Like humans, orangutans nap after a rough night

Whether it’s been a stressful morning, a big Christmas lunch or just a bad night’s sleep, sometimes there’s nothing better than a nice afternoon nap.  

In a recent collaboration, researchers from the University of Konstanz, Germany, Universitas Nasional, Indonesia and the Max Planck Institute of Animal Behaviour (MPI-AB) have uncovered that orangutans also enjoy dozing off for a bit during the day. 

“Moving through the canopy, finding food, solving problems, navigating social relationships; these are all tiring and cognitively demanding tasks,” says the study’s first author, Alison Ashbury.

“When an orangutan doesn’t get enough sleep, it does what any sleep-deprived human might do: it climbs into bed, lies down, and takes a nap.” – Alison Ashbury

Until now, studies have never focused on how orangutans cope with sleep deprivation and getting enough hours of rest. 

But in tracking the sleep and napping patterns of some of our closest evolutionary ancestors, the research team has opened a door into developing a better understanding of how sleep, including human sleep, evolved. 

“Studying sleep in the wild, in the natural social and ecological conditions under which it evolved, is important to broadening our understanding of the evolutionary origins and the ultimate functions of sleep,” says co-author, Meg Crofoot, director at MPI-AB and a professor at the University of Konstanz.

To conduct this study, the researchers monitored 53 adult orangutans from the Suaq Balimbing Monitoring Station in Sumatra, Indonesia. Over the course of 14 years, the team recorded a total of 455 days and nights of orangutan sleep behaviour. 

How to track an orangutan’s sleep patterns

A big challenge the study had to overcome was to develop a technique to track the sleep patterns of orangutans in the wild, without disrupting them. 

 “From our point of view on the ground, we usually can’t see orangutans at all in their night nests,” says Caroline Schuppli from MPI-AB. “But we can hear them rustling around, getting comfortable.” 

Orangutans sleep in the rainforest canopy where each night they build their ‘nests’ so the research team were able to listen out for a long stretch of silence in between the orangutans building their nests and then leaving these nests in the morning. 

This stretch of silence, which on average was almost 13 hours long, was referred to as the ‘sleep period’. 

But not every orangutan was getting a full 13 hours of sleep. 

“We thought it was really interesting that just being near other orangutans when building a night nest was linked to shorter sleep periods,” says Ashbury.

“Imagine you stay up late hanging out with your friends, or your roommate is snoring so loudly in the morning that you get up early. I think it’s a bit like that. They’re prioritizing being social over sleeping, or their sleep is being disrupted by others nearby, or even both.”

For these orangutans that lost sleep, the team noticed a trend. An orangutan would nap for 5 to 10 minutes longer for every hour of sleep they missed from the night before.

 “For people, even a short nap can have significant restorative effects,” says co-author Meg Crofoot, director at MPI-AB and a professor at the University of Konstanz.

“It’s possible that these naps are helping orangutans reset physiologically and cognitively after a poor night’s sleep, just like they do in humans.”

Orangutan naps wikicommons
A Borean orangutan in a tree nest. Credit: Joeavison1/Wikimedia Commons.

High workload, more naps

Among the observed orangutans, the Suaq orangutans built the highest number of nests during the day and displayed the highest amount of cognitively demanding behaviours. 

“This may be linked to their relatively high propensity for daytime nest use,” says Schuppli. 

“Either they need these high-quality naps to meet their cognitive demands, or their cognitive abilities can come about because they take high-quality naps in day nests so often.”

While the research contributes to a growing field of sleep studies, Crofoot hopes this inspires more sleep studies to step out into the wild. 

“Why did animals, from humans to primates to spiders to jellyfish, evolve to spend such large portions of their lives in this vulnerable unconscious state?,” says MPI-AB’s director. 

“If we’re going to answer this question, we need to bring sleep research out of the lab and into the field. Studies such as this one contribute to that effort.”

The study was published in Current Biology.

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