In bears and other carnivores, it's primarily the males that disperse. Social carnivores like wolves, and coyotes have a different social structure; both sexes will disperse. But in bears and mustelids (the weasel family), and the cats: generally the female will establish a smaller home range than males, and she'll raise her young and she'll keep her female offspring nearby. Female bear cubs will often overlap with their mother's home range for their entire life. Otherwise, they'll live fairly close.

But the mothers actively chase the males away and males often travel long distances. I mentioned the longest known distance was 800 kilometers. They just take off and look around for a place to live and they look for good habitat and they get chased away from places with other, bigger, meaner bears. The males therefore are always the first to migrate into new habitat. When they reach a new area like that, there are no females so there's not much point in sticking around. Females, though, only colonize areas very slowly. The rate of expansion of the bear population is limited by one female home range at a time.

This is why bears only made it as far as Central Mexico, during the same time that humans made it all the way down to South America, established complex civilizations and then invaded again from Europe. Bears were limited by their dispersal rate, one female home range at a time.

But if you look at these areas in the Northern Rockies, that's why we're talking about trying to reintroduce bears into the Salmon-Selway, because if we have to rely on female dispersal, even if the habitat to disperse through were available, it could possibly take hundreds of years for females to get there on their own.

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