed sloths living in rainforest trees slept at least six hours less than the 16 hours a day previously recorded for sloths in a lab, says Niels Rattenborg of the Max Planck Institute for Ornithology in Seewiesen, Germany.

To clock sloth sleep, Rattenborg and his colleagues took advantage of a new, battery-operated, miniaturized EEG and EMG recorder weighing only 11 grams. Small enough for animals to wear, the device allows monitoring of electrical impulses from the brain, but in the wild. “This is the first step out of the laboratory,” Rattenborg says.

The researchers lifted three adult female Bradypus variegatus out of their trees around the Smithsonian Tropical Research Institute’s field station on Barro Colorado Island in Panama. Each sloth got a recorder glued to her hair. “Sloths are not known for grooming their hair,” Rattenborg says. Then the animals were freed to climb back up the trees.

Three to five days later, the researchers retrieved the devices and checked the results, which they report in an upcoming Biology Letters.

“I think this is the first to really do sleep in free-roaming animals — that’s what makes the paper interesting,” says evolutionary neuroscientist Paul Manger of the University of the Witwatersrand in Johannesburg, South Africa. The approach holds promise since miniaturization and data retrieval technologies are improving so rapidly, he adds.

Comparative sleep researcher Jerome Siegel of the University of California, Los Angeles, says the move into the wild “is what is needed.”

Tests in the wild could yield different results from those in the lab because wild animals need to keep an eye out for lurking predators and for potential dinner, Rattenborg says. Or for some animals, the lab may be so stressful that tests there yield sleep numbers that are unrealistically low.

The sloths didn’t show obvious signs of stress from having recorders glued to their heads, Rattenborg says. After a day, their behaviors stabilized. And they even showed REM sleep patterns for about 20 percent of the time, much like people.

Also, the recorders picked up signs of sloths chewing leaves. Even when the brain wave patterns indicated sleep, the sloths made mild chewing motions. “If they dream,” says Rattenborg, “I think they’re dreaming about eating leaves.”

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