Scientists curious about the winter time vanishing act of a Madagascar dwarf lemur type were astonished to find the animals burrowed deep into the soil, curled up for a months-long sleep.
The discovery, announced in a study published in Nature Scientific Reports today, makes the island country's eastern dwarf lemurs the only primates in the world known to hibernate underground.
The fat-tailed lemur, a cousin from the slightly warmer, drier forests of western Madagascar, was already known to hibernate in tree holes for about seven months of the year.
Researchers long suspected the eastern lemurs may be doing the same, but could never find them.
"You don't see them, trap them or find them during the dry season (winter time) while walking the forests at night," study co-author Marina Blanco of Germany's Hamburg University told AFP.
"They had to go somewhere..."
And so the team fitted radio-transmitter collars on 12 lemurs from two eastern species in summer, and waited.
The species -- Sibree's dwarf lemur and Crossley's dwarf lemur -- live in the forest of Tsinjoarivo.
Setting out in winter with signal trackers, the team fully expected to find the lemurs sleeping in tree holes.
"We were tracking the collar's signal and pointing our antenna up in the air, towards the tip of a tree. But the signal was coming from the ground, so we thought the animal had lost the collar," said Blanco.
"We looked around and didn't see anything so we started to dig up the area and found a furry ball, the dwarf lemur was curled up and cold to the touch, still wearing its collar."
The tiny bundles weighed about 250 gram to 350 gram depending on which species they belonged to.
They hibernated for anything from three to six months buried 10 to 40 centimetres under a spongy layer of tree roots, soil and decaying plant matter.
It is uncommon for primates to hibernate -- in fact the western fat-tailed lemur was previously the only primate known to do so.
It is also rare behaviour for animals in tropical regions -- residents of colder climes like polar bears, hedgehogs or squirrels are usually the ones who find it necessary to hide out from winter.
During hibernation, the metabolism slows down and the core body temperature reaches ambient levels -- meaning the body has to work less hard to stay alive.
During the Madagascar winter, lemurs are exposed to drastic daily temperature fluctuations of as much as 30 degrees Celsius.
