Bear Reproduction

January and February are important times for black and grizzly bears. While popular opinion has them sleeping away the winter months in a state of dormancy, females wake up briefly in January or February to give birth to one (often two) tiny bear cubs. This ritual of mating and reproduction has been going on for as long as there has been life on Earth.

Black Bear - Click for more informationBears mate in late May or June, shortly after emerging from their winter dens. Female black bears usually become sexually mature by their fourth summer, while grizzlies may wait until they are almost 7 years old before mating for the first time.

Mating is a highly evolved ritual. Once females come into season, they may leave scent trails, and male bears quickly hone in on the scent. More than one male may catch the female's scent and this can lead to some potentially violent showdowns between competing males.

The mating pair spend several days getting to know each other. Normally wary sows need to be very cautious about letting a larger, more powerful boar nearby. As time passes, they may get closer, and even nuzzle each other. In some areas, particularly Waterton Lakes and Banff National Parks, males have been observed 'herding' females towards summit areas prior to mating. Isolating the sow in this way may have the effect of reducing the likelihood of her scent trail being discovered by other boars. Biologists in Banff watched a boar restrict a sow to an area covering only 2-3 hectares for 13 days.

With the preliminaries out of the way, mating begins, usually lasting only a few minutes at first. The pair mate repeatedly over several days, and some of the later copulations may last as long as an hour. Sows  may mate with one or several bears over the course of about a week. With the end of mating, the sow will have nothing more to do with the male.

After mating, the female may be pregnant, but that does not mean she will give birth to cubs. There is an old joke that you can't be half pregnant, but bears have proven this statement to be false. Bears, weasels and some seals have developed a process called delayed implantation. The fertilized egg develops into a small embryo called a blastocyst. This is where the interesting stuff begins. After this brief period of development, of the fertilized egg suddenly stops growing and simply floats freely in the uterus for several months.

If a sow is in peak condition when she heads into her winter den, the embryo implants in the uterus and begin to develop. She'll wake up during January or February to give birth . Black bears give birth to between one and four cubs, with two being the most common. Grizzlies also average two cubs, with rarely more being produced.

If the sow is not in peak condition at the onset of hibernation, her body will reabsorb the embryo and not give birth that year. This gives bears more control over their reproductive rate than just about any other animal. Once an elk or deer is pregnant, they are pregnant, and winter pregnancy can be fatal. Animals diverting energy to reproduction during the difficult winter months run the risk of falling victim to predation.

Grizzly cubs weigh approximately 500 grams at birth, and are blind and helpless. While grizzly cubs are covered with hair at birth, black bear cubs are hairless. The cubs will grow to weigh 3-4 kg prior to emerging from the den in spring.

Bear reproductive rates are being studied by groups like the Eastern Slopes Grizzly Project. Some data suggests that the reproductive rates in the eastern slopes are one of the lowest in North America. Their data indicates that grizzlies average 6.8 years before mating for the first time with an average interval between litters of 4 years.

This must be factored with the importance of protecting the habitat of breeding female bears. If we think about the lives of female bears, the most critical aspects of their life are reliable food supplies, safe areas to raise their young and safe denning sites. Lake Louise represents the highest concentration of breeding female grizzlies in the Central Rockies, but is also the most heavily impacted by tourism. Over the next few years, we may see more and more restrictions on access to certain areas in order to protect these key habitats.