As the weather cools down, animals must find ways to stay warm. This is especially important for small animals as they lose body heat faster than larger animals. One way to stay warm is by increasing your metabolism to create body heat, although this process requires a lot of energy. That can be a problem in areas where food may be scarce in the winter. Although some animals, like the grey squirrel below, anticipate this need and stock up on foods for the winter – much to the dismay of many homeowners.
Other animals, like the 13-lined ground squirrel, simply hibernate through the winter. Hibernation allows them to reduce their metabolism and body temperature to conserve energy. In fact, their body temperature can drop down to -2.9degC (27degF) – like a furry popsicle.
These bouts of torpor are interrupted by regular periods of arousal during which time body temperature, metabolism, and blood flow increase. In fact, heart rate increases from about 2 beats per min to greater than 300 beats per min in the matter of only hours. In non-hibernators, such an increase in blood flow would be accompanied by an increase in oxygen delivery to tissues and oxidative stress-induced tissue damage.
Mitochondria are the structure inside cells that produce energy in the form of ATP, or heat if energy production becomes uncoupled. Although the process of making ATP can result in oxidative stress, hibernators are protected because they reduce their metabolism (and hence ATP production) during hibernation. To further combat oxidative stress induced tissue damage, hibernators appear to reduce the ability for mitochondria to use oxygen during hibernation and only slowly start utilizing oxygen again during arousal to prevent a sudden increase in the generation of reactive oxygen species that cause oxidative stress. Researchers suspect that a build-up of reactive oxygen species while in torpor may in fact be what triggers the animals to arouse periodically.
Scientists are trying to learn a trick or two from 13-lined ground squirrels and other hibernators about how to protect human blood vessels from ischemia reperfusion injury – i.e. damage to blood vessels caused by restoring blood flow after a stroke or blood clot for example.
JF Staples, KE Mathers, BM Duffy. Mitochondrial metabolism in hibernation: Regulation and implications. Physiology. 37(5): 260-271, 2022. https://doi.org/10.1152/physiol/00006.2022