- New research based on more than a decade of field observations indicates that wild Sumatran orangutans balance their sleep needs much like humans do, by napping during the day.
- The team found the shorter an individual’s sleep period overnight, the longer was its cumulative nap period the next day.
- Orangutan sleep also varied with temperature, activity levels and proximity of others, indicating the great apes have to trade off between foraging, socializing, traveling and sleep.
- Deforestation, habitat fragmentation and climate change are all likely to influence the capacity of orangutans to manage their sleep needs, with clear consequences for their overall health and survival.
When it comes to sleeping, few animals are as fastidious as orangutans. Each evening, as twilight approaches, the tree-dwelling great apes meticulously weave a cup-shaped nest out of branches, twigs and leaves, line it with comfortable soft foliage, and bed down for the night. The nest secures them high in the forest canopy and allows them to grab up to 12 or 13 hours of shut-eye per night.
For more than a decade, researchers studying a group of Sumatran orangutans (Pongo abelii) in the forests of Indonesia’s Aceh province had observed individuals also taking daytime naps in more makeshift nests. They wondered whether the napping orangutans were catching up on poor sleep the previous night, in much the same way humans might flop down in an armchair or sofa for a quick power nap.
In a new study published in Current Biology, the researchers show that the orangutans do indeed compensate for a poor night’s sleep by napping more the following day. They found the shorter an individual’s sleep period overnight, the longer its cumulative nap period the next day.
“We were always wondering over many years, is this really what is happening? Are they really napping more when they had a shorter night’s sleep?” Caroline Schuppli, study co-author and group leader at the Max Planck Institute of Animal Behavior at the University of Konstanz in Germany, told Mongabay in an interview. “To see it materialize in the data was really nice.”
The findings are based on 14 years of field observation data collected at the Suaq Balimbing research station in Gunung Leuser National Park, and represent the first quantitative assessment of factors that shape orangutan sleep patterns in the wild.
Schuppli and her colleagues from Germany, Canada, Indonesia and the U.K. also investigated how a range of social and ecological factors affected the orangutans’ sleep patterns.
They found that the presence of other orangutans sleeping nearby correlated to shorter night sleep and daytime nap periods, suggesting that social interactions might interfere with sleep. They also observed that individuals that traveled farther during the day slept for shorter durations at night, and that higher temperatures during the day resulted in shorter nap times. According to the study, these results indicate possible trade-offs between foraging, socializing, traveling and sleep.
Significant energetic demands
To arrive at their results, the team observed the sleeping habits of 53 adult orangutans over 276 nights and 455 days between 2007 and 2021. They collected data on the length of time the orangutans were settled in their overnight nests and the cumulative time they spent napping during the day.
The time the orangutans spent resting in their sleeping nests isn’t a direct measure of actual sleep, the authors point out. Rather, it’s the best available proxy for their “sleep period” given the challenges of studying wild animals in dense forests. “We can hear them rustling around, getting comfortable, and then everything becomes quiet,” Schuppli said. “For us, these were clear indicators of them going to bed and falling asleep.” In the morning, the researchers would observe them waking up and leaving their nests.
Sumatran orangutans make a particularly good subject to study sleep because of the energetic demands of their large brains, hefty bodies and arboreal lifestyle, the authors note. Activities like climbing through the canopy, finding seasonally variable food, and navigating social relationships are also tiring and require plenty of rest to recover. Because orangutans mostly live along, they can choose when and where to rest. This makes them different from social primates like baboons or chimpanzees, allowing the researchers to study how they make these choices.
Research has shown sleep deprivation weakens the immune system and can impair mental performance. With their steep energetic demands, “reduced sleep may be particularly detrimental for orangutans,” the study says.
Daytime napping is one way the negative effects of lost sleep can be remedied, according to Schuppli. However, the finding that orangutans napped for shorter periods on hotter days is concerning – the climate crisis and associated rising temperatures could make it harder for them to catch up on lost sleep, with knock on consequences for their health and survival. “It’s not just habitat fragmentation and habitat loss that’s affecting orangutans, it’s also climate change,” she said.
When it came to compensating for poor sleep, the researchers were surprised to find the orangutans increased their napping by roughly 10 minutes for every hour of sleep lost the previous night. “This is surprisingly similar to what is recommended for humans to catch up on their lost sleep.” Studies have suggested a power nap of five to 15 minutes can restore how we perform mentally, for instance.
Another finding that intrigued the authors was the reduced time spent sleeping when orangutans nest near others. Sumatran orangutans are a semi-solitary species, Schuppli said, so sharing nests is rare, occurring only between mothers and their nursing young. But even the impact of sleeping within 50 meters (160 feet) of another orangutan seems to be enough to reduce their length of time in slumber, the team found.
This adds to what’s already known about the costs of being social for orangutans. As a large-bodied fruit eater, competition can arise when several orangutans dine on ripe fruit in the same tree, for instance. Despite this, and the new evidence that sleeping near others can reduce rest, orangutans still choose to be social, indicating there might be deeper reasons for sociality that are vital to orangutan survival that we don’t yet fully understand, the authors say.
Forests give animals flexibility to survive
The new study offers a “rare and valuable window” into how wild orangutans balance their need for sleep with environmental pressures, said Kathleen Reinhardt, an evolutionary anthropologist at the University of Calgary, Canada, who wasn’t involved in the study.
“By showing that even large-bodied apes use naps to maintain sleep homeostasis, it not only deepens our understanding of primate evolution but also underscores the importance of protecting habitats that allow these animals the flexibility they need to thrive,” Reinhardt told Mongabay in an email.
Previous studies looking at how primates manage their sleep needs in the wild have focused on the much smaller lorises and baboons, Reinhardt added. The new findings on orangutans offer valuable insights into how great apes regulate their sleep, adding to our understanding of the adaptive and evolutionary role of sleep.
While the ability to compensate for lost sleep through naps suggests orangutans have a certain resilience to disturbances, Reinhardt warned that increasing anthropogenic pressures in their forest environments could push them too far.
“If orangutans have to travel longer distances due to deforestation and habitat fragmentation, they might have less time or fewer safe places to nap and rest, leading to cumulative sleep debt over time. The study highlights just how interconnected sleep, energy balance, and movement ecology really are for these animals,” she said.
Sumatran orangutans are considered critically endangered by the IUCN, the global wildlife conservation authority. Fewer than 13,900 individuals are thought to remain in the wild, confined to tracts of intact forest in the north of Sumatra — mostly in the Leuser Ecosystem, where Gunung Leuser National Park is located. Even here, however, they’re threatened by habitat loss driven by logging, mining and conversion of the supposedly protected forests to plantations.
Why sleep?
A lot is known about how animals sleep in the wild, from African elephants that sleep mostly standing up for as little as two hours a day, to dolphins that rest just half of their brain at a time, to migratory birds that shut off their brains for 10-second bursts mid-flight, to lions and tigers than can snooze for up to 20 hours a day. However, there’s still a lack of scientific consensus on exactly why animals sleep, although many theories exist, from physiological repair and functioning, to memory generation and learning.
This is partly due to the challenges of studying sleep patterns in the wild, according to Meg Crofoot, a professor of animal behavior at the University of Konstanz and co-author of the new study. It’s tricky, for instance, to measure the electrical activity in the brain of wild animals using devices traditionally used in laboratory settings. “No self-respecting wild animal is going to let you stick one of those caps on its head,” she said.
Crofoot said this is what makes the orangutan study so fascinating. The long-term direct observations of the wild population around the Suaq Balimbing research station in Sumatra are a “unique and powerful data set” that could enable many further sleep questions to be answered, she said, such as the effects of lack of sleep on wild animals.
As a next step, the team aims to understand more about the consequences of a good versus a poor night’s sleep in wild orangutans. This would entail studying changes in their mental performance or social abilities during key tasks such as foraging. “Some foods orangutans eat are quite complex — they require a lot of steps of processing, sometimes using tools, before they get to the part they can ingest,” Schuppli said. “So we could look at this as a measure of cognitive performance, among other things.”
For Crofoot, the major strength of the orangutan study is its use of long-term data on the behavior of the Suaq Balimbing orangutans — more than a decade of data in this case. However, many such programs are facing uncertainty amid global conservation funding cuts, she noted. “These data sets become more and more valuable each year that data is accumulated. Once there’s a data lapse, we can’t go back and recover it.”
Studying the behavior and perceptual realities of great apes through such studies is valuable not only to understand how anthropogenic pressures will likely impact other species, but also in revealing insights into our own humanity. As Schuppli puts it: “We risk losing a global heritage of what those data can tell us about ourselves and who we are as a species.”
Carolyn Cowan is a staff writer for Mongabay.
Banner image: An adult Sumatran orangutan. Image by Rhett A. Butler / Mongabay.
Citations:
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