Beavers are large rodents adapted to live and work both on land and in the water, weighing up to 60 pounds in Mississippi and moving clumsily on land because of their squat bodies and low centers of gravity. In the water they are powerful swimmers, using webbed hind feet and a flat tail that serves as a rudder, a prop when sitting up, and a danger signal when slapped on the water. Their chisel-like cutting teeth are strong, sharp, prominent, and self-sharpening, and they can gnaw underwater because their lips seal out the water. Their nose and ears have valves that close when they dive and open when their heads rise out of the water, and they have keen hearing, an acute sense of smell, and supposedly excellent vision underwater though only fair vision on land. Beavers instinctively keep busy, felling trees, floating logs through canals they dig, building lodges and dens with underwater entrances and tunnels, and constructing dams to form ponds; one such dam in Mississippi measured 1,080 feet in length. Their service to good conservation practice far outweighs the damage they do. Beavers usually pair in February and stay mated for life, and their young, called kits, are born in May completely furred and with eyes open, with an average litter of four. Their diet, mostly bark, changes with the seasons, including sweetgum, pine, and hophombeam in spring; willow, cottonwood, and tender green plants in summer; acorns, buttonbush, willow, and giant cane in fall; and ash, elm, and miscellaneous plants in winter.