When we talk about the different methods that animals use to survive the winter, hibernation is often at the top of the list. But in reality, not that many animals truly hibernate. Many enter a lighter state of sleep called torpor. Others utilize a similar strategy called estivation in the summer months. So what is the difference between these survival tactics called hibernation, torpor, and estivation?
Hibernation is a voluntary state that an animal enters into in order to conserve energy, survive when food is scarce, and minimize their need to face the elements in the cold winter months. Think of it as a truly deep sleep. It's a body state marked by low body temperature, slow breathing and heart rate, and low metabolic rate. It can last for several days, weeks, or months depending upon the species. The state is triggered by day length and hormone changes within the animal that indicate the need to conserve energy.
Before entering the hibernation stage, animals generally store fat to help them survive the long winter. They may wake up for brief periods to eat, drink, or defecate during hibernation, but for the most part, hibernators remain in this low-energy state for as long as possible. Arousal from hibernation takes several hours and uses up much of an animal's conserved energy reserve.
True hibernation was once a term reserved for only a short list of animals such as deer mice, ground squirrels, snakes, bees, woodchucks, and some bats. But today, the term has been redefined to include some animals that really enter a lighter state activity called torpor.
Like hibernation, torpor is a survival tactic used by animals to survive the winter months. It also involves a lower body temperature, breathing rate, heart rate, and metabolic rate. But unlike hibernation, torpor appears to be an involuntary state that an animal enters into as the conditions dictate. Also unlike hibernation, torpor lasts for short periods of time - sometimes just through the night or day depending upon the feeding pattern of the animal. Think of it as "hibernation light."
During their active period of the day, these animals maintain a normal body temperature and physiological rates. But while they are inactive, they enter into a deeper sleep that allows them to conserve energy and survive the winter.
Arousal from torpor takes around one hour and involves violent shaking and muscle contractions. It expends energy, but this energy loss is offset by how much energy is saved in the torpid state. This state is triggered by ambient temperature and the availability of food. Bears, raccoons, and skunks are all "light hibernators" that use torpor to survive the winter.
Estivation-also called aestivation-is another strategy used by animals to survive extreme temperatures and weather conditions. But unlike hibernation and torpor, which are used to survive shortened days and colder temperatures, estivation is used by some animals to survive the hottest and driest months of summer.
Similar to hibernation and torpor, estivation is characterized by a period of inactivity and a lowered metabolic rate. Many animals, both invertebrates and vertebrates, use this tactic to stay cool and prevent desiccation when the temperatures are high and water levels are low. Animals that estivate include mollusks, crabs, crocodiles, some salamanders, mosquitos, desert tortoises, the dwarf lemur, and some hedgehogs.