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.

Sunday, November 29, 2009

A Year in the Life of a Black Bear

JANUARY: Birth of cubs begin in dens. Mothers lick the blind and nearly hairless cubs clean, and keeps them warm. After giving birth, the mother resumes her deep sleep, waking now and then to tend to the cubs' needs. She eats their feces and moves into position so cubs can easily nurse. Cubs do not hibernate. They snuggle under their mother, who makes a special cove for them under her chest by folding her front paws and drawing her hind legs alongside her body. The cubs aren't strong enough to wander away. They just want to stay with their warm mother.

FEBRUARY: Hibernation, birth and care of cubs continues. A hibernating bear sleeps in a curled-up position so that its crown is against the den floor and its nose is near its tail. This position minimizes a bear's surface area and reduces heat loss from the areas with thinnest fur (muzzle, legs, and underside).

MARCH: Hibernation and care of cubs continues. In adult males, levels of the hormone testosterone begin to rise.

APRIL: Most bears come out of their dens. Adult males are first to emerge from hibernation. Mothers with cubs come last. Food is very scarce. Adult males begin to roam. Most other bears remain lethargic and slow-moving, eating mainly aspen catkins and willow catkins. All but baby bears have been losing weight.

MAY: The forest habitat begins to green up. Lethargy ends. Bears eat sprouting grass, emerging herbs and young tree leaves. Cubs taste what their mother eats, but swallow little except milk. Lactating (nursing) mothers are losing weight. Other bears are slowly gaining weight.

JUNE: Green plants become mature and bears don't prefer them as much, but ant pupae become abundant. Bears switch to a favorite diet of ant pupae. Bears now begin to fatten up for winter as carbohydrate-rich berries get ripe. Mating season begins. Males roam widely to find females without cubs. Cubs begin eating solid food. Lactating mothers stop losing weight, and others are gaining weight slowly.

JULY: Cherries, blueberries, serviceberries, wild sarsaparilla berries and raspberries begin ripening and become the major bear foods. All bears gain weight rapidly if their natural foods are in good supply. Mating season ends towards the end of the month.

AUGUST: Viburnum berries, dogwood berries, wild plums, hawthorn berries, mountain-ash berries, and hazelnuts ripen. Bears switch to hazelnuts if the nuts are abundant, otherwise continue feeding on berries. They continue to gain weight.

SEPTEMBER: Acorns ripen. Berries and hazelnuts become scarce. Bears who are eating acorns continue to feed and fatten. Other bears begin losing weight from expending energy to look for digestible food, or perhaps traveling back to their home range to hibernate. Nursing ends. Lethargy begins. The bear's fur more than doubles in insulative value during the fall in preparation for hibernation. Some bears will enter dens in September, especially as their digestible foods disappear.

OCTOBER: Most bears enter dens. Bears now spend some of their precious energy by preparing a different den each year, even when their previous dens would work. They may choose burrows, caves, rock crevices, hollow trees, or depressions under fallen trees or brush piles. These locations provide insulation and also protection from hunters and predators. The entrances are just large enough for the bear to squeeze through, opening into a chamber about 2.5 to 5 feet wide and 2 to 3 feet high. For extra insulation, bears sleep on a nest of leaves, grass and other material that they rake into the den. These nests also help keep the cubs off the cold ground. Hibernation is light.

NOVEMBER: Denning continues for bears that are still out. Hibernation deepens and the heart rate slows to as low as 8 beats per minute. Breathing slows to one breath in 45 seconds. Fertilized eggs now implant in the uterus of female bears that became pregnant in June.

DECEMBER: Hibernation continues. Last year's cubs sleep through their first birthday, cozy in the den nest with their warm mother.

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

Sunday, October 18, 2009

The Difference Between Male And Female Black Bears

How can you tell when a Black Bear is a male or a female?

This gender determination cannot always be distinguished easily or with certainty while in the field, unless of course it is a female accompanied by cubs or the bear is in a position to expose its underside.

The females of the Black Bear species (sows) are typically smaller and leaner than the males. Their heads are more narrow with a flat looking forehead, plus their ears are generally bigger than a males ears. Female Black Bears are usually more shy and cautious when entering an area.

Black Bear Sow

Black Bear Sow

Black Bear Sow

Black Bear Sow

The males of the Black Bear species (boars) are typically bigger than the females. Adult males have more heavily muscled heads, necks, and shoulders than do females. The male head is wider and rounder than the females and the ears are generally smaller than a females ears. Male Black Bears are not as cautious when entering an area.

Black Bear Boar

Black Bear Boar

Black Bear Boar

Black Bear Boar

I personally enjoy whenever I get to see the adult boars, they are much more reclusive and nocturnal than sows, yearlings and young boars. They are the equivalent to big whitetail bucks as far as keeping a low profile (unless of course it is mating season). Most of the adult boars tend to have somewhat of an attitude the majority of the time, but that's one of the traits that makes them the superior beast of the mountains.

As with all things in nature, there are ALWAYS exceptions to every rule. Note the use of the word 'typically' above. The weight information is only useful for trying to make a quick determination in the field. I have personally seen several large sows that could mistakenly be identified as males (based upon the weight theory), which shows that hereditary traits can cause exceptions. The ears theory has exceptions also because nearly all juvennile bears seem to sport the 'big ears' for the first 3 years of their lives.

Note the highlighted chest nipple on this sow

Another method of gender identification is the presence of nipples, most notably in the chest area (see above photo). Both male and female bears have nipples (as all mammals do), but the males have only two and the females have 6 functional nipples. With a female who has cubs, the chest nipples are much easier to detect. While they are lactating it is much easier to detect them as the area around the nipple will be swollen with milk. Another way to tell if a female bear has been actively nursing cubs is that the fur around the nipples will be slightly discolored from the saliva of the cubs (see below photo).

Note the exposed nipple on this sow

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

Monday, September 7, 2009

Black Bears In Trees

Shenandoah National Park Black Bears

With the end of Summer approaching at Shenandoah National Park, the forest floor stops producing abundant plant life and the once thick vegetation begins drying out. Even the trees begin thinning somewhat and signs of Autumn's approach is everywhere.

For the black bears, it signals that another year will soon be coming to an end for them. Each year around this time, hyperphagia kicks in and the bears become 'eating machines'. A bear may eat 20,000 calories and gain several pounds or more per day. Adult bears can pile on more than 100 pounds before denning. It is literally the fruition of the fall that provides the black bear with the calories needed to add weight for the winter famine.

They will eat non-stop throughout the day and night until the denning period begins, which for most bears in SNP starts in late October. Contrary to popular belief, hibernation is triggered due to lack of food, not cold Winter weather. Once the food supply is gone, bears have no choice but to go into a den. Due to the park's dense population of black bear, the available food is consumed much faster than in areas with smaller populations (such as national forests).

Although this time of year can be a bit sad if you are a bear enthusiast, it does provide the best opportunities to witness bears climb trees and feed on nuts (namely acorns and hickories). I particularly like to view the large adults scale a tree, it is so fascinating that an animal with such mass can move around and balance itself so easily on tree limbs. And of course, watching cubs manuever their way around a tree is quite a treat as well (see the short video clip at bottom of a cub feeding on acorns).

A black bear cub foraging in an oak tree

Oak trees are not the only trees that the bears forage in, they also love apple trees in mid-summer, as well as hickory trees that also produce towards the end of summer. However I personally believe that the unripened green acorns are their top favorite, because they tend to go to great lengths to get at them.

A big sow climbing up a huge oak tree


A short video clip of a black bear cub eating acorns

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