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Hibernation and Torpor explained!
There's no denying that winter can be rough- even for us! Colder temperatures, limited daylight and less food availability combine to make winter a challenging season for many of our animal friends. There are a lot of different strategies that have evolved for overcoming these winter woes! The PNW has a temperate climate, which means we have relatively mild winters but temperatures can still get pretty cold, especially on the mountain! Food is generally more scarce in the winter in this region and less daylight hours means less time spent foraging for most animals.
Animals evolved different strategies for dealing with winter based on a few different factors, notably, thermoregulation strategies and body size. Endothermic animals are animals that maintain their own internal body heat, as opposed to exothermic animals which rely on external conditions to regulate their body temperature. These two thermoregulation strategies are often colloquially referred to as warm-blooded and cold-blooded. Endothermic animals (mammals, birds and some fish!), have higher energy needs than do exothermic animals because of the energy demands of maintaining their body temperature. In the winter, when temperatures drop and food can be scarce, some endotherms are not able to obtain the amount of energy needed to maintain their internal body temperature and will use hibernation or torpor to conserve energy. Exotherms may also enter a period of reduced physiological activity, called brumation, at cold temperatures. Similar to hibernation, during brumation an exotherm's body temperature, heart rate, respiratory rate are significantly lowered.
Body size is also a factor in determining how an animal deals with winter weather. An animal's surface area to volume ratio determines how fast they lose heat; really small animals (like a squirrel or a hummingbird) have high surface area to volume ratios, which means they lose heat more rapidly than a larger animal (say a deer or bear) would. This means that smaller animals end up using more energy to maintain their body heat than larger animals that are able to retain more of their body heat. Which explains why lots of small mammals in cold climates resort to hibernation or periods of torpor during the winter!
How much body fat an animal is able to hold will also play a role in determining an animals winter survival plan! Animals that are able to hold a lot of fat on their body (like marmots) are able to hibernate for much longer than animals with limited fat stores.
In this post we are going to discuss who is hibernating right now in the PNW, explore the difference between hibernation and torpor, and try to unpack what the *heck* bears are doing in those dens all winter long!
Some endothermic animals hibernate over winter in response to cold temperatures and a lack of available food. Hibernation allows animals to conserve energy by lowering their body temperatures and metabolism. During this period, animals breath rate and heart rate slow down and they appear to be in a deep sleep. True hibernators are the animals that are unable to be woken up during this period, regardless of outside stimulus. True hibernators in the Pacific Northwest include yellow-bellied marmots, ground squirrels, bees, and some bats!
A yellow-bellied marmot soaking up some summer sun.
Marmots are extreme hibernators- they spend over half the year in hibernation! In Eastern Oregon, yellow-bellied marmots enter hibernation towards the end of July and emerge from hibernation in late February/ early March. If July seems a little early for a hibernation to begin, keep in mind that in this region, all the plants die back midsummer, so a lack of food availability spurs their early retreat! Those little bodies actually store a lot of fat, which is how they are able to hibernate for such long periods!
A golden-mantled ground squirrel running across a rocky landscape.
Golden-mantled ground squirrels are active from early spring to mid August, after which they hibernate for a period of 5-8 months. Unlike marmots, who subsist off of stored body fat, these ground squirrels have brief periods of activity during their long winter hibernation during which they will gorge on cached seeds.
A California ground squirrel caught in action!
California ground squirrels are facultative hibernators, meaning that they will hibernate if they experience cold or food stress but in warmer parts of their ranges are active year-round.
A bat darting through the night.
There are 15 bat species in Oregon! Some of these species migrate to escape the colder winter months, while others, such as the little brown myotis, hibernate in caves through the winter.
Torpor is another physiological mechanism for conserving energy which similarly involves a reduced body temperature and metabolic rate. The main differences between torpor and hibernation is that torpor is short term (hours, days or weeks), compared to months of hibernation, and animals can be easily roused from torpor, which is not the case for hibernating animals.
Animals that utilize torpor to help survive the winter months in the PNW include hummingbirds, chipmunks, skunks, badgers, deer mice! Hummingbirds often enter a state of torpor nightly, especially in colder climates. Since their surface area to volume ratio is so high, hummingbirds lose heat quickly and must eat almost constantly throughout the day to keep up with the demands of maintaining their body temperature, as well as the energy demands of flight. At night they enter a torpid state, lowering their body temperature, metabolism, heartbeat and respiration rate to conserve energy.
A striped skunk sniffing around!
Skunks will go into torpor when temperatures are particularly low, and food supplies are scarce, as a way to conserve energy. Since they are nocturnal, they go into torpor during the day!
An American badger moseying around in the woods.
Similarly, badgers will go into a state of torpor if temperatures are low. They are rarely seen above ground when temperatures are below freezing- preferring to stay in their dens in this dormant state.
A chipmunk perfectly perched in view of the trail camera.
Chipmunks typically spend the winter snuggled up in their burrows in a state of prolonged torpor- rising occasionally to snack on food they cached in the fall.
A little mouse making their way across a fallen branch.
Deer mice utilize a variety of strategies to keep their tiny bodies warm in the winter: huddling, nesting, and torpor. While huddling together or building a deep nest is likely enough for most deer mice to withstand Oregon's winter temperatures, if temperatures become too cold then they will resort to torpor.
So, What are Bears doing?
There has been a lot of debate over the years as to what the *heck* bears are doing all winter long! Many scientists have argued that bears are not true hibernators because their body temperatures do not lower to the same extremes as other true hibernators (marmots, ground squirrels) do. This means that they are more responsive during the denning period than a true hibernator would be. Some scientists now distinguish what bears do as super hibernation, because they do not eat, drink, defecate or urinate during the hibernation period. Other people take issue with that classification and believe that bears are actually in a long-term torpid state because they remain semi-alert and responsive throughout the season.
A bear taking a break!
Many black bears in Oregon don't retreat to their dens for periods of dormancy, anyways! Our temperate climate means that bears at lower elevations can find enough food throughout the winter to remain active.