Animals in the national parks have many different strategies to survive the winter. Some migrate to warmer weather, others adapt with thicker or camouflage winter coats, and still others go into a period of winter dormancy with temperatures and food sources are low.
While some animals are easier to spot in the winter months as they are more active and easily distinguished against the white backdrop of snow, hibernating animals will be rarely seen. The combination of cold temperatures and food scarcity make winter dormancy critical to the survival of many species.
Remember learning about how all rectangles are squares, but not all squares are rectangles? Well, you can think similarly about hibernation and winter dormancy. While hibernation is a form of winter dormancy, not all types of winter dormancy are considered true hibernation.
Hibernation is often used as a broad term for any animal that goes into a period of winter dormancy, but to be considered a “true hibernator” a species needs to meet a specific set of requirements: reduced metabolism, slower heart rate, and lowered body temperature.
So which animals go into winter dormancy and which are true hibernators?
Bears
Bears are perhaps the most well-known winter nappers, and they are good at what they do. After preparing for months by going into a state called hyperphagia, they are ready to find their dens and enter a lighter state of sleep called torpor.
Torpor is sometimes called “light” hibernation, but the largest difference is that torpor is an involuntary response to low temperatures. During torpor, a bear’s body temperature reduces and it can sleep for more than 100 days without eating, drinking or passing waste.
During this phase, bears are a self-contained system. Their bodies recycle their own water, synthesize proteins to maintain muscle and organ mass, and metabolize fat. Although they emerge in the spring with 15-30% less body mass, often their bones and overall health is remarkably unchanged.
Bears begin and end their hibernation at different times depending on their environment. In the colder parks, such as Yellowstone and Grand Teton, black and grizzly bears’ hibernation period begins in October/November and ends in March/April.
In warmer parks such as Big Bend and Yosemite, black bears are dormant for only three to four months (January–March) each year. While their metabolism does slow down and they spend time resting in dens, they are awake much of the time, and may periodically emerge to forage.
Rodents
Rodent dormancy can span anywhere from five to eight months, beginning in late September or October and ending between March and May.
Many rodents, such as squirrels and chipmunks will wake up periodically if their body temperature gets too low or to eat some food from their stored cache.
Rodents are classified as true hibernators because they reduce their heart rate and metabolism and can significantly drop their body temperature while waiting out the winter months.
Bats
While some species of bats migrate to warmer climates in winter months, others roost in places like caves, old mine shafts or under bridges. These bats are considered “true hibernators,” with their metabolism, heart rate and body temperature all dropping while they’re waiting out the cold or waiting for food to become plentiful in tWinter Survival in the Parks: How Animals Hibernate and Adapthe spring.
During hibernation, a bat’s heart rate drops from 200-300 beats per minute to 10 beats per minute, and it can go minutes without taking a breath.
Some species even combine winter survival strategies and hibernate for parts of the season before migrating to warmer areas.
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