The conversation
Is winter miserable for wildlife?
Yeah, I’m a little cool, why? tim elliott / Shutterstock.com While the weather outside this winter may indeed be scary, a parka, knitted hat, wool socks, insulated boots, and perhaps a roaring fire make things bearable for people living in cold climates. But what about all the wildlife out there? Won’t they freeze? Pets are often suitable for protection against the cold. Photology1971 / Shutterstock.com Anyone who has walked his dog when the temperature is cool knows that canines will shiver and favor a cold paw – which partly explains the boom in the pet clothing industry. But chipmunks and cardinals do not wear fashionable coats or loot. In fact, wildlife, like humans and pets, can succumb to freezing and hypothermia. In the northern United States, the immaculate tails of opossums are a common victim of exposure to cold. Sometimes an unusual cold in Florida causes iguanas to fall off trees and hippos to die from cold stress. Avoiding cold is important for preserving life or limbs (or, in the case of the opossum, tail) and the opportunity to reproduce. These biological imperatives mean that nature must feel cold in order to avoid the harmful effects of its extremes. Animal species have their own equivalent of what humans experience as the unpleasant bite, mixed with the pen-and-needle sensation that encourages us to warm up soon or bear the consequences. In fact, the nervous system mechanisms for sensing a variety of temperatures are almost the same for all vertebrates. One winter challenge for warm-blooded animals, or endotherms, as they are scientifically known, is to maintain their internal body temperature in cold conditions. Interestingly, the temperature sensing thresholds may depend on physiology. For example, a cold-blooded – that is, ectothermic – frog will feel cold starting at a lower temperature compared to a mouse. Recent research shows that mammals in hibernation, such as the thirteen-line squirrel, do not feel the cold to lower temperatures than endotherms that do not sleep. So animals know when it is cold, only at varying temperatures. If the mercury drops, does nature suffer or does it just go with the icy flow? Some animals find a sheltered place to wait out the worst, like this chipmunk. Michael Himbeault, CC BY One solution: slow down and go check. Very cold climatic endotherms exhibit conspiracy: a state of diminished activity. They seem to be sleeping. Because animals that are able to work together vary between regulating their body temperature and the environment’s influence on it, scientists consider them “heterothermic”. In difficult conditions, this flexibility offers the advantage of a lower body temperature – noticeable in some species, even below the freezing point of 32 degrees Fahrenheit – which is not compatible with many physiological functions. The result is a lower metabolic rate, and therefore a lower demand for energy and food. Hibernation is a long version of torpor. Torpor especially holds energy savings for smaller body life, especially bats, songbirds and rodents. They naturally lose heat faster because the surface area of their body is large compared to their overall size. To maintain their body temperature within normal range, they need to spend more energy compared to a larger body. This is especially true for birds that maintain higher average body temperatures compared to mammals. Unfortunately, torpor is not a perfect solution for surviving icy conditions, as it involves compromises, such as a greater risk of becoming a lunch of another animal. Adaptations that help are not surprising that animals have developed other adaptations to weather the winter months. The large ears of a fennec fox would be a liability in a cold climate such as where the polar fox lives. Jonatan Pie / Unsplash and Kkonstan / Wikimedia Commons, CC BY Wildlife species in northern latitudes are usually larger with smaller attachments than their relatives closer to the tropics. Many animals have developed behaviors to help them overcome the cold: tending, wrapping, digging and supporting in cavities is a good defense. Some animals experience physiological changes as winter approaches, fat reserves build up, thicker fur grows and an insulating layer of air traps against the skin under the fur or feathers. Nature has devised other neat tricks to help different animals deal with conditions that humans, for example, could not tolerate. An animal standing in cold water or on ice benefits from the countercurrent heat exchange (1). Warm arterial blood (2) flowing from the heart warms the cooler venous blood (3) to the heart. Ekann, CC BY-SA Have you ever wondered how geese can stand comfortably on ice or squirrels in snow in their bare feet? The secret is the proximity of the arteries and veins in their limbs that create a gradient of warming and cooling. As blood moves from the heart to the toes, the heat from the artery transfers to the vein that carries cold blood from the toes back to the heart. This countercurrent heat exchange allows the core of the body to stay warm while limiting heat loss when the limbs are cold but not so cold that tissue damage occurs. This efficient system is used by many land and water birds and mammals, and even explains how oxygen exchange takes place in the gills of fish. Carp in a partially frozen pond do well. Starkov Roma / Shutterstock.com Speaking of fish, how come they do not freeze from the inside out in icy waters? Fortunately, ice floats because water is the densest as a liquid, allowing fish to swim freely in not-quite-freezing temperatures below the solidified surface. In addition, fish may not have the cold-acting receptor shared by other vertebrates. However, they have unique enzymes that allow physiological functions to continue at colder temperatures. In polar regions, fish even have special “antifreeze proteins” that bind to ice crystals in their blood to prevent widespread crystallization. Another secret weapon in mammals and birds during long periods of exposure to cold is brown adipose tissue or ‘brown fat’, which is rich in mitochondria. Even in humans, these cellular structures can release energy as heat, which generates heat without the muscle contractions and energy inefficiency involved in shivering, another way the body tries to heat up. This non-vibrating heat production probably explains why people in Anchorage can be satisfied on a 40-day Fahrenheit spring day. Of course, migration can be an option – although it is expensive in terms of energy costs for wildlife, and financially for people who want to get closer to the equator. As a species, humans have the ability to acclimatize to some degree – some of us more than others – but we are not particularly coldly adapted. Maybe that’s why it’s hard to look out the window on an icy day and not feel bad for a squirrel being knocked down while the winter wind sweeps through its fur. We may never know whether animals fear winter – it is difficult to determine their subjective experience. The wildlife does have a variety of strategies that enhance their ability to withstand the cold and ensure they can see another spring. Read more: Offices are too hot or too cold – is there a better way to control room temperature? As Arctic sea ice shrinks, new research shows how much energy polar bears use to find food How old is my pet in dog or cat years? A veterinarian explains Bridget B. Baker does not work, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has not disclosed any applicable commitments outside of their academic appointment.