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. The population varies widely from year to year, but tends to show a peak in abundance about every 10 years.
Muskrats are tenacious fighters.
Minks prefer to tackle young or sick 'rats, because a mature adult puts up a brisk defense. Females defending young have been observed driving off attacking minks. Muskrats are parasitized by mites, fleas, flatworms, roundworms and tapeworms. While the average life span is under 12 months, some individuals may live as long as five or six years. Muskrats also damage agricultural and ornamental crops near water, and their tunnels riddle dams, dikes, canal banks, etc.