Bears are fascinating creatures. There are about seven hundred grizzlies in the Greater Yellowstone Ecosystem. Seeing a mom and cubs is always a rewarding sight, but do you know what it takes to be a grizzly mother? If you love bears and want to learn more about the Yellowstone grizzly, then you have come to the right place.
Let’s start at the beginning. No, I’m not talking about the Big Bang Theory. I’m referring to the reproductive age of female grizzlies. After growing up and leaving their mothers, a female bear has a couple of years of freedom before the boys start knocking on her door. A female will reach sexual maturity at age four. However, some North American bears have managed to be cub free until they reach ten years old. In Yellowstone, the maximum age reached without having cubs is seven. When a bear goes into estrus, the mating season can last up to 63 days, between May and July. During this time, a female can mate with multiple males. However, when a male grizzly finds a suitable female, he’ll follow her around for days, even weeks, guarding her against other potential suitors. This increases his chances for copulation. When copulation does occur, it can last anywhere from 25 minutes to an hour.
After a job well done, the breeding season comes to an end and the embryo begins to develop, right? Wrong! Grizzly bears have an adaptation known as delayed implantation. What this means is that the fertilized egg, known as the blastocyst, will not be implanted into the uterine until late November to early December. As a result, the development of the embryo has been stalled. When autumn rolls around, the bear’s hunger goes into overdrive as they go through a phase known as hyperphagia. This is a period when the bears are driven to eat more carbohydrates so they can quickly gain body mass in preparation for hibernation. A pregnant female must obtain at least 20% body fat in order to have cubs. More than 30% is better. If she succeeds, she’ll waddle into hibernation as the embryo becomes implanted in the uterine. Then the development begins.
For a grizzly bear, the gestation period is 60 days. After this time, the bear will give birth sometime between late January to early February while in hibernation. The combination period of delayed implantation and gestation is about 235 days. When the cubs are born, they’re about eight inches long and weigh about 1.1 pounds. They’re also blind and covered with very little fur and are highly dependent on their mother. By the time the bears emerge from their den in April, the cubs will be weighing about 8 pounds. Newborn cubs are known as COYS which is an acronym for ‘cubs of the year.’ The average number of cubs a grizzly bear will have in a single litter is two. But the bears have been known to have anywhere from one to three cubs per litter. Occasionally, they can have four which is a remarkable sight, especially if all four cubs make it to adulthood.