Males may impregnate several females, and play no part in raising young. Muskrats have a high reproductive potential, giving birth to large litters and breeding from spring to fall. In a month they are weaned and fully furred, and the female drives them off, especially if she is about to bear another litter. A female may overwinter with her final litter of the year, the family breaking up in the spring. Young disperse along streams or colonize new sections of marsh.
Muskrats are sexually mature the year following their birth, but few survive long enough to breed.
Young 'rats and dispersing immatures are especially vulnerable to minks, hawks, owls, foxes, snapping turtles and snakes. Surplus individuals are more vulnerable to predation, starvation and disease than are members of the secure, basic population. Some prey populations may limit their own numbers by failing to breed in crowded conditions, by aggressively defending a territory in overpopulated areas, or by some other type of behavior. Overcrowded muskrats are strongly territorial, and predation by minks is just a handy way of reducing the excess population. A bad winter, an outbreak of disease such as coccidiosis, or a flood during the height of breeding season may also cut muskrat numbers.