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.