Black Bears In The Winter: All About Hibernation

Winter is a great time to get out and do some scouting, especially in the snow because of the high number of animal tracks you can find and identify. But one species’ tracks will most likely be missing: black bears! Hibernation season is upon us, and the bears that we may see commonly during the warmer months are a little bit out of sight, out of mind. But there’s actually a lot going on in those wintry bear dens, especially when it comes to sows. 

Black bears are true hibernators, and are notable because they are one of the largest species to undergo hibernation. Hibernation looks a little different for each bear. Bears need to hibernate during times of low food availability– temperature has little to do with the need to hibernate. Some zoo bears do not hibernate as long as they are fed a sufficient diet year round, but this can cause health problems like unhealthy body weight. Because bear food sources such as mast producing plants become so scarce in the winter, it benefits the bear to go into a state of reduced metabolic activity– hibernation! 

The best way to define black bear hibernation is a reduction in metabolism, and therefore calorie demands. While some species’ body temperatures plummet to near freezing (i.e., some species of squirrels), bear body temperatures are only reduced by around 6 degrees. This allows them to be able to move around, have cubs and react in case of danger. Bears actually move around quite a bit in their dens to help prevent sores from a static position all winter. 

Other body functions that are affected include heart rate, which drops from an average of 55 down to 9 beats per minute. Breathing rate goes down to 1-2 breaths per minute and overall metabolism is reduced by 75%. Hibernating bears do not eat or drink, and rarely urinate or defecate. They survive off of the fat stores they accumulated during the warmer months when food availability was high– this is why that hyperphagia period prior to fall is so important!

Going into hibernation with a high body fat percentage is especially important for sows that were bred during the spring. When bears breed, the eggs are fertilized but do not implant into the uterine wall until fall, if the sow has good enough body condition to support the energy demands of pregnancy and lactation. She gives birth in the den in January or February, and the cubs grow rapidly. Black bear cubs are born altricial, but grow from a birth weight of ~0.7 pounds to 6 pounds when they emerge from the den with their mother. Bear milk is very high in fat content– 20-25% fat compared to a human or cow at 3-5% fat. The sow is awake to give birth and care for her cubs, all with a reduced metabolic rate! 

Bear dens can be just about anything, from a hollowed out tree, depression in the ground, a nest up in a tree, a spot in a rock pile, under your porch, the list goes on. The timing that bears emerge from hibernation is mostly temperature dependent. When the average daily temperature in the spring is above 40 degrees, you can expect to see bears on the move. However, their metabolic rate is still functioning at a lower than normal capacity for about 3 weeks. Bears will ease back into eating and drinking to prepare their gut for a long spring and summer of packing on the pounds for next year’s hibernation!

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