Arctic ground squirrels hibernate with bellies colder than freezing water
A hibernating Arctic ground squirrel can hold its abdomen at minus 2.9 °C for more than three weeks. A Madagascan lemur hibernates through winters topping 30 °C. And bears, once dismissed as mere deep sleepers, turn out to recycle their own urine to keep muscles and bones intact for months.
Hibernation is a seasonal slowdown in which an animal cuts its metabolism, body temperature, breathing and heart rate, usually to survive winter when food is short. The term once covered only deep hibernators such as rodents, but it is now defined by active metabolic suppression rather than a particular temperature drop, and many experts see daily torpor and hibernation as points on one continuum. Its summer counterpart is aestivation. Beforehand, large species gorge and lay down fat, while many small ones stash food instead.
Obligate hibernators, including many ground squirrels, European hedgehogs, monotremes and marsupials, shut down every year whatever the weather or food supply, their bodies cooling almost to ambient levels. Their torpor is broken by periodic warm-ups whose purpose is still debated; one favoured idea, backed by Arctic ground squirrel data, is that they must rewarm to pay off a sleep debt. Facultative hibernators do it only when cold or hungry. Chipmunks fall in this camp, and among prairie dogs the white-tailed species is obligate while its black-tailed cousin is facultative.
No primate was known to hibernate until the fat-tailed dwarf lemur of Madagascar was found spending seven months a year in tree holes. In poorly insulated holes its temperature tracks the air; in snug ones it stays steady with regular arousals, showing that slowed metabolism need not mean a cold body. Cold-blooded animals have their own dormancy, called diapause in many invertebrates, and some people call reptile winter rest brumation.
Bears were long denied the title because their temperature falls only 3 to 5 °C, against drops of 32 °C or more in other species. A 1977 heart-rhythm study by the Folks, then work on black bears in 2011 and brown bears in 2016, settled the matter. Hibernating bears neither eat nor drink, getting water from burning fat, recycle proteins and urine so they need not urinate for months, and protect their skeletons by raising the hormone CART and damping a protein that blocks bone growth. Gut microbes turn urea into ammonia that helps rebuild amino acids like leucine. Pregnant black bears give birth during the season and can lose 15 to 27% of their weight.
Source: Hibernation
