Bears throughout the Year

The undeveloped, immature cubs — usually three but sometimes as many as four to a litter — arrive to the world early by necessity. Pregnant female bears in hibernation have only their own body fat to use as nourishment for the placenta that feeds their unborn young. If bear cubs were carried to what humans consider "full-term," the mother would emerge from hibernation completely depleted — a state that would serve neither her nor the cubs well. Instead, bears experience a shortened gestation period and give birth to premature young.

Fortunately, the protective environment of the den, coupled with the mother's rich, nourishing milk (it boasts a full 22 to 24 percent fat content!) will help the cubs to grow and mature to the point where they will be prepared to leave the den when spring arrives. They feed every few hours, guided to their food source by the heat radiating from their mother's nipples. The sounds of their contented humming fill the den — it is the sound of well-fed, well-cared for bear cubs.

This regimen of feeding will help the cubs develop a full coat of fur and the boisterous energy that will help them keep up with their mother once they leave the den. The mother bear will be in dire need of food come spring, after several long months hibernating and caring for her young without food — simply relying on her stores of fat from the fall.

Spring doesn't burst into full bloom in the regions where bears make their homes. It trickles slowly to life, offering scant relief for bears emerging from hibernation. In many ways, spring is the hardest season of the year for bears — particularly bear cubs. They emerge from their dens to a world that they must share with juvenile and adult bears that have also just surfaced after a long and depleting sleep. They are hungry, but the land is not yet offering an abundance of nourishment.

Adult and juvenile bears will sometimes scavenge the carcasses of bighorn sheep or mule deer that succumbed to avalanches or the cold. But the opportunistic bears do not discriminate when feeling the sharp pangs of hunger, and it is not uncommon for them to turn to cannibalism during the lean spring months.

The cubs are still small — no more than 12 pounds. They are already attractive prey for wolves, lions, lynx, bobcats and even golden eagles. But these predators pose less of a concern than other bears. Some consider infanticide — the killing of one's own young — to be the most common cause of mortality in cubs. This cannibalistic behavior may the result of hunger alone, or it may be related to the ever important objective of reproductive success.

Regardless of the motives, one-third of the cubs born this year will never live to see their first birthday. In addition to those felled by disease and starvation, many will likely perish at the claws of hungry members of their own species.

As spring progressively moves into Bear Country and the winter snows clear from the valleys, new sources of food begin to unfurl and appear. This is good news for bears, as 80 percent of their diet is usually based on plant matter. Grizzly bear mothers are meticulous foragers, teaching their cubs by example how to recognize the grasses, sages, berries and roots that will be nutritious and won't cause harm.

The cubs have grown to be boisterous and playful, and their attention span is short. Fortunately, they will stay with their mother for approximately 2-½ years (black bear cubs are sometimes weaned as early as 1-½ years), and she will only drive them away when she is prepared to mate again. In the interim, they will have plenty of time to learn what they need to know in order to survive on their own.

For now, the mother still takes great care to avoid other bears. Though the passing months bring a more abundant food supply, she still avoids situations that might threaten her offspring.

The month of May brings with it heavy rains, and these serve to water vast sedge flats. The tips of these grasses contain large amounts of protein, and bears flock to feed on them from May to July. But they have more than eating on their mind during that time frame.

May marks the beginning of the mating season for brown bears, while most black bears don’t begin mating until June. Although the mating season extends for a handful of months, females are only able to become pregnant for about three weeks of that time period. Furthermore, they only consent to amorous activities during the first few days of the estrus cycle. For bears living fairly solitary lives, it can be tricky to find a mating partner at precisely the appropriate moment. Male grizzly bears have a home range of between 800 and 2,000 square miles, and sometimes they have to wander far across their home ranges to find a breeding partner. The same is sometimes true for female grizzlies, which have a home range of 300 to 550 square miles (black bears have a much smaller home range — up to 15 square miles for females and 40 for males).

In order to find their way to a female in estrus, males simply follow their noses. Bears have an acute sense of smell, equivalent to that of a dog. Males have been witnessed to walk 30 miles in a straight line, directly to a female in estrus, simply by following her scent.

Pronounced sexual dimorphism exists among bears, meaning that the males are much larger than the females. The largest bears have an advantage when it comes to mating because their size allows them to ward off competing males. However, it can also intimidate females and make them more difficult to woo. The act of courting an estrus female may take several days of simply shadowing her movements before she allows the male to come closer. Once they have established intimate behavior — grazing, playing and resting together — the female will permit her suitor to mount.

Competition among male suitors can become fierce, and it can result in serious injuries — lacerations, broken jaws and broken canine teeth, to name a few. But, although males sometimes fight viciously over females, this does not mean that females copulate with only one male during the mating season. On the contrary, each cub in a litter can potentially have a different father. Of course, the fathers' involvement lasts only as long as the courting process. Once the animals go their separate ways, male bears have played out the entirety of their role in their offspring's lives.

Even as spring changes to summer, many bears — particularly juveniles in their first year away from their mother — still struggle to survive. In fact, as many as one-third of juvenile bears never reaches sexual maturity, due to death from starvation. Starving juveniles can be dangerous and unpredictable, and they are responsible for most bear attacks on humans.

As adults mate and mothers with cubs in tow do their best to steer clear of predatory bears, the juveniles are left to fend for themselves. Fortunately, the most plentiful months of summer are not far off.

Mother bears and their cubs enjoy the summer weather and abundance of roots, berries and other plants available for their consumption. The cubs also continue to nurse, although this puts a tremendous strain on the mother. In addition to the calories required to feed her cubs, she must also be working to put on the layer of fat that will keep her alive during the winter. For this purpose, she must consume 20,000 calories per day during the abundant summer months.

July provides plenty of food for bears, but its bounty is nothing compared to what August has in store.

The month of August heralds the migration of salmon, and this represents a dream come true for hungry bears. Male bears can land as many as 50 salmon per day, each fish providing 2,500 to 6,000 calories. When the fish are most abundant and the pickings are easy, bears can afford to be choosy and feed on the prime bits — the skin, brain and caviar, all high in fat. Bears augment this rich diet with nuts and berries, also high in protein. Bears can eat up to 16 hours each day, consuming incredible quantities of soapberries, buffaloberries or huckleberries. The goal is a condition known as hyperphagia — excessive eating and weight gain. It would be considered an eating disorder among humans, but for bears it is an absolute necessity if they hope to make it through the long, deep sleep of winter.

The extra fat, however, can make it difficult for bears to stay cool in the August heat. Bears slow down considerably in the summer sun — they can't move faster than 3 miles-per-hour without running a fever. Much of their time is spent swimming, lying in cool water, and resting in the shade. A bear's hot spots include the ears, muzzle, nose, footpads and especially the inner thighs and armpits. The combination of huge meals and the heat make it a commonplace sight to see bears lying spread-eagle — obviously content to simply rest and digest.

The fall is a season of great importance for all bears, but this holds most true for pregnant bears. September represents their last chance to fatten themselves up before retreating to the den where they will bear their young in a matter of months. A pregnant female in the autumn months may consume as much as 90 pounds of food each day, and she can eat more than 200,000 berries in a 24-hour period.

The pregnant female has no choice but to indulge in an eating frenzy at this early stage of autumn, if she wants her pregnancy to continue. Bears experience delayed implantation after mating, which means the fertilized egg will not implant on the uterus wall until the female goes into hibernation. If she has not gained enough weight to allow her to safely begin hibernating by November, the pregnancy will terminate.

The feel of winter is in the air now, and the bears sense it. As the days grow colder and the time for winter's deep slumber approaches, the bears become more active. Juvenile and adult bears alike grow more playful, engaging in intense (but friendly) wrestling matches that can last for hours. It's almost as though they want to "get the wiggles out" before descending into the confines of their dens.

As October turns to November, the bears begin moving towards their "denning area." While their den from the previous year may not be intact, most bears remain faithful to a general location for their den with the coming of each winter.

The time to retreat to the insular world of the den has arrived. Bears are not driven into dens because they can't handle the cold, harsh weather of winter. Rather, bears seek the sanctuary of dens because they cannot find enough food to survive during the winter. Instead of wandering the wasteland starving, bears take advantage of an amazing adaptation that allows them to conserve their energy and survive without food for the duration of the cold, dark winter months. In areas where the food source is abundant enough to survive, some bears actually practice a "walking hibernation," in which their metabolism slows but they continue to move about searching for food. For most bears, however, the den is the safest and most secure place to be once November rolls around.

Grizzlies generally excavate dens, with an entrance tunnel approximately 6-feet long but only 25-inches wide. This tunnel leads to a small chamber, 6-feet long, 5-feet wide and 3-feet high. These dens are dug into hill or mountain slopes.

Black bears use a wider variety of sites for dens, including excavations, hollow trees, or even highway culverts and basements (the human inhabitants sometimes have no idea they have a black bear for a roommate!). Tree hollows provide the most insulated, secure dens, but logging practices have rendered them a rare option.

Once a satisfactory den has been located or excavated, the bears retreat and hunker down for the long winter ahead. Often, they enter the den during a snowstorm, possibly to hide their tracks and avoid being ambushed in such a vulnerable position. Once in the den, their metabolic rate slows by 50 percent. Their heart rate drops from 50 beats per minute to 10. Their body temperature falls by several degrees. The bears are now in a state of hibernation.

The hibernation of a bear differs from that of ground squirrels or other small rodents. These smaller animals lower their body temperature to within a few degrees of freezing or even slightly below the freezing point. They shiver violently every two weeks to raise their body temperature and awaken themselves long enough to urinate, defecate and eat a small snack. Bears are simply too large to successfully hibernate in this fashion — it would take too long to warm and cool their bodies to such extremes. Instead, they do not eat, drink, urinate or defecate. They burn more than 4,000 calories each day while in hibernation, but somehow they do not experience a significant loss in muscle mass or bone density. This remarkable adaptation allows bear to slumber deeply for months on end as winter rages beyond the entrance to their small, safe, warm haven.

The darkest night of the year — the winter equinox — carries little meaning for bears wedged snugly into dim dens. For them, each day and night runs together and forms the whole of a season of hibernation. Adult and juvenile bears snooze alone; mother bears and their cubs snuggle together for added warmth; and, in some dens, expectant mothers slumber, conserving their energy and calories for January and the beginning of yet another season of new life in Bear Country.

Primary Source: Breiter, Matthias, Bears: A Year in the Life, Firefly Books, 2005.