Of Being Endothermic

For most of us, the emotional toll of winter’s cold weather and darkness is the biggest seasonal concern we face, putting into perspective our unique human intelligence. Winter may feel emotionally challenging to us humans at times, but it can be a much harder time for our friends in the animal world. Plummeting temperatures present a much more immediate threat for the world’s non human creatures that live outside and are unable to visit grocery stores. In cold climates, plants cease to grow as snow blankets the ground, meaning food sources can become scarce. Cold temperatures can cause even the furriest of animals to freeze, as they have no heated homes to retreat to. Their survival instincts kick into overdrive, causing more risky behavior that these animals would never undertake during the comparatively easy and luxurious summertime. 

It is no wonder then that so many cold weather creatures have adapted to hibernate during the winter. On the surface it may seem like ectotherms, animals that do not heat their own bodies and instead rely on sunlight and warm temperatures to stay alive, like most amphibians and reptiles, would be more affected by outside temperatures than endotherms, which includes some reptiles and all birds as well as all of our fellow mammals that create their own personal body heat. It’s easy to think that not depending on the weather to heat your body would make an animal less affected by it, but this is debatable when taking into account that creating this personal heat requires an animal to expend far more energy than if they simply relied on the outside temperature to be warm enough to heat them.

While endotherms are less reliant on outside heat, they have the disadvantage of needing more food, something that can be hard to come by in the cold winter, when “thermoregulatory costs…may become prohibitively high in small endotherms, especially when energy availability  is low,” according to Fritz Geiser’s paper in the journal Current Biology. This is the principle reason that many endothermic animals have adapted to hibernate: food, or really the lack of food that occurs in the winter. Endotherms require a lot of energy to both stay awake and regulate their body heat, so cutting out one of these processes is a smart way to avoid the dearth of available food during the winter.

While it is often thought of as simply animals going to sleep for months at time, hibernation is a much more extreme process than would be required for just taking a quick nap. Hibernation is “characterized by pronounced temporal reductions in body temperature, energy expenditure, water loss, and other physiological functions and are the most effective means for energy conservation available to endotherms,” according to Geiser’s paper. As they rest, the heart rates of these animals slow significantly, and through this process animals like bears, groundhogs and chipmunks are able to stay in this coma-like state of reduction to go sometimes more than half the year without eating, drinking or expelling waste. And although endotherms do require energy to heat themselves, they use this to their advantage during hibernation; even in a hibernation state, endothermic creatures can use their internal heat production to prevent damage to their tissues from cold temperatures, whereas most ectotherms, who are physically unable to hibernate, would “simply freeze to death.”

If you ever have to fortune to see a moose, what will likely strike you first is how unimaginably large they are. These huge, distinguished boreal forest animals are simply fascinating, and they offer an interesting case of what can happen to one specific cold weather animal when they are not adapted to hibernate. Moose live exclusively in very cold climates, either in northern forests or at high mountain elevations, or both, and are endothermic like all mammals. Yet they have no adaption to hibernate, and have to face a significant loss of their plant based food sources every winter.

In the summer, these enormous herbivores, the largest of the deer family, have the pleasure of a varied diet when things like “leaves of deciduous shrubs, some forbs, and aquatic plants become available.” In the harsher winter environment these food sources are not accessible and moose are limited to consuming twigs and evergreen needles that stay available and above the snowpack when ponds freeze and deciduous leaves fall. Moose can survive on just these food sources, as opposed to using them only as a substitution for their other food options as they do in the summer, but these foods are much lower quality and provide less energy. During the winter, moose have to spend far more energy finding this less abundant food that provides them less nutrition.

As a result, moose try to find  ways to conserve energy during this time; one such way is by sticking to areas cleared of snow like roads, trails and train tracks so they won’t waste energy trudging through deep snowpacks. This is understandable to anyone who’s ever walked through snowdrifts, as they can be quite exerting to traverse through, but the inevitable result of this risky energy conserving method is an uptick in moose fatalities as a result of collisions with cars, trains and snowmobiles during the winter months. Even if they can both conserve their energy and avoid collision, many moose, unlike creatures that hibernate, are failed by their endothermy. During the harsh winters of their natural environment, “moose expend more energy than they take in and many can starve.”

Winter may feel long and hard for us humans living in cold environments, and some of us may even wish we were designed to take a cue from some of our fellow endothermic creatures and avoid the winter altogether. As challenging as it can feel for us in certain ways, we really have no trouble getting through the winter when compared to wild animals. For cold weather animals, merely staying alive during the winter can end up being quite the challenge. When looking at animals like the moose that do not hibernate and instead tough it out by spending all winter awake, it becomes even more clear how useful of a strategy hibernation really is for dealing with winter food shortages. The poor moose, who have to scrounge around for less nutritious food and sometimes fail at doing so, have few winter food options and can likely end up being killed by a combination of the only climate they are able to live in and their endothermic need for large amounts of food. It is easy then to see why so many animals have adapted to spend the winter in hibernation; it’s much easier to survive the winter when you’re not even awake for it.