During Alaskan winters, monitored Black Bears dropped their temperatures only a modest 5.5 degrees Celsius on average. Using a standard physiologist's calculation, such a temperature drop should yield a 65% slower metabolism. But measurements demonstrate that the bears' metabolisms is reduced to only 25% of the basic summer metabolism rate. Also, the heart rate went from 55 beats per minute before hibernation to 14 erratic beats per minute in winter. This unexpected response has not been found in any other hibernating mammal.
The recent study that found this result was the first to successfully undertake continuous monitoring of metabolic rate and body temperature throughout the bear's hibernation. Earlier studies used only intermittent sampling with older instruments, indirect evidence, or studies of bear in circumstances in which lots of people were nearby.
The researchers also found that when the bears started moving again in Spring, their metabolism took several weeks to move back to the "normal" range. Even with half-speed metabolic rates, the bears still displayed "normal bearish behavior."
The irony of the result is that until this study, many researchers considered bear hibernation as a "different" and "lesser form" compared to the hibernation of other mammals, such as ground squirrels.
The study can be found (in 2 parts) at http://www.sciencemag.org/content/331/6019/906.figures-only and http://www.sciencemag.org/content/331/6019/906/suppl/DC1.