Inside the secret world of the black bear den

One of my most surprising encounters with a black bear was finding a set of paw prints in fresh snow one January afternoon. Why surprising? Because the bear should’ve been hibernating. To better understand these Lyons residents I’d like to share some facts about hibernation.

In Colorado, bears heavy from their fall gorging, called hyperphagia, begin to settle into their dens in early October. Over the winter: a bear’s metabolic rate will drop 50% to 60%; its heart rate will decrease from 40 bpm to only 8 or 10 bpm; and, its body temperature will fall 7 or 8 degrees celsius. The bear will spend about 200 days in this state without food or water. A fecal plug will block the intestines to prevent defecation. Urination will be halted, causing the amount of nitrogen in the bear’s blood to rise, though amazingly, without damaging its kidneys. Over the winter it will lose 16% to 37% of its body weight and 10% to 20% of its muscle fiber. Even its foot pads will shed and regrow.

Perhaps even more miraculously, while in the hibernation females may give birth. Black bears breed between June and August at which time the egg is fertilized but not implanted in the uterus. It divides into a hollow sphere of cells called a blastocyst which will, then, remain dormant for months, allowing the bear to devote her full energy to eating. It only implants in the uterus around November. This delay provides her with a fail-safe. If for some reason she doesn’t put on the weight needed to survive hibernation and pregnancy in good shape, the blastocyst won’t implant, and she won’t become pregnant. On good years, a litter of two or three cubs is born in the den in late winter while the mother is still in hibernation. The mother and cubs won’t actually emerge for another couple of months.

When she does wake — and realize she has cubs — it will still take two or three weeks before she comes to terms with the new situation and begins to eat or drink again. This dazed state is called “walking hibernation.”

I hope that learning about the miraculous process of hibernation has helped you appreciate our fellow Lyons residents a little more. And I also hope we can give them a little break here and there. They go through a lot trying to survive, just like us.