For those who dread the cold, dark, dreary days of winter, the idea of curling up into a nice warm ball and sleeping for six months sounds joyous. Imagine enjoying the sun-filled days of the summer months each year, eating as much food as you can stomach, and then settling down for one long nap. Sounds nice, doesn’t it?
Still, there are several practicalities to consider. How do you stay nourished and hydrated? And what about going to the toilet? Holding your bladder and bowel movements for six months seems a little extreme. Discover how bears overcome these obstacles to navigate a yearly hibernation successfully.
Why Do Bears Hibernate?
Most bears hibernate through the winter months, meaning they sleep or retreat to a den where they stay for a prolonged period (anywhere from a few weeks to several months). Bears generally make these dens in hollow trees or rock crevices.
We should be careful to distinguish the torpor of a bear from the deep hibernation of small mammals, though. Take the marmot; during hibernation, it breathes only once to twice per minute and drops its body temperature as low as three degrees. But most bears do not reduce their bodily functions to this level. For example, the polar bear must remain alert enough to defend its den if necessary. If they were to reduce their temperature to just three degrees, they would have no way to “get up and get going” quickly enough if a predator were to attack.
In addition, there are many types of bears, and each treats hibernation - or torpor - slightly differently. Specific figures within this article (such as food consumption, heart rates, etc.) refer to the black bear. In contrast, the general facts (such as hibernation stages and what happens during pregnancy) can apply to most bear species.
What Do Bears Do Before They Hibernate?
We can break the annual hibernation cycle down into five main stages:
- Regular activity
- Hyperphagia
- Transition to fall
- Hibernation
- Waking hibernation
During each stage of the cycle, the bear experiences different biochemistry and physiology along with changes in appetite and exercise levels.
Regular activity sustains bears through the green months of spring and summer. During this time, a black bear will consume between 5,000 and 8,000 calories provided they have a plentiful food source. This stage is vital for bears to prepare for hibernation. If bears don’t receive enough water and nutrition during this time, they will fail to create hibernation responses. Instead, dehydration and malnutrition will cause them to use muscle mass as energy and cause nitrogenous waste accumulation in the blood.
During hyperphagia, a bear will eat and drink excessively (consuming up to four times as many calories as a “regular” day) to fatten itself for hibernation. During this time, the bear must drink sufficient water to rid its body of nitrogenous waste, meaning it can eliminate up to four gallons of water daily.
Following hyperphagia, the fall transition is when the metabolic changes in a bear’s body prepare it for winter. At this stage, the bear chooses to eat less food but continues drinking large amounts of fluid to flush waste products from its body. As they transition through fall, lethargy increases; their heart rate drops from 80-100 bpm to 50-60 bpm, and the bear can start to sleep for up to 22 hours a day.
How Much Do Bears Eat to Prepare for Hibernation?
A bear lives and eats in moderation through the summer months, but as summer draws to a close and the leaves start to fall from the trees, these mammals engage in hyperphagia. During this time, a bear is at its most active seeking nonstop sources of food and drink to prepare them for months of steady slumber.
At this time, a black bear can consume more than 15,000 - 20,000 kcal a day and drink several gallons of water.
What Happens to a Bear’s Body During Hibernation?
When bears enter hibernation, their body temperature drops, pulse rate, and respiration slow, and they rely on the stores of fat they have accumulated for sustenance.
During this time, bears can use up to 4,000 kcal of stored energy though they do not drink, eat, urinate, or defecate. Both metabolic rate and oxygen consumption can drop as low as 25% of the summer rates, and the bears will only breathe once every 15 to 45 seconds. Also, the heart rate can drop as low as eight beats per minute, and blood flow is reduced by 45%.
Pregnant bears give birth to cubs within the confines of the den, and the entire family stays there until spring arrives. The mother sleeps while the cubs feed, nurse, and grow. Once winter passes, the mother and cubs emerge from their den and head out searching for food. New mums often maintain an average body temperature and engage in many normal behaviors. They consume the cubs’ urine and feces, plus snow, icicles, and meltwater, which allows them to continue urinating and defecating themselves.
What Happens When Bears Wake Up from Hibernation?
Waking hibernation refers to the weeks following hibernation when bears must adjust to normal metabolic levels and bodily processes. During this time, bears tend to eat and drink less than they do during regular activity, and they excrete less urine, nitrogen, magnesium, calcium, and phosphorus.
What Is a Fecal Plug?
The fecal plug is often referred to as a mystery of hibernation, whereby bears excrete a dense composition of plant material, hair, and other unidentified materials. Still, the fecal plug isn’t too mysterious when you observe a bear’s hibernation behaviors.
Even though the bear doesn’t consume any food during hibernation, it will continue to produce a small amount of fecal matter (the same as starving humans do), which collects in the lower part of the intestine to create a sort of plug. Though bears sleep through most of this time, they sometimes groom themselves by licking their fur or paw pads. The bear ingests these materials and passes them through the body relatively unchanged.
So, when the bear emerges from hibernation and excretes this material, it can look a little strange. It is just the waste that accumulated while they were sleeping.
