Warm hibernation

Source: nationalgeographic.com
American black bears hibernate from five to seven months without eating, drinking, urinating or defecating. They then emerge from their dens, leaner but fully healthy. We have known for quite a while that these animals slow their metabolism in order to get through this extended period.
The expectation has been that bears would have to lower their body temperature drastically in order to slow their metabolism, as occurs in many other animal species. New research, however, has discovered that this is not the case. Four ‘nuisance bears’ – problem bears that live too close to people and are usually euthanized – were fitted with sensors to measure variables such as temperature and heart rate. The bears were brought to artificial dens, wooden crates in an undisturbed forest, where all four went into hibernation and the measurements could be taken.

It was discovered that the bears’ temperature (normally 33 °C) dropped only 5-6 degrees, while their metabolism was reduced up to 25 per cent. The heart rate dropped from its norm of 55 to a mere 9 beats per minute. In intervals between breaths, heartbeats could be as much as 20 seconds apart. The reduction in temperature is actually variable. The temperature of a pregnant black bear in the study, however, did not fluctuate as much as that of the other bears, probably to protect the foetus.

How the black bear can slow its metabolism so dramatically is not completely understood, though there are theories. The same mechanism could be used, for example, as in the marmot: the mass of the digestive tract is drastically shrunk down, to be built up again after hibernation.

During hibernation the black bears don’t actually sleep the whole time. Periods of deep sleep alternated with some activity. The researchers heard the bears during these periods, licking themselves or moving around.