In the first study of its kind, scientists have discovered proof that ring-tailed lemurs prefer to sleep in the same bed night after night, usually in caves and crevices. It marks the first time this sort of behavior has been observed in primates. Though Fusui langurs have been found sleeping in caves, this behavior has been documented as a direct result of deforestation, with the monkeys traveling from cave to cave.

University of Colorado Boulder anthropology Associate Professor Michelle Sauther led the study published in the journal Madagascar Conservation and Development.

“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,” she said. “What we are seeing is a consistent, habitual use of caves as sleeping sites by these primates, a wonderful behavioral adaptation we had not known about before.”

Most primates (including chimpanzees, our nearest human relative) sleep in nests, which they build and rebuild night after night as they move from place to place. The researchers anticipated the same from the lemurs in Madagascar, expecting to see them descend from the trees each morning. They were in for a surprise.

“They seemed to come out of nowhere, and it was not from the trees,” she said. “We were baffled. But when we began arriving at the study sites earlier and earlier in the mornings, we observed them climbing out of the limestone caves.”

The caves proved to be what tipped the researchers off that something was different. The caves are located next to the so-called “Spiny Forest” which, as its name implies, isn’t the most comfortable place to settle down for the night. On top of that, the forest is relatively open, making it an easy hunting ground for the fossa, a mongoose-like animal and top predator to the lemurs.

“We think cave-sleeping is something ring-tailed lemurs have been doing for a long time,” she said. “The behavior may be characteristic of a deep primate heritage that goes back millions of years.”

It turns out, the ring-tailed lemurs still need the trees: Deforestation in Madagascar has them placed on the IUCN’s endangered species list.