Animals that achieve true hibernation, which is an inactive, near-death state characterized by low body temperature and slow heart rate and breathing, include bats, hedgehogs and certain species of rodents such as ground squirrels, hamsters, marmots and hazel mice. Animals that achieve a less intense state of dormancy that is commonly called hibernation include bears and other mammals, as well as various species of reptiles, amphibians and fish.
True hibernating animals are sometimes in a state so close to death that they may even appear to be dead. The temperature of their bodies is near zero, they take only a few breaths per minute, and their heartbeats are so slow as to be virtually undetectable. Some depend upon a reserve of body fat for nutrition, while others, such as some types of rodents, wake up every few weeks, ingest stored food, and go back to sleep.
On the other hand, the body temperature of bears does not drop drastically when they hibernate, but their metabolic rate drops to about one-quarter of normal. During hibernation, black bears can go over three months without drinking, eating, urinating or defecating. Their warm pelts help them to maintain body heat, so their temperature drops only slightly. They receive water from the breaking down of fat tissues and protein from the breaking down of muscle and organ tissues. The urea in their urine does not become toxic but somehow forms new protein. They do not become dehydrated, but maintain almost perfect water balance.