Black and brown bears hibernate, but polar bears do not. Bears are not the only creatures to sleep for long periods of time during the colder months of the year. Chipmunks, ground squirrels, hedgehogs, skunks, raccoons, bats, turtles, frogs, snakes, ladybugs and some fish also hibernate during the winter.
According to Yellowstone National Park Service, hibernation lengths vary greatly, depending on the latitude of the bears’ location. For instance, black bears in Mexico may only hibernate for a few days or weeks, while the same type of bear hibernates for at least half of the year in Alaska.
A bear has to eat a lot before settling down to sleep for the winter and will eat all day and night to bulk up. According to ReadWriteThink.org website, brown bears can eat up to 90 pounds of food a day during this time. They will eat grass, roots, berries, fish, insects and small animals. Scholastic states that some black bears can gain up to 30 pounds per week during this pre-hibernation time. Some bears even collect some food to store inside their den. Grizzly bears and black bears do not get up to eat or eliminate during hibernation months. Instead, they use body fat to sustain themselves. Their bodies actually recycle fat metabolism byproducts and use it to build protein for their muscles and tissues. Their bodies create a kind of plug to keep the digested food inside, preventing wastes from leaving the body while they sleep.
When bears are not hibernating, their body temperature is around 100 to 101 degrees Fahrenheit. However, when they settle in for the long, deep sleep, their body temperature will drop to about 88 degrees. Some other animals drop their body temperature way below their normal active temperatures, and they must allow their bodies to warm up upon waking before they can move. Sometimes these other animals take days before they can become active again.
During bears’ normal awake period, they will breathe about six to ten times a minute. Once they are in hibernation mode, they will only take a breath once every 45 seconds or so. A bear's heart rate will also go from 40 to 50 beats a minute in non-hibernation mode to 8 to 19 per minute during hibernation.
When the bears are heading into the den to hibernate, they follow an order. The “line leaders” are the pregnant females. Then come the females with cubs, followed by the teenage bears and lastly the male bears settle in. When it’s time to come out of the den around the first couple weeks of March, the bears come out in the reverse order that they went in, with the adult males first and the mothers with new cubs coming out last.
Pregnant black bears may or may not wake up when their babies are being born during hibernation. She goes back to sleep afterward and only wakes occasionally to take care of the baby or babies. She can have one to five babies at a time. The cubs are blind and have no fur, but keep themselves warm in the mother’s fur and nurse when they need to. They grow and get fat pretty quickly.