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Why do bears hibernate in winter? Hibernation is one of the most exciting and misunderstood behaviors in the animal kingdom. Contrary to popular belief, bears don’t actually sleep all the time they hibernate. During “real hibernation” it is almost impossible to wake up animals. Marmots, ground squirrels and some species of bats go into real hibernation. In this state, their body temperature and heart rate drop dramatically, and they wake up only on rare occasions.
On the other hand, bears belong to the category of animals that simply sleep often and for a long time in winter. Skunks, possums and raccoons also belong to this group of hibernating animals that sleep on demand, but only lightly. Given these common misconceptions, it’s important to understand exactly why bears hibernate. In this article, we’ll cover three reasons why bears hibernate, the science behind hibernation, and answer a few frequently asked questions about bears.
Why do bears sleep
First, it’s worth noting that not all bear species hibernate. Pandas, sun bears, Mexican black bears, and polar bears do not hibernate or hibernate only under certain circumstances. In the case of pandas and sun bears, food is present throughout the year in their natural habitat, which explains why they do not hibernate. As for the Mexican black bears, they can hibernate for a maximum of several weeks. Oddly enough, polar bears do not hibernate because they have adapted to the cold and can find food throughout the summer and winter. However, female polar bears will be in the den during pregnancy and feeding their newborn cubs. Species that hibernate include American black bears in colder climates, brown bears, grizzly bears, and Kodiak bears.
Even after much research, scientists still don’t understand everything about hibernation. What we do know is that hibernating bears get a biological trigger in the fall to put on weight. They enter a state of excessive overeating known as hyperphagia, during which they consume up to 20,000 calories a day. When it’s time to hibernate, the bears receive a signal telling them to seek shelter. Although this exact process is not entirely clear, scientists call this signal the hibernation induction trigger (HIT). This unique substance in their blood causes numbness, which causes the bears to seek shelter. Upon receiving this signal, the bears will build a den, find a cave, or make a house out of a hollowed-out tree trunk to survive the winter. On average, bears hibernate for three reasons: to avoid cold temperatures, to give birth and to raise cubs, and to conserve energy when food is scarce.
Due to low temperatures
To clarify, bears do not hibernate to clearly avoid the cold. Many real sleeping animals hibernate to avoid the harsh weather, but not bears. In contrast, cold weather is one of the triggers that bears use to signal when to hibernate. The ecological signal of shorter and colder days indicates to the bear that its habitat is changing and it will soon become more difficult to find food. This helps explain why some bears hibernate longer than others, and some do not hibernate at all. For example, Mexican black bears do not hibernate or hibernate for only a few weeks. North American black bears, meanwhile, can hibernate for up to seven and a half months. The colder the climate, the longer the bears sleep.
What’s more, recent research shows that bears sleep less when temperatures rise, or do not hibernate entirely. A 2017 study found that bears end their hibernation six days earlier for every degree of Celsius in temperature. In addition, when temperatures are too high, it becomes more difficult for bears to fall asleep. They will wake up more often and may just stay awake. As it gets warmer, the bears wake up from their sleep, believing that food is now available. In short, cold weather makes bears hibernate not because they want to avoid the cold, but because it makes them know that now is the right time to hibernate. Likewise, warm weather warns bears to get up and look for food.
Birth and raising of newborn babies
Another reason bears hibernate is for the birth and rearing of newborn cubs. The gestation period for female bears is about 250 days. Given this, pregnant bears in the spring will give birth to their cubs in the winter. To protect their cubs from bad weather and predators, bears will take refuge in dens and wait out the winter. Typically, female bears hibernate for two months. During labor, women’s heart rate increases and they wake up to give birth. After the birth of the young, they hibernate again.
At birth, cubs cannot take care of themselves. In fact, babies usually don’t grow big enough to feed themselves until they are six months old. While in the den, the cubs will breastfeed their mother’s milk, which is about 30% fat. This rich fat provides them with all the nutrients they need while their mother continues to hibernate.
To save energy when food is scarce
The main reason bears hibernate is to save energy in the winter when food is scarce. To survive the winter, bears hibernate in cold climates to support themselves for several months while living in severe calorie deficits. Once hibernating, bears enter a deep sleep cycle. During hibernation, most bears do not eat or drink, and rarely, if ever, need to urinate or defecate. On average, they lower the body temperature by 8-12 degrees, and the heart rate and respiration slow down significantly. In deep sleep, the bear can only breathe twice a minute. When these basic body functions are turned off, their metabolism decreases and they burn fewer calories.
While bears do wake up during hibernation, they usually only wake up to change posture, keep warm, or eat readily available food, if available. During hibernation, bears begin to break down their fat stores and burn them for energy. This fat, called brown fat, is very dense and allows bears to survive for months without any other means of subsistence. By not consuming protein, bears can maintain their strength, preventing them from waking up too exhausted or weak in the spring. In addition, bears can completely metabolize calcium in their bones during hibernation. This ability also prevents bears from over-weakening, allowing them to wake up in the spring with enough strength to find food. By the time they wake up, most bears have lost 15% to 30% of their weight.
Frequently asked questions about bears and hibernation
When do bears hibernate?
Traditionally, bears hibernate from late September to early October. The exact time depends on the local conditions in which the bear lives.
When do bears stop hibernating?
Most bears end their hibernation around April. However, bears may wake up earlier depending on the temperature in their area.
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