Hibernation is a biological mechanism whereby animals conserve energy, especially during winter when food resources are scarce. During hibernation, stored body fat is used up at a slower rate. Hibernation is characterized by a state of inactivity and low metabolic functioning, low body temperature, and slow breathing.
Many animals hibernate, including bats, ground squirrels and other rodents, mouse lemurs, many insectivores, monotremes, some snakes, and marsupials. There is some discussion as to whether the most famous hibernator, the bear, actually hibernates. During the bear’s denning period, his metabolic state is significantly less depressed than that of other smaller hibernating mammals. As a consequence, many people use the term “winter lethargy” to describe this state for bears, rather than hibernation.
During winter lethargy, a variety of physiological changes occur in red blood cell shape and size as well as in the recirculation of metabolic water. During winter lethargy for bears, the body temperature remains relatively stable at 31° C, down from 37° C. Additionally, the bear can be aroused easily.
The time of winter lethargy is a good opportunity for bear biologists to gather data about bears, replace batteries in radio collars on research subjects, and better understand the species.
Delayed Implantation
Female bears demonstrate delayed implantation. Although bears mate during the summer, the implantation of the female’s embryo “waits” until late fall. If a female has not acquired sufficient stores of fat and other energy during the year, the embryo will not implant and, therefore, the mother will not become pregnant. Lactation is enormously energetically expensive for mother bears, so delaying embryo implantation ensures that mother and cubs have a good chance to survive the following spring, summer, and fall.