Updated: Dec 7, 2020
Post by Walking Shadow Naturalist Mike Rosekrans
A set of large tracks imprint upon a dusting of snow leading up the north face of a slope in the Gallatin Mountains of Southwestern Montana. The tracks have a large 6’ width with five digits just above the main pad. Claw marks protrude past the digits and it becomes evident that this is the track of a great bruin, the Yellowstone grizzly bear, making his way upslope as daylight lingers in the waning months of the year. Leaves have mostly fallen, and the berries have long-since withered. Plants have gone to seed, and the invertebrates are largely in their larval stage. The ground has frozen making it difficult for the grizzly to penetrate the now icy surface, even with his massive hump of muscle specifically evolved for digging starchy roots. The bruin grows lethargic after months of seeking out the highest caloric foods possible, and now that the food sources are all but gone, it is time for the deep winter slumber known as hibernation.
The bruin finds a prime location at 8500’ in a stand of Engelman Spruce. He digs and digs and digs despite the frozen ground until he has excavated a nice depression in the ground not much larger than his body. He collects some needles and duff from the surrounding forest and lines the inside of his den. He spends a few more days, searching and excavating until he settles on one that is just right, and finally, at the end of November enters his lair for the next few months of his life, living off stored fat from a spring, summer, and fall of foraging, fishing, hunting, and grazing.
Hibernation is a fascinating natural adaptation that has long attracted the attention, curiosities, and mythologies of our human minds and cultures. Many animals hibernate, but it is the bear who is so ingrained in our minds when we humans tend to think of the word, “hibernation.” But why do bears hibernate? What physiological adaptations occur that allow them to go months without eating, drinking, or dispelling waste? And how is it that cubs are born in the den, weighing scarcely a pound, while mother continues here amazing act of self-preservation? These are questions that have so intrigued people for millennia and has influenced such mythical folklore around the bear from cultures around the world.
To begin with, hibernation is an evolutionary response to a lack of food. Now one may ask, can’t the bear just hunt elk and deer in the winter? Well, the truth is that bears are not very good hunters and have a much more diverse diet than other carnivores such as the cougar or the wolf. They are what is known as a generalist omnivore, and in Yellowstone have been known to eat 266 different types of food ranging from plants to animals to various fungi. And being as big as they are, the bear requires a lot of food (in the preceding months leading up to hibernation; a bear can consume >20,000 calories in a single day. So, when the snows start to fall and the good roots and grasses and insects and elk calves are no longer available, the bear starts to slow its metabolism and seek out a nice place to excavate (dig) a den to spend the winter months when food is at its most scarce.
It is early December now, and the females with cubs have been in their dens for over a month. The big, male bruin is the last to seek out a den when the snows settle in for good in the Yellowstone ecosystem. The bruin climbs in his den that he has lined with spruce, fir, and pines boughs along with other duff from the forest and begins his 5-month period of dormancy. Black bears are more opportunistic and in addition to excavating will also den in hollowed out trees, uprooted depressions, or simply just lie down on the duff in the forest. Snows fall outside as the bruin settles in for the winter.
How do these bears survive for 5 to 6 months without eating, dispelling waste, and not exercising their muscles? Well, those are questions that have puzzled biologists for years and are big reasons why we study bears and the physiological adaptations that result from hibernation. First of all, to conserve calories which the bear has worked so hard to accumulate over the spring, summer and fall months, the bears will slow their heart rate from about 60 to 90 beats per minute down to around 20. Respiratory rate (breaths per minute) go from 12 to 2, and the metabolic rate drops significantly. All of these decreases in physiological processes help to conserve those hard-earned calories because even things like digesting food, heavy breathing, and a normal resting heart rate takes calories. So, the bear will live off of the fat reserves from caloric intake throughout the course of hibernation. Even though hibernation is an efficient conservation strategy, the bear will still lose up to 30% of its weight during hibernation.
The key is, it is all body fat and none of the weight loss results from reduced muscle mass. Incredible! If we humans were to lie in bed or 6 months are muscles would severely atrophy and we would need to relearn how to walk after such an extended period of inactivity. Somehow, a bear not only eludes muscle atrophy, but actually gains muscle during hibernation, but how?
Have you ever woken up in the middle of the night and had to go to the bathroom? Well, when bears are hibernating, they deal with no such annoyances. Other hibernators like ground squirrels and marmots wake every so often to expel waste (urine and feces) but bears are a different story. If waste builds up in the body and is not excreted it can lead to kidney failure. But during hibernation, the bear has a special enzyme in its saliva that when swallowed triggers a chemical reaction that converts urea (waste) into lean muscle. If I could go to sleep in November looking like Jabba the Hut and wake up looking like Dwayne “The Rock” Johnson” I would be a pretty happy hiker!
Now one more astounding feature concerning hibernation is the cubs. If you have ever watched bears, you might have noticed that single mothers enter the den in the fall and emerge with one to four 10-pound cubs when they emerge from the den in the spring. This means that our mother bears are giving birth when they are in their dens during hibernation!
Grizzly bear mothers will separate from their cubs (kick them out) when they are 2 ½ years old. What triggers this separation is the females urge to mate and raise another set of cubs to ensure the survival of future generations of grizzlies. From mid-May to mid-June each spring, mothers will separate with their two-year-old cubs (sometimes 3-years old’s as well) and allow a male or two to court them. Impregnation occurs during these months, as does egg fertilization. But, because fetal development takes a lot of calories and energy, the egg remains in a suspended state and does not move into the uterus for development, a physiological adaptation known as delayed implantation. This allows the female to eat, eat, and eat, and to gain as much weight as possible prior to hibernation. When she has obtained enough calories to but on a good fat layer before winter, she enters the den and the physiological processes of hibernation described above triggers a response for the mother bear to start developing her next batch of cubs.
After 6 short weeks, a pair of twins are born in the month of February in the den weighing less than 1 pound and being bald, blind and helpless, crawl up to mom and begin nursing. Thanks to delayed implantation and a short gestation period, mom’s calories obtained over the summer allows her to remain healthy and produce a rich, fatty milk that helps the cubs develop in the den until they are ready to emerge in late-April, now weighing over 10 pounds.
As the days grow longer and the snow melts in the valleys, new life emerges. Bison calves are born, migrating birds begin to show up in greater abundance, and lush new vegetation with spring flowers blanket the grasslands in the Yellowstone ecosystem. An abundance of food in all shapes and sizes is now available to the bruin as he rouses from his slumber and emerges from his den in early April. After five months of decreased metabolism, heart rate, and respiratory rate, the bruin takes a bit of time to get the physiological systems primed and he enters a stage known as “walking hibernation.” Contrary to some beliefs, the bruin does not emerge from the den a ravenous beast, and will spend a week or two meandering around his den site while his body adjusts. Eventually he walks down the mountain, finds an elk carcass that had died from the harsh environs of Yellowstone winter, begins to eat, and the cycle of a bear’s annual year begins again.
In the spring, summer, and fall months, join Walking Shadow Ecology on a tour to look for this fascinating creature with one of our experienced naturalists!