What is a large semi-aquatic rodent native to Canada, much of the United States, and parts of northern Mexico that constructs homes out of sticks, twigs, and mud in lakes or streams?

Option D

Beaver, (genus Castor), either of two species of amphibious rodents native to North America (Canada), Europe, and Asia. Beavers are the largest rodents in North America and Eurasia and the second largest rodents worldwide. Their bodies extend up to 80 cm (31 inches) long and generally weigh 16–30 kg (35–66 pounds, with the heaviest recorded at more than 85 pounds). They live in streams, rivers, marshes, ponds, and shorelines of large lakes and construct dams of branches, stones, and mud, forming ponds that often cover many hectares. Ecologists often refer to beavers as “ecosystem engineers” because of their ability to alter the landscapes in which they live.

Beavers have short legs and a stout body with a small, broad, and blunt head. Massive chisel-shaped incisor teeth have orange outer enamel because iron has replaced calcium, and this makes them stronger than most rodent incisors. Upon submergence, folds of skin (valves) close the nostrils and the stubby rounded ears, and the eyes are protected by a membrane that keeps water out (nictitating membrane). The fur-lined lips close behind the incisors, blocking water from the mouth and lungs and allowing the animal to cut, peel, and carry branches underwater. Small front feet with five clawed digits dexterously manipulate food. The hind feet are quite large, and the five digits are connected by webbing, which makes them useful as paddles for propulsion underwater. Claws of the second hind digits are split and have serrated edges used for grooming the fur. Fur consists of a grayish to brown layer of short, fine, and dense underfur that keeps water from reaching the skin. Over this layer are long, coarse, glossy guard hairs ranging in colour from yellowish brown through reddish brown to black; underparts of the animal are paler. The distinctive tail is scaly, flat, and paddle-shaped and measures up to 45 cm (about 18 inches) long and 13 cm (5 inches) wide. Both sexes possess castor glands that exude a musky secretion (castoreum), which is deposited on mud or rocks to mark territorial boundaries. Anal glands secrete oil through skin pores to hair roots. From there it is distributed with the front feet and grooming claws over the whole body to keep the fur sleek, oily, and water-repellent.

Beavers are colonial and primarily nocturnal. Their characteristically dome-shaped island lodges are built of branches plastered with mud. In marshes, lakes, and small rivers, beavers may instead construct bank lodges, and in large rivers and lakes they excavate bank dens with an underwater entrance beneath tree roots or overhanging ledges. Each lodge is occupied by an extended family group of up to eight individuals: an adult pair, young of the year (kits), and yearlings from the previous litter. Lodges are usually 3 metres (10 feet) high and 6 metres (20 feet) across the base but can be as large as 5 metres (16 feet) high and 12 metres (39 feet) wide. One or more tunnel entrances open below the water’s surface into a spacious central chamber above water level; the floor is covered with vegetation. An entry tunnel leads to the nest chamber above the waterline. In winter the moist walls freeze, adding insulation and making the lodge impenetrable to predators.