Sexual maturity in young otters usually occurs at 2 years of age for both sexes. Reproduction in river otters is complex because of delayed implantation. Otters mate between December and April and the fertilized eggs exist in a free-floating state until the following winter. When the eggs implant in the spring, growth of the fetus lasts 60 to 65 days, until kits are born between February and April. In the south, birth dates are earlier. One litter is produced each year and litters usually contain 2 to 4 kits. Only the female cares for the young; they usually remain together as a family group though the fall and winter. Young disperse between April and May at 12 to 13 months of age.
River otters select sites for denning based on availability and convenience, including hollow logs, crevices formed from rocks, and abandoned lodges and bank dens of beavers and nutria. River otters have been found living in lodges with beavers. They frequent unused human-made structures. Natal dens tend to be located on small waterways that lead to major drainages or lakes.
Otters have immense aesthetic and recreational value. Their morphology enables them to move through water with agility, grace, and speed. Seasonally, they may travel 50 to 60 miles along streams and lake shores and have home ranges up to 60 square miles. Males may travel up to 10 miles in a single night.
River otters are nocturnal but frequently are active during the day in areas that are undisturbed. The basic social group is a female and her offspring. Otters spend much of their time feeding and playing in groups, repeatedly sliding down steep banks of mud or snow. River otters are active year round. Otters tend to reduce daily movements during winter months, often by 50%.
River otters are always associated with water (fresh, brackish, and salt water), although they may travel overland for considerable distances. They occur at much higher densities in the Great Lakes region, brackish marshes and inlets, and other coastal habitats inland. In colder climates, otters frequent rapids, waterfalls, and other areas that remain free of ice. Vegetative cover and altitude do not appear to influence distribution as much as water quality, availability of fish, and suitable sites for denning.
River otters eat about 2½ pounds of food per day, including several species of fresh and anadromous fishes (born in freshwater, migrate to the ocean to grow into adults, then migrate back into freshwater to spawn), and crayfish. Otters opportunistically prey on amphibians, reptiles, and crustaceans. They rarely eat birds. Otters sometimes prey on beaver. Consumption of game fish in comparison to non-game rough fish generally is proportional to the difficulty or ease with which the fish are caught. The otter-caused losses of warm-water sport fish are believed to be minor in comparison to the losses river otters can inflict on species of fish in cold water, such as trout and salmon. Warm water provides an abundance of alternate species of prey.