Common Muskrat
(Ondatra zibethicus) #63-112

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Physical characteristics and distribution

Common Muskrat Ondatra zibethicus

The head and body length of the single species O. zibethicus is 229-325 mm, tail length is about 180-295 mm, and adult weight is 681-1,861 g. The coloration of the upper parts of the pelage are dominated by the long, shiny guard hairs which provide a protective layer over the soft dense underfur. The primary color ranges from medium silvery brown to dark brown to almost black. The underparts are generally lighter and the feet and nearly hairless tail are dark brown to black.

O. zibethicus is particularly well suited to swimming. The hind foot is partially webbed with a row of short stiff hairs along its edge which is known as the swimming fringe. In addition to being almost hairless, the tail is scaly and flattened and serves as a rudder. The name Muskrat derives the musky odor secreted from the musk glands in the animal's perineal area. In males, these glands are larger and open within the foreskin of the penis. During urination, the secretions mix and are deposited along the muskrat's route. Females usually have two pair of mammae; one inguinal pair and one pectoral.

They prefer aquatic habitats and can be found in fresh and saltwater marshes, lakes, ponds, rivers and sloughs. The depth of the water should be enough to keep it from freezing completely in the winter, but shallow enough to support a sufficient amount of vegetation which is not submerged.

When threatened, O. zibethicus can remain underwater for as long as 17 minutes. Average submersions are 2-3 minutes. Largely nocturnal and crepuscular, the Muskrat may be seen foraging during the day, especially in winter. Depending on its chosen habitat, the Muskrat builds one of two types of shelter. Along flowing streams or canals, a burrow is dug in the adjacent bank or dike. The entrance is located below the lowest depth of where the water freezes, and a tunnel extends for as long as 10 meters to a dry chamber above the high water level. Several burrows may be interconnected.

In marsh or swamp habitats, a mounded house of vegetation such as cattails and grasses, is built and covered with mud, sheltering an interior dry nest. The house is built on a clump of vegetation or directly in the ground and may have tunnels which exit underwater. The structure is about 1-2 meters wide and 1 meter tall. In northern climates, O. zibethicus may build a dome of vegetation over the ice where When the animal can surface through a hole in the ice to eat.

The most valued foods are cattails and bulrushes in North America and water lilies in Europe. The diet may also include other vegetable matter and animals such as crayfish, crabs, mussels, and small fish.

A mated pair appears to be monogamous, though the male will live in a separate nest while the female is suckling her young. The offspring stay with their parents, helping to maintain the nest, until they reach sexual maturity. Females are polyestrous. In the northern part of its range, O. zibethicus usually has 2 litters per year, while in the south the breeding season is continuous, producing as many as 5 or 6 litters per year. Gestation last 25-30 days and litter size ranges from 1-11 young. Average litter sizes may be as low as 2.4 in some southern areas and as high as 7.1 in northern areas. Sexual maturity differs according to geography as well, and is achieved much earlier in southern areas.

O. zibethicus is found in North America, north to the treeline, including Newfoundland; south to the Gulf of Mexico, Rio Grande and lower Colorado River valleys. Intorduced to Czech Republic and now widespreaad in the Palearctic, including C and N Europe, most of the Ukraine and Russia, and Siberia, and adjacent parts of Mongolia and scattered throughout China, NE Korea, and Honshu Isl, Japan; also into southernmost Argentina.

Description of the brain

Animal source and preparation
All specimens collected followed the same preparation and histological procedure.

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