As winter approaches, the last leaves fall gently from the trees, and the frigid morning air reminds you to grab a coat before you leave the house. Over the past few weeks, the animal kingdom has been full of the hustle and bustle of many species preparing for winter. Squirrels are frantically trying to hoard enough food to make it through the winter, birds are trying to put on extra layers of body fat, and foxes are moving back to winter den sites. All of the species are preparing to face the yearly struggle for survival, as the winter air moves in and food resources become scare. While the challenges of winter are universal, the strategies animals use for surviving them are not. Each species has a unique strategy for combating the loss of food and the freezing winter temperatures.
Some mammals like the bear, skunk, chipmunk, and bat go into a state called hibernation. Hibernation is a resting state characterized by inactivity and metabolic depression. During this phase, the animal has a lower breathing rate, pulse, metabolic activity, and body temperature. The lowering of these vital signs decreases the animal’s energy requirements and allows it to make it through the long winter without food. Some animals enter a stage called torpor. Torpor is a less permanent form of hibernation, where the animal can be woken up much more easily. Many mammals will spend the fall constantly foraging in order to store up enough fat to overwinter. While in hibernation, the animal will constantly draw energy from its fat stores instead of eating. Despite being relatively common in mammals, some mammal species do not hibernate. Deer, rabbits, foxes, mice, beaver, most birds, and squirrels all forgo true hibernation for other winter survival strategies. Deer and rabbits forage during the winter just like they do in the summer. Squirrels increase their fat stores and also store food in caches that they can then visit during the winter to feed on.
Animals that do not regulate their own body temperature have an even more difficult challenge of surviving the winter. Reptile, amphibians, fish, and insects all are cold-blooded. Being cold-blooded means that whatever the outside (ambient) temperature is, that’s what temperature you are. This is why snakes, frogs, and turtles bask, and why fish swim in specific portions of streams. In fish, dormancy is the main way to cope with winter temperatures. Like hibernation, the fish will become inactive and greatly reduce its metabolic activity in order to reduce food and oxygen use. Unlike hibernation, the fish cannot lower its body temperature and instead stays the same temperature as the surrounding water. Frogs, like the wood frog, burrow down into the soil to overwinter. Because they cannot dig too far down they are extremely susceptible to freezing temperatures. In order to combat the cold, frogs accumulate urea (from metabolic waste) and the liver converts large quantities of glycogen into glucose and circulates it through the body. Both urea and glucose act as cryoprotectants to limit the amount of ice that can form and prevents the rupturing of cells and blood vessels. Aquatic turtles move to the bottom of the pond or the stream that they are inhabiting and enter a period of dormancy. The water temperature in most larger bodies of water will not drop beneath freezing during the winter because of the high heat retention capacity of water (due to the hydrogen bonds, polar bonds, dipole-dipole interactions, and London dispersion forces) and because colder water will rise to the surface, keeping the bottom warmer.
As you can see, there are a multitude of adaptations to allow organisms to survive in the harsh winter climates. From hibernation and torpor, to advanced chemical reactants, life finds many unique ways to survive.
