Not living up to their name

Sloths in the wild sleep a lot less than previously thought

May 21, 2008

In the first experiment to record the electrophysiology of sleep in a wild animal, three-toed sloths carrying miniature electroencephalogram recorders slept 9.63 hours per day - 6 hours less than captive sloths did, reports an international team of researchers working on the Smithsonian Tropical Research Institute’s Barro Colorado Island in Panama. Niels Rattenborg, head of the "Sleep and Flight-Group" of the Max Planck Institute for Ornithology in Germany and his research colleagues developed a new technique for the recording of electroencephalograms (Biology Letters, May 14th 2008).

"We are fascinated that some species sleep far longer than others. If we can determine the reasons for variations in sleep patterns, we will gain insight into the function of sleep in mammals, including humans," said first author Niels Rattenborg, group leader of the Sleep and Flight Group at the Max Planck Institute for Ornithology. "If animals behave differently in captivity (where all previous comparative studies were performed) than they do in the wild, measuring their brain activity in captivity can lead to the wrong conclusions."

The research team got around this problem by using a technique developed for monitoring brain activity in humans in conjunction with a newly developed miniature recorder for neurophysiological data in order to monitor sleep in the wild.

In addition to two brain activity sensors worn as a cap on their heads, three adult three-toed sloths, Bradypus variegatus, also were fitted with radio-telemetry collars and accelerometers so that their exact locations and movements could be monitored during the next three to five days. The activity of two other sloths was monitored via radio-telemetry collar alone, for approximately seven months using a unique Automated Radio Telemetry System in place on the island. "The beauty of the automated telemetry system is that it makes a new suite of animal behaviour studies possible," said Martin Wikelski, one of the initiators of the system and co-author of this study. Wikelski, now director at the Max Planck Institute for Ornithology, is researcher at Princeton University and research associate at the Smithsonian. "We certainly encourage researchers who would like to monitor animals in the wild to join our ongoing studies or initiate work using the system," says Wikelski.

The placement of sensors on sloths living in treetops demonstrates the feasibility of understanding a complex behaviour, such as sleep, in the intricate tropical forest environment. It also will lead to more refined, comparative sleep research.

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