MUSKRATS....  Biology and behavior

Reproduction

        Muskrats are very prolific.  Most females have two or three litters per year (sometimes more depending on location), with an average of about 6 or 7 kits per litter.  The gestation period varies between 25 and 30 days.  Muskrats in California probably have 2 litters per year in the spring and summer months.  The young are able to swim within 14 days, and are weaned at 28 days.  Most become sexually active the spring after their birth.

        Muskrats mate while partially submerged.  Young are born inside nest chambers of houses or burrows.  They may also be born in nests of coots or diving ducks.  Until the young are weaned, the mother cares for them.  Males only care for young in the event of the female’s death.

        Depending on the environment and season, muskrats either construct houses or burrow into banks.  Soil type and slope of the bank are important in determining the permanence and complexity of a burrow system.  Burrows often begin in the water, from 6- to 8-inches below the surface, and penetrate the embankment on an upward slant.  At about 12 to 15 feet from the entrance and above the water level, a dry chamber is hollowed out.  Muskrats may also dig feeder tunnels into nearby pastures and yard areas.  These tunnels may be up to 80-feet long, are shallow and enable muskrats to feed on the roots of plants.  Burrows are most often found in pools, backwater coves, and other areas of slow-moving water.  Burrows are not typically found when banks are less than 0.2 meters high, slope is less than 10 degrees, or when combined sand/gravel content is less than 90 percent.  Houses are usually constructed from river vegetation including cattails and bulrushes.  They are built at water level with several underwater tunnels for entrances.  Muskrats build primary houses or feeding houses.  Feeding houses tend to be much smaller than primary dwellings.  Primary dwellings usually comprise one or more nest chambers lined with fresh plant material.  Water depth, soil texture, and amount of aquatic vegetation influence the site selection for houses.

        Muskrats are primarily herbivorous although have been known to consume crayfish, mollusks, fish and turtles during food shortages or when these prey are especially abundant.  Signs of muskrat feeding activity include food platforms and feeding houses.  Muskrats occasionally cache food in their houses.

        Most activity occurs at night, with peaks at dusk and dawn.  Small home ranges are typical with most activity occurring within a 50- to 80-ft radius of a "home burrow".  Territoriality is common, especially during breeding season.  Most muskrats form temporary sexual pairs, but polygyny does occur.  Male home ranges often overlap two female ranges.

        Muskrats usually disperse in the spring to establish breeding territories or to colonize vacant territories.  Males more frequently disperse than females. Muskrats are most aggressive prior to and during the breeding season.  Females may be more aggressive than males in defending their breeding territory, often killing any intruder.  Competition between muskrats for breeding sites is intense.  The larger or older members of the population usually occupy the best burrow sites.

        Muskrat populations generally cycle every 5 to 10 years depending on location.  Maximum lifespan is probably about 5 years.  Disease, predators, and displacement of young are causes of mortality.  Mink, raccoons, foxes, coyotes and raptors are predators of muskrats.

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