Muskrats (Ondatra zibethicus) are easily recognized by their moderate size, their blunt head, and small non-descript ears and eyes. Adult muskrat weigh between 2.5 and 4 pounds, and total length may range from 23-26 inches, with a tail length of 8-11 inches. A scaly, laterally compressed tail with a fringe of coarse hair along the underside of the tail is a feature muskrats share with no other New York State mammal. They possess large hind feet with partial webbing in between their toes with a row of coarse hairs along the outer edge of each foot. Their fur can vary in shades of brown and in some cases black and consists of a soft, dense undercoat with an interspersion of longer, coarse guard hairs.
Found throughout New York State, muskrats occupy a variety of aquatic habitats including ponds, lakes, marshes and streams, and can also occur in brackish habitats. They prefer marshlands, but are found to occupy wetlands and waterways that are heavily vegetated, particularly with cattails, bur-reeds, and bulrushes.
Food and Feeding
The roots and stems of aquatic vegetation are the muskrats' dietary staple. Animal matter is also consumed in times of vegetation shortages or peaks in abundance of invertebrate species. They have been known to eat mollusks, fish, various invertebrates and even turtles. Classic signs of the presence of muskrat are well-matted resting and feeding platforms such as the bare edges of stream banks, the tops of tussock grass clumps, or nestled within aquatic plants. These are often littered with piles of vegetative debris and occasionally crayfish or mussel remains, as well as droppings. Muskrat foods will vary with the type of habitat. Marsh dwellers may eat aquatic plants such as cattails almost exclusively, whereas animals in large bodies of water such as lakes and ponds may be more opportunistic, thus accounting for a more omnivorous diet.
Primarily nocturnal, muskrats may also be active during daylight hours and remain active year round, as they do not hibernate. Muskrat will defend their territory vigorously from other muskrat and potential predators, especially prior to and during the breeding season. Territory holders are usually older adults, while younger animals remain subordinate and are more likely to fall victim to predators as they are forced into sub-optimal habitats by territorial adults.
Den construction is dependent upon the type of habitat occupied. When in a stream habitat, muskrat burrow into the banks to create dens. One or more entrances are hidden underwater and lead to chambers located above the waterline. They excavate channels or runways in shallow water leading from den entrances for ease of mobility. In marshy habitats, a dome-shaped hut is constructed on a firm substrate using emergent vegetation in the immediate area. Regardless of den type, muskrat activity may be destructive to the banks of waterways and plant communities in the immediate area of a den site.
During winter months, another type of structure created by muskrats is referred to as 'push-ups' or 'breathers'. These are masses of vegetation collected from underwater and pushed up through cracks or holes in the ice. Ultimately, these freeze solid and serve as resting places and are maintained as breathing holes.
When muskrats manipulate vegetation during feeding or while constructing dens, they impact many other species that share these habitats. Some species, such as turtles, use muskrat houses as winter hibernacula. Canada geese and mallards will nest on top of muskrat huts. A unique ecological situation occurs in western New York that includes muskrat, bur-reed, and the state endangered black tern. As muskrat consume the bur-reed, a primary food item, they create open matted areas on the water surface that black terns can use as courtship and nesting areas, thereby increasing tern reproductive success.
The breeding season starts in April, with the first litter born in early May. After a gestation period of 25-30 days, muskrats give birth to 4-8 young or kits, and can have up to three litters a year.
Nearly hairless at birth, kits are blind for about the first two weeks of their lives, after which they venture out of the den for their first swim. Females born in early spring may mate in autumn of the same year; however, muskrat in the northern parts of the species range do not reach sexual maturity or adulthood until the April following birth.
Predators, Parasites, and Diseases
Automobile collisions and trapping are two major sources of muskrat mortality. In addition, muskrats are a valuable food source for a wide variety of predatory wildlife. Raccoon and mink are their primary predators and other carnivores such as fox, coyote, red-tailed hawks and great horned owls readily prey on muskrat.
Historically, muskrat are susceptible to and have been ravaged by a variety of diseases such as tularemia, leptospira, salmonella, and hemorrhagic fever, but these diseases have not been extensively reported in New York State. As with most wildlife, muskrats can act as hosts to a wide variety of endoparasites such as intestinal roundworms and tapeworms that do not necessarily have a negative impact on the animal's overall condition. External parasites such as fleas, mites and ticks, which inhabit the soft underfur, also take up residence in the warm, dry interior of muskrat houses.