Thursday, December 31, 2009

Black Bears In Winter Dens

Shenandoah National Park Black Bears

In Autumn, black bears eat heavily to fatten themselves for Winter. It is a critical time for them, as they enter a state of hyperphagia, attempting to pack on the pounds for their long Winter sleep.


A Sow that has packed on the needed pounds during Autumn.


Also in the Fall, bears start to look for a Winter den. This den will be safe and somewhere that the bear doesn't go at other times of the year. Bears like to make their dens in small caves, tree cavities, hollow logs, rock outcrops, the upturned root mass of a blown-down tree and areas beneath large roots, stumps, and logs. Tree dens predominate here in Virginia. Bears will prepare a different den every year.

When they make their den, they drag in leaves and tree branches for a bed. The den will be big enough for the bear to move around and stretch but small enough that the bear's body heat warms it. Typically the bears at SNP enter dens between mid-October and late November and emerge from them during late March or early April.


The above Sow's 10 month old cub that also packed on the pounds.


During the Winter months, many animals are doing some type of sleeping. This kind of sleep has been split into two main types: hibernation and torpor. Sleeping is the way that animals adapt to the climate and land around them. Certain animals must be able to live through the Winter or die. To do this, most will sleep because it is hard to find food during the Winter. Hibernation is an adaptation to escape starvation, not cold. The timing of hibernation varies with region and is genetically set to the period when natural food typically becomes unavailable. In the north, black bears enter dens in September or October. In the south, they will enter dens between late October and December, and some do not enter dens at all if food is available.

Hibernation occurs when an animal's body temperature and heart rate drop drastically. With true hibernation, the animal can be moved around or touched and not know it, and appears dead. There is no movement and it takes a long time for it to wake up enough to even walk around. Hibernators such as chipmunks lower body temperature to near freezing but wake up every few days to raise body temperature to near normal, eat stored food, and eliminate body wastes. Then they lower body temperature and repeat the cycle.

Most people associate bears with hibernation, but they actually go into a similar state called torpor. This occurs when an animal's body temperature, breathing and heart rate drop slightly. Some other animals that enter the state of torpor are raccoons, badgers and skunks. Animals in torpor wake from their sleep more quickly than those in hibernation. True hibernators take a long time to wake up from their Winter sleep but bears are alert and know what is going on around them. They can get up right away.

Animals that sleep during Winter store food as body fat during the end of Summer and during Fall. This body fat runs their bodies all Winter. This would be hard to do if they stayed awake, moved around a lot, or ran around because this activity would use up the body fat before Winter was over.

A sleeping animal's body saves energy by doing a couple of interesting things. For instance, a bear's heartbeat and breathing slow down, but their body temperature doesn't drop very much (as it does for a true hibernator). The heart rate slows to as low as 8 beats per minute. Breathing becomes as slow as one breath every 45 seconds.

This is when that stored fat that the bear packed on in the Fall comes in handy. This stored fat lasts longer because their bodies are slowed down so much that they don’t need much energy. This is how the bear makes it through the whole Winter on the fat it has stored in its body. This is why it's important for bears to get enough food stored in the Fall. If there is a shortage of food at this critical time, the bear might not live until Spring when it can find food again.

During torpor, a bear's eyes will be open when it is awake but it might be groggy. Its body gets a little cooler (but not as cold as hibernating animals). Since it mostly sleeps and lays around, the body fat that it stored in the Summer and Fall lasts longer. It doesn't take too much body fuel to sleep. Its body does lots of things while it is in torpor. It makes its own water and recycles wastes. Bears won't drink or get rid of wastes for a few months. They will lose about eight pounds a week during torpor. The weight they lose will be from stored fat, not muscle. This means that when they leave the den in the Spring, they are still strong. In Spring, they will wake up and leave the den. They will be somewhat thinner and very hungry.

Here's something interesting. Bears don't get rid of wastes or go to the bathroom for the few months during torpor. The urine is broken down inside their bodies and reused as protein to help the bear keep its muscles healthy. A 'plug' of feces, hair, and nest stuff forms at the end of the bear's digestive tract. This comes out when the bear leaves the den and goes to the bathroom. Disgusting, but interesting.


UPDATE: when physiologists and biologists previously defined hibernation simply in terms of temperature reduction, bears were not considered hibernators. However, when biologists discovered the many metabolic changes that let black and grizzly bears hibernate up to 7 ½ months without eating, drinking, urinating, or defecating, they realized that body temperature was only a small part of hibernation. They redefined mammalian hibernation as a specialized, seasonal reduction in metabolism concurrent with scarce food and cold weather. Bears are now considered highly efficient hibernators.


How bears remain healthy through a long hibernation may have important human medicinal implications. During hibernation, black bears produce a bile fluid called tauro ursodeoxycholic acid (UCDA) that prevents gallstones which could develop from the unusually high levels of cholesterol sustained during the long sedentary winter. Their retained urine does not poison them, but rather is broken down to produce nitrogen for the creation of proteins that serve to maintain their muscle mass.

Click here to read about an interesting study being conducted on snoozing bears at the Virginia Bear Research Center, Virginia Tech - Blacksburg, Virgnia.



Did you know? During torpor (hibernation), a black bear will lose 15%-30% of its body weight. For a 450 pound bear, that is a weight loss of about 90 pounds!

Did you know? A majority of black bear dens have short openings which allows for Winter elements such as snow to accumulate on the bear's body (usually the back). However, black bears have two kinds of fur on their back—visible guard hairs about 3 inches long and, in Winter, a hidden layer of fine underfur so dense that water can scarcely penetrate it.

Did you know? Lethargy is actually a lowered period of bear activity (sluggishness and abnormal drowsiness), not the deep sleep popularly associated with bears (hibernation). Lethargy occurs right before denning and also directly after emerging, not while in the den.

Did you know? Newborn cubs do not hibernate. Their job is to eat, sleep, and grow.

For more black bear photos, information and video clips, visit the black bears section of my Web site.