Hibernation is one of the most amazing physiological adaptations in nature, and the more I learn about bear hibernation, the more the process fascinates me. Most bears have recently left their dens here on Kodiak, and they are slowly restarting their digestive systems as they prepare for the summer feeding season. Over the next two weeks, I’ll post about denning and hibernation for Kodiak bears. I wrote a post a year ago about den emergence on Kodiak, but this time, I will discuss hibernation in more detail.

Bears hibernate in the winter to conserve energy when weather conditions are harsh, and food is scarce. While hibernating bears experience a degree of dormancy, it is nowhere near as extreme as it is in many other species such as bats, squirrels, and rodents. On Kodiak where winters are relatively mild, bears often go into and out of hibernation and some bears (mostly boars) do not hibernate at all. It is common to see bear tracks in the snow all winter on Kodiak.

Bears typically enter their dens in the order of pregnant females followed by lone females and then females with cubs. Males are the last to enter their dens, and large, old boars in particular may not hibernate at all. Denning conditions vary from year to year depending on the weather conditions and the availability of food.

The time of den emergence in the spring is also dependent upon temperature and weather conditions and varies by sex and age. Males typically leave their dens first, followed by single females and then sows with cubs. Sows with newborn cubs are the last to emerge   Males usually spend three to five months in hibernation while pregnant females may hibernate as long as seven months.

While there is an instinctual aspect to denning, it also appears to be a learned behavior that sows teach their cubs. Perhaps the most important information the mother bear relays to her cubs is how to choose a den site. On Kodiak, there are few natural rock caves, so bears must dig their dens into the sides of the mountains or the sides of snow banks. Lawrence Van Daele, Victor Barnes Jr., and Roger Smith studied and compared denning behaviors on the northern part of the island to those on southwestern Kodiak. On the northern portion of Kodiak where the mountains are taller, they determined that the bears in their study group denned at an average elevation of 2180 ft. (665m). The bears in this region preferred steep slopes in alpine habitat for their dens, probably because the dens were high enough that the soil remained frozen throughout the winter, and the den structure remained stable. On southwestern Kodiak Island with its gentler topography, bears denned at an average elevation of 1499 ft. (457 m) and preferred midslope habitats near alder thickets. The researchers believe that the alder roots help stabilize these dens that were dug in loose, unfrozen soil. It is important that a bear chooses an area for his den that will remain stable throughout the winter and won’t collapse, but dens often do collapse in the spring and summer after the bears have emerged, and the bears are forced to dig new dens the following fall.

Before a bear digs and enters his den, he eats a large amount of food to build his fat layer. Berries provide natural sugars, and a bear may gain as many as 20,000 calories per day from eating berries in the late summer and fall. Bears also drink large quantities of water and consume foods high in protein, such as salmon. Brown bears add six to eight inches (15.24 to 20.32 cm) of fat before hibernation. They stop eating shortly before they enter their dens.

During hibernation, bears do not eat, urinate, or defecate. They curl up to conserve heat, but they may change position in the den. They are sensitive to their surroundings, and hey may awaken and move about or even temporarily leave the den. Pregnant sows give birth while in hibernation, and they can lactate to nurse their cubs. They can also lick and groom their cubs.

Bears emerge slowly from their dens in the spring. A bear may go in and out of his den many times over the course of several days before leaving his den for good. This is especially true of sows with newborn cubs who may stay near the den for one to two weeks, slowly allowing the small cubs to adapt to the outside world. Upon emergence, bears are groggy and sluggish, and it takes time for their bodily functions to return to normal. Soon after leaving the den, a bear will pass a fecal plug that may be as long as two feet in length (61 cm). This plug consists of digested food that has accumulated in the lower intestine as a result of not defecating all winter. After leaving the den, bears drink large quantities of water and slowly begin to eat new plant shoots.

Next week I will write about the amazing physiological adaptations bears employ during hibernation and how and why these adaptations are being studied by human medical researchers.

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