Wild Sumatran orangutans made up for lost nighttime sleep with longer daytime naps, according to a study published in the journal Current Biology. Researchers analyzed 14 years of data on 53 adults from the Suaq Balimbing monitoring station in Sumatra’s rain forest. For every lost hour of the nearly 13-hour night-time sleep period, an orangutan napped five to ten minutes longer the following day.
“We logged 455 full day–night cycles and saw that on 41 percent of the days the apes took at least one nap, averaging 76 minutes,” said Alison Ashbury of the Max Planck Institute of Animal Behavior (MPI-AB), according to Phys.org.
Each evening an adult orangutan spent about ten minutes weaving branches into a sturdy platform, then slept for almost 13 hours. After leaving the nest at dawn to forage and travel, many animals built simpler “day nests” in less than two minutes for midday rest.
“From our vantage point we can’t see them in the night nests, but we hear rustling until everything goes silent; in daylight they sometimes nap two, three, even four times,” said Caroline Schuppli, a co-author, in an email cited by Phys.org.
Day nests were less elaborate than night nests yet still provided a secure place to sleep. The MPI-AB team identified three consistent reasons for shorter night sleep: colder temperatures, longer travel distances, and the presence of another orangutan near the nest. “Simply being near others when building a nighttime nest seemed to shorten sleep—rather like staying up late with friends or waking early because a roommate snores,” Ashbury said, according to El Mundo.
“Many animals nap, but we have not found such a strong link between daytime napping and poor night sleep in group-living primates,” said Meg Crofoot, director at MPI-AB, in comments to El Mundo. Crofoot suggested that coordinating nap times in groups can be difficult, whereas the semi-solitary lifestyle of Sumatran orangutans allows flexible rest.
The Suaq population is known for complex tool use and foraging behaviors. “These naps may help orangutans recover physiologically and cognitively after a bad night’s sleep, much as they do in humans,” Crofoot added.
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