Scientists have discovered that some ring-tailed lemurs in Madagascar regularly retire to limestone chambers for their nightly snoozes.
This is the first evidence of the consistent, daily use of the same caves and crevices for sleeping among the world's wild primates, researchers said.
The ring-tailed lemurs may be opting to sleep in caves for several reasons, said University of Colorado Boulder anthropology Associate Professor Michelle Sauther, who led the study.
While the cave-sleeping behaviour is likely important because it provides safety from potential predators, it also can provide the primates with access to water and nutrients, help to regulate their body temperatures during cold or hot weather and provide refuge from encroaching human activities like deforestation, she said.
"The remarkable thing about our study was that over a six-year period, the same troops of ring-tailed lemurs used the same sleeping caves on a regular, daily basis," Sauther said.
"What we are seeing is a consistent, habitual use of caves as sleeping sites by these primates, a wonderful behavioural adaptation we had not known about before," said Sauther.
Although sleeping in caves by ring-tailed lemurs - which are found only in Madagascar - has likely been going on for millennia, it is only now being recognised as a regular behaviour, said Sauther.
The endangered Fusui langurs, slender, long-tailed Asian monkeys roughly 2 feet tall, also have been documented sleeping in caves but as a direct result of extreme deforestation, moving from cave to cave every few days.
There also have been isolated reports of South African baboons sleeping in caves, researchers said.
The study was published in the journal Madagascar Conservation and Development.
