They also tunnel into earth banks. Their nests are well above high water and have tunnels exiting underwater below the lowest freezing level. Muskrats construct slides and make fairly well-defined channels through vegetation along banks of streams and ponds. Mature females usually have two litters per year and annually give birth to 15 young, or 7 to 8 per litter. Mating begins as soon as there is open water in the spring.
Females mate again three to five days after the first litter.
Their second litter is born about 25 days later. Thus there are two peaks in breeding activity separated by about 30 days. Evidence indicates that male muskrats remain at the same den with the female for several weeks. The young are weaned at about 1 month of age, but may stay with the parents for awhile longer. Second litters often overwinter in the same den with their parents.