Benjamin Hess is collections manager of mammals for the N.C. Museum of Natural Sciences. Here he explains how mammals big and small combat the cold. Questions and answers have been edited.
Q. How do animals get ready to face the winter months?
A. A number of physical changes can occur in animals to improve survival in cold temperatures. For mammals like reindeer or caribou, adding a winter coat that has more densely packed under-fur and longer guard hair helps to trap air for insulation.
A seasonal color change from brown to white occurs in a number of mammals, like collared lemmings and snowshoe hares. Black or dark-colored animals lose heat faster by radiation than white-colored animals.
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A large mammal has a smaller surface area to volume ratio than small mammals, which enhances their heat conservation. Curling the body into a ball, like a pet dog tightly curled up on a cold day, can decrease the exposed surface area and reduce the amount of heat lost. According to a scientific postulate known as Bergmann’s rule, the body size of most mammals tends to increase with increased latitude.
Other adaptations such as voluntary muscle contractions will produce heat, and involuntary muscle contractions, such as shivering, help an animal’s core temperature for short-term survival. Adding a layer of fat or blubber will also insulate and provide a site for storing energy.
Q. Some animals do more than adjust their coat color and fat stores. How does hibernation work?
A. Many animals hibernate, or go through a prolonged period of dormancy that can last multiple weeks during the winter. These hibernators generally have a number of things in common. Their body temperature can drop close to the ambient temperature. Their heart rate slows, their blood volume increases, and they become more resistant to chilling. They even experience sleep apnea or periods of time where they stop breathing – in fact, some bats can go three to eight minutes without breathing.
Bears are not true hibernators. They do experience a lethargic period over the winter characterized by a slight decrease in body temperature. For a bear to reduce its body temperature close to the ambient temperature, the energy required to arouse the bear out of a true hibernation would be incredible.
Q. How do animals manage to go so long without eating, drinking, or urinating during these winter’s naps?
A. Most hibernating mammals will arouse from their dormancy period to feed, urinate or defecate, but bears are an exception. Rather than excreting the waste they produce, they recycle the urea produced from metabolizing those fat stores and use it to build protein and maintain muscle mass.