Of all the things you wouldn't expect to find in a tropical rainforest, hibernating primates may be the cutest.It was less than 10 years ago that scientists first discovered dwarf lemurs in western Madagascar hibernate the dry season away in hollow trees. Now, they report the hibernation habits of the island's eastern dwarf lemurs: They spend 3 - 6 months of the year buried underground.Dwarf lemurs (genus Cheirogaleus) are the only primates known to regularly hibernate. In a 2004 study, a German team reported fat-tailed dwarf lemurs (Cheirogaleus medius) in the western forests of Madagascar hibernate in tree holes for seven months of the year. They found the hibernating lemurs' body temperature depends on how well its tree hole is insulated. In well-insulated hollows, lemurs maintained a relatively constant body temperature and woke up every 10 - 14 days for brief spells of arousal. The body temperature of lemurs in poorly insulated holes, however, fluctuated widely with the ambient temperature, and they did not go through any periods of arousal. It appears that hibernating fat-tailed lemurs regulate their temperature the reptilian way: hitching their thermostat to the environment instead of maintaining it internally. Since waking up during hibernation is energetically costly, this strategy allows them to conserve energy stores.But why do tropical animals need to hibernate in the first place? In the western deciduous forests of Madagascar where fat-tailed lemurs live, winter temperatures can rise to over 86 °F, so it's not a matter of cold temperatures. For western dwarf lemurs, hibernation is probably a response to the scarcity of fruit (a critical food source) and water during the winter.Madagascar is a land with varied habitats, so eastern dwarf lemurs face different challenges. Unlike the fat-tailed lemur, eastern dwarf lemurs have to cope with (relatively) cold weather. The rainforests on the high plateaus of Madagascar are home to the island's coldest environments. During the winter, temperatures consistently drop to nearly 40 °F and never rise above 86 °F during the day. Fruits are also scarce in eastern forests during the winter.A team of researchers from the Duke University Lemur Center, the University of Hamburg, and the University of Antananarivo in Madagascar set out to uncover the hibernation habits of the island's eastern dwarf lemurs. Specifically, they looked at two species that live in Tsinjoarivo, a high-altitude rainforest in central-eastern Madagascar. Sibree's dwarf lemur (C. sibreei) and Crossley's dwarf lemur (C. crossleyi, also known as the furry-eared dwarf lemur) are between 7.5 and 10.5 inches long, with another 6-7 inches of tail. Sibree's dwarf lemur is slightly smaller, weighing in at around half a pound, while Crossley's dwarf lemur gets up to about three-quarters of a pound.Twelve dwarf lemurs (eight C. crossleyi and four C. sibreei) were fitted with radio transmitter collars that allowed the researchers to track the animals to their hidden hibernation spots. The collars also recorded the skin temperature of their wearers every 60 minutes.The researchers discovered both species of eastern dwarf lemur hibernated between three and six months of the year. And unlike the tree-hibernating western dwarf lemurs, the eastern dwarf lemurs all hibernated in underground burrows. The researchers found the dwarf lemurs buried 4- 16 inches below ground, beneath a spongy later of secondary roots and root hairs, humus, and leaf matter. The lemurs always hibernated individually (one lemur per burrow) and rarely changed locations over the course of hibernation season.  The underground burrows are better insulated than the tree holes used by western dwarf lemurs. The soil temperature is generally lower than the ambient temperature during the day but higher during the night. Underground hibernation might protect the lemurs from exposure to drastic temperature changes during the day and reduce the potential risk of freezing at night.In their underground burrows, dwarf lemurs are able to maintain low, relatively constant body temperatures of about 59 °F for more than 10 days before undergoing energetically costly arousals. During the winter, the ambient temperature in high-altitude forests like Tsinjoarivo does not get high enough for the dwarf lemurs to use passive warming to raise their body temperatures. Since passive warming is not an option, maintaining a stable body temperature and avoiding mid-hibernation arousals as much as possible might be the most energetically efficient strategy to escape the challenges of winter.In addition to the thermal advantages of hibernating underground, there may also be safety advantages. When they're not hibernating, both Crossley's and Sibree's dwarf lemurs spend most of their time high in the canopy. During the non-hibernation period, the researchers observed large predatory birds flying by tree holes used by the lemurs as daytime sleeping sites and at least two dwarf lemurs were killed by such birds. While there is currently no evidence of predation of hibernating underground dwarf lemurs, not enough observations have been gathered to say with certainty whether lemurs are safer underground. The researchers report that the burrows they found were inconspicuous and may be too deep for predators to sniff out.Given the thermal advantages of hibernating underground, why don't western dwarf lemurs do it? The researchers believe what it might come down to is the type of soil. The soil in western Madagascar is harder and drier than the soil in eastern rainforests. Dwarf lemurs don't have claws, so burying themselves might only be possible in spongy, soft, moist soil. [youtube height="HEIGHT" width="WIDTH"]http://www.youtube.com/watch?v=EKtlyAw2NEw&feature=youtu.be[/youtube]Video: Sibree's dwarf lemur is retrieved from underground hibernaculum at Tsinjoarivo forestCredit: J.F. RanaivoarisoaReference: Blanco, M.B., Dausmann, K.H., Ranaivoarisoa, J. F. and Yoder, A.D. (2013). Underground hibernation in a primate. Scientific Reports 3: 1768. DOI: 10.1038/srep01768.