How Do Bears Sleep All Winter Without Food?

Question: How are bear sows able to sleep through the winter without food or water, especially when they’re pregnant? (Lori G., Carmel Valley)

Answer: California black bear cubs are typically born in early February while the sow is hibernating. The newborn cubs weigh less than a pound at birth and continue developing while suckling. They emerge with the sow from their dens in April or May at five to seven pounds. According to Bob Stafford, a Department of Fish and Game (DFG) biologist who has worked with bears for almost 20 years, wild female bears in California reproduce when they are 4 ½ years old and generally breed every other year, producing around two cubs per litter.

One of the remarkable adaptations that black bears exhibit is that of “delayed implantation.” In this case, an adult female will carry a fertilized egg in her womb for many months. The egg is ready to attach itself to the uterine wall and begin developing into a fetus, but it does not do so until the female’s body gives some unknown signal.

This adaptation allows bears to time the birth of their cubs so they are not born too early or too late. It also gives the mother a way out if food is scarce. If she has not accumulated enough fat by the time she settles into her den to hibernate, the egg will spontaneously abort. This interesting phenomenon appears to be a natural mechanism that bears have developed so they can avoid producing young when environmental conditions are not favorable.

California black bears typically mate in June and July and their reproductive success is related to the abundance of high quality summer and fall foods. Although black bears are opportunists and utilize a wide variety of plant and animal foods, their simple stomachs are inefficient at extracting nutrients from plant matter. They require berries, acorns and other highly digestible plant foods to provide them with sufficient nutrition to meet their reproductive requirements.

Once a black bear begins hibernating, it can doze for many months with a body temperature of 88 degrees or higher. They can go on slumbering because their warm pelts, lower tendency to lose heat and large body mass allow them to better retain body heat. This, in turn, enables them to cut their metabolic rate in half. Black bears keep their heads and torsos warm enough during hibernation so that they can wake if disturbed, although they require a few minutes to awaken.

During hibernation, black bears live off their own fat; their cholesterol levels are more than twice what they are in summer (more than twice as high as most people). But bears show no signs of hardening of the arteries or the formation of cholesterol gallstones. Research has shown that hibernating bears generate a form of bile acid that, when administered to people, dissolves gallstones, eliminating the need for surgery.

Weight loss during hibernation is extreme. Male black bears will typically drop between 15 and 30 percent of their body weight, while reproductive sows can lose up to 40 percent. Despite this grave weight loss, more than 90 percent of black bears survive the winter.

Bears appear to maintain their muscle mass and tone during the three- to four-month hibernation period. Even though they are meeting all their energy requirements by metabolizing fats, they do not lose muscle in the process. They seem to be able to use urea (a nitrogen rich waste product in the blood) to make new protein. For humans, unlocking this biochemical mystery would greatly assist with dieting and long term fasting to lose weight.

Another fascinating physiological adaptation that bears possess is the ability to avoid bone loss and actually rebuild bones during hibernation. Bears regenerate and repair bones by a mysterious mechanism that researchers hope may someday help give us a cure for degenerative arthritis and other bone diseases. Additionally, the means by which bears maintain muscle and bone mass during extended periods of inactivity may hold the keys to better treatment of people with long term incapacitation, and even space travel.

Once spring rolls around and the black bear sow and her cubs emerge from their dens, the cubs follow their mother around and learn from everything she does, including how and where to find food and what is dangerous and to be avoided. Unruly cubs are often disciplined by their mother’s growling, grunting and she even swats cubs who have not responded to her vocalizations. Some cubs remain with the sow for up to two years before they become independent and drift away.

For more fascinating information on California black bears, please check out DFG’s Web site at www.dfg.ca.gov/wildlife/hunting/bear/biology.html.

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Carrie Wilson is a marine biologist with the California Department of Fish and Game. While she cannot personally answer everyone’s questions, she will select a few to answer each week. Please contact her at CalOutdoors@dfg.ca.gov.

One response to “How Do Bears Sleep All Winter Without Food?

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