It’s just one of the amazing things that characterize bear hibernation, a months-long sleep that really has no parallels in the animal kingdom.
Bears aren’t considered true hibernators; what they do is technically known as “torpor,” a period of deep sleep. True hibernators like bats and groundhogs won’t wake when they hear a noise – you can even move them around without them waking. Bears, on the other hand, can go very quickly from sleep to a very groggy alertness.
Still, it’s a very deep sleep, so let’s call it hibernation. A bear’s heart rate slows to as little as 8 beats a minute, and it breathes as little as once every 45 seconds. It’s body temperature drops, but not as dramatically.
Once of the things that sets bears apart from other deep sleepers is that they don’t pee or defecate during hibernation. Instead, a bear’s body recycles urine into fresh proteins, using the water to avoid dehydration. Before going into its den in the fall, the bear fasted for a week or so and then ate a meal of dry grasses, pine needles, twigs and so on to create a nice constipation-inducing bung.
For moms, hibernation is when the fetuses are being formed. Black bears mate in June or July, but the fertilized eggs don’t start growing until about ten weeks before the cubs are born, a remarkable feat of timing known as “delayed implantation.” If the mom didn’t manage to build up enough fat reserve to be able to nurse the cubs, the eggs won’t be implanted.
In late January or February, the cubs are born – typically two or three tiny little things around the size of a chipmunk. Medieval Europeans believed that bears were born as shapeless lumps and were literally “licked into shape” by their parents. An old French term for an uncouth person was “un ours mal léché” – a badly-licked bear.
The tiny, blind cubs crawl toward their mother’s nipples and do little but eat and sleep and grow until spring. A bear’s milk is 20 to 40 per cent fat (human’s is four percent), so the cubs grow quickly. By the time spring arrives, the cubs will weigh four to six pounds – roughly eight times their birth weight – while the mom will have lost at least a third of her body weight. Bears that aren’t nursing lose 15 to 20 per cent of their weight in the winter.
A month or so after emerging from the den, the cubs will develop new teeth and start eating solid food. But their mother will continue nursing them through the summer, helping them grow quickly. Some cubs are able to climb trees as soon as they emerge from the den, while the others learn quickly.
Next winter, mom and the cubs will all den up together – mom will dig the den, but the cubs will often help by dragging in grass, leaves and other dry bedding.
They will stay together until mid-summer of the cubs’ second year, when the mother is ready to mate again. Then she’ll chase them away. Females will stay nearby – often occupying a portion of their mother’s home range, which she tolerates – but the male cubs will head off on a journey, traveling as much as 200 km to establish their own range.
Interested in learning more about bears and supporting their conservation? Visit Bear With Us, a rehabilitation and conservation group in this area focused solely on black bears. Aspen Valley Wildlife Sanctuary also does amazing work with bears, as well as other animals.