Winter is the most difficult time of year for most animals and they have developed various strategies for surviving these challenging months, with hibernation probably the most well-known.
Hibernation is a complex and poorly understood process. The body continues to work; it just works in slow motion. Rather than just a way to avoid frigid temperatures, hibernation likely evolved also as a way for animals to survive during times of food shortage.
Hibernators include ground squirrels, groundhogs and mice. Hibernation can be triggered by a variety of factors, including lower ambient (surrounding air) temperatures, decreased food availability and shorter days.
During hibernation, physical processes in the body undergo profound changes. For example, in the little brown bat, a common Wyoming resident, researchers have found that hibernating individuals drop their heart rate from about 200 beats per minute to 20 and can commonly go 45 minutes without taking a breath.
But perhaps the most amazing feature is the lowering of body temperature, often several degrees. For most mammals, normal internal body temperature is between 98 and 100 degrees Fahrenheit. Because ambient temperatures are usually lower and fluctuate often, maintaining this temperature requires substantial amounts of energy.
Small mammals such as shrews and bats use a large amount of their energy intake just maintaining body temperature. Since food is often limited during winter months and the temperature may drop substantially, it makes sense to conserve as much energy as possible and one way to do this is to lower internal body temperature closer to the surrounding air temperature. One incredible example is the arctic ground squirrel, which can lower its body temperature to 27 degrees Fahrenheit during hibernation.
In contrast, the human body can tolerate very little change in body temperature without catastrophic consequences. If the body temperature of a human drops even a few degrees, he or she begins to lose the ability to function and death can occur if the internal body temperature drops below 85 degrees Fahrenheit.
In addition to hibernation, there is a condition called torpor that has less dramatic bodily changes and is characteristic of larger mammals such as bears and badgers. The difference between torpor and hibernation is complicated, with torpor sometimes occurring on a daily basis in some animals (some hummingbirds) or for more extended periods. During torpor, breathing and heart rates slow down and appetite decreases, but body temperature drops only slightly, just 12 degrees in bears. This is because it would waste large amounts of energy to drop and raise temperatures in these larger bodies.
Torpor in bears can last several months. They can lose up to 30 percent of their body weight during this time, but remarkably, do not lose much, if any, muscle mass or bone density, due to complicated chemical recycling and conversion of waste products into protein.
Bears can go for up to 100 days without waking, eating, drinking, urinating or defecating. However, they are more alert and can be aroused more quickly if they sense danger than animals in hibernation. Hibernators such as ground squirrels will awaken every few days to raise their body temperature, move around, eliminate bodily waste and eat before reentering their slowed metabolic state.
Of course, hibernation is not the only way animals survive during times of cold temperatures and food shortage. Many animals, such as elk and mule deer, survive the winter months by moving to areas with more food availability, sometimes traveling dozens of miles or simply moving from higher to lower elevations. In 2012, researchers discovered the longest documented mule deer herd migration in the world in Wyoming, with hundreds of deer traveling 150 miles one way between their winter range in the Red Desert and their summer range in the Hoback Basin, a journey documented with maps and narrative on the Wyoming Migration Initiative’s website.
Other animals store food throughout the year to use it during the lean times. The American pika, found in the highest elevations of the Bighorns and other mountain ranges in Wyoming, is a champion food stasher. A member of the rabbit family, this small mammal lives year round between 8,000 and 13,000 feet, one of the harshest environments on earth. While other animals such as marmots, ground squirrels and chipmunks also live in these high elevations, they hibernate to escape the travails of the winter months. However, pika remain active, scurrying beneath the snow in tunnels and eating on stashes of vegetation they accumulate during the summer. Known as haypiles, these stashes of food can be as large as a bathtub.
All of these life strategies show the extreme measures animals are capable of when adapting to their environments.
Christina Schmidt is public information specialist for Wyoming Game and Fish Department — Sheridan Region.