Chapter 5 Reproduction and Young

The Mating Season

Grizzly bear cubs are born to hibernating females in the dead of winter. But their life story actually begins the previous summer during the mating season. In late spring or early summer, male bears that are solitary during the rest of the year begin to seek and keep company with females. How do two grizzly bears, both of which might have a home range of several hundred square miles, find each other for mating? Scientists believe females of sufficient age and fitness for breeding leave scent trails in late spring that are discovered and followed by male bears. Once a male and female meet, they get acquainted. The prospective pair may engage in a game of chase, or wrestle. During these courtship activities the female is assessing the male’s health and strength. If she finds him acceptable, the two will become a pair. Evidence of a female’s acceptance of her suitor may be displayed by both bears through nuzzling and licking. These pair bonds, however, are short-lived, usually lasting a few days or weeks.

The time of actual breeding varies considerably depending on the location, the age and physical state of the female, and the particular year. A research study conducted in Yellowstone National Park reported the earliest date of breeding on May 18 and the latest on July 11. In Katmai National Park in Alaska, mating occurs from late May to early July. No matter where they live, most grizzly bears probably breed during the month of June.

During this time male bears will try to hide a female away from other males, keeping her in an isolated area where she’s less likely to be seen or smelled by a rival. Grizzly bears are not monogamous, which means that during any given year a female may mate with more than one male and a male may mate with more than one female. Thus, a male bear will expend considerable energy trying to keep a female away from other males. In Waterton Lakes and Banff National Parks in Canada, researchers have documented boars essentially herding sows to high mountaintops. There they forcibly repel the female’s attempts to return to lower elevations until breeding is complete. Evidently, some instinctual drive prompts them to isolate females in these lofty areas where their scent is much less likely to be dispersed to rival males.

In cases where a female mates with more than one male, each of the boars may father offspring in the same litter. Female grizzlies may produce up to four cubs in a litter, although one to three is most typical. In a litter of three, each of the cubs may be sired by a different father if the female mated with several different males during the breeding season. After breeding, a male may stay with a female for a short period of time. But by early August the pairs have normally broken up and bears of both sexes have gone back to their solitary ways.

Grizzly sows may mate with more than one boar. “Twin” cubs might actually have different fathers. Photo William Mullins