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On occasion, they will eat fresh- water clams, snails, frogs, crayfish, fish and salamanders. In hard times, they can subsist on dried stems and leaves and may resort to cannibalism.
Reproduction:
Muskrats produce large numbers of young, but offer little parental care. They can potentially have up to four litters and 45 young a year, but Nebraska muskrats normally have two to three litters of five to seven young each. Additionally, first-litter females will sometimes have a litter late in the same year they were born. In the spring, dispersal and mating are the two most noticeable activities of muskrats. Dispersal is associated with ice break-up and is primarily the result of increased aggression between animals. Dispersal movements can be long, cross-country treks or shorter, less dramatic movements within a marsh. Dispersing muskrats experience extremely high mortality from predators, exposure, starvation and accidents, but the movements play a vital role in repopulating vacant habitat.
In Nebraska, the breeding season begins in March and runs through summer. After pair bonds are established, muskrats establish territories and defend them fiercely against neighbors. Peaks in litter production occur in April, May and June, and there is another small peak during August-September. Litters are born blind and helpless, four weeks after mating occurs. The young go on short excursions within two weeks and are weaned at about three weeks. Adult females breed soon after the birth of a litter and prepare their dens for the new arrivals soon after the litter is weaned.
Mortality:
While muskrats are prolific, they are short-lived. Most muskrats live less than one year, and two- to three-year-olds are extremely rare. Muskrats are prey for a wide range of predators, including hawks, owls, mink and northern pike. Many muskrats are killed or seriously wounded in territorial battles throughout spring and summer, and few spring dispersers make it safely to a new home. Muskrat populations can fluctuate wildly from year to year, based on vegetation and water conditions, and populations can actually be too successful for their own good. Sometimes a marsh produces so many muskrats during the summer that there is insufficient vegetation left in the fall for food and lodge construction. This is called an "eat out'-the muskrats eat themselves out of house and home. When muskrat numbers get too high in late summer and fall, the population becomes stressed. Fighting and cannibalism increase, reproduction decreases and mal- nutrition increases. Two contagious diseases, Tyzzer's disease and tularemia, can have devastating effects on local muskrat populations under these conditions. Muskrat numbers can be reduced to a fraction of the original population in just a few weeks by these diseases. Unfortunately, these diseases will persist in a marsh and continue to depress muskrat populations for a number of years. Regulated trapping can reduce the impacts of disease, starvation and fighting on muskrat populations, by removing surplus animals when numbers are high and populations are under stress.
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