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tossing and diving for rocks and clamshells, the repeated capture and release of live prey and, occasionally, sliding. Although sliding can be a play activity, and otters will repeatedly slide down a wet bank, sliding is more commonly a wintertime mode of overland travel. Otters will bound several times, then use their momentum to slide in the snow for 10 feet or more.
Reproduction
River otters generally reach sexual maturity when they are two years old. Most females do not become pregnant until their third year. Although males become sexually mature at about two years of age, in high density situations they often are not successful breeders until about five years of age. Litter size varies from one to six, but litters of two or three are most common. The small litter size, along with the fairly late maturity, results in a low reproductive potential typical of predatory animals.
In most temperate areas, breeding occurs in March or April, but the breeding season is quite variable. Otters exhibit a phenomenon called delayed implantation, common in the Mustelidae family, which involves discontinuous development of the fertilized egg. After conception, the fertilized egg remains floating in the uterus for about nine months. The egg then implants in the uterine wall, and following a gestation period of about 60 days, the young are born nearly a year after conception. Young otters are helpless at birth, and their eyes do not open until they are at least three weeks old. Young otters grow quickly, and when they are about 60 days old, they venture from the den and are introduced to the water. The females are devoted parents, teaching the young to swim and even catching and releasing prey to improve the young otters' foraging skills. Adult male otters are not known to participate in caring for the young. Young otters can care for themselves in about five or six months, but the family usually stays together for at least two months longer, often until the birth of a new litter. Although dispersal of young is quite variable, it generally occurs at about 12 to 13 months of age.
Food
Fish make up the greatest portion of the otter's diet. Crayfish are also an important food when available. Other foods include amphibians, insects, mammals and birds. Foods and foraging techniques vary in different areas and at different times of the year. In clear water, otters use their excellent swimming ability to capture fish by sight and direct pursuit. In murky water, they use their whiskers to locate prey.
Otters are opportunistic and will take foods that are most available. The most available fish tend to be the slower-swimming species and those that are most abundant. Although anglers are sometimes concerned that otters will have a detrimental effect on game fish, the fishing activity of otters has not been found to be a problem with sport fisheries. Food-habit studies have generally found game fish to be taken less often than the slower-swimming rough fish. Several authors suggest that otters may even improve a fishery by removing rough fish.
Otters have occasionally caused depredation problems at fish hatcheries and rearing areas. There, only game fish at artificially high densities are present, and the fish cannot escape capture. Otters have also been accused of damaging fur resources in some areas, but studies have shown predation on other furbearers to be very unusual.
River otters are quite adaptable, utilizing a variety of habitat types. Although they frequent lakes and ponds, they typically live in marshes and along wooded rivers and streams with sloughs and backwater areas.
Otters live in dens in the ground most of the year. Otters rarely dig dens themselves; instead, they utilize dens built by beavers or other animals. Brush piles, root areas under large trees and similar sites are used as temporary homes. The presence of beavers in an area is important to otters, not only because of the dens they build, but also because the ponds created by beaver dams make ideal otter habitat.
Limiting Factors
River otters have been reported to live nearly 25 years in captivity and about 15 years in the wild. They have few natural enemies. especially while they are in water. On land, young otters are vulnerable to a variety of predators.
Most otter mortality is related to the actions of humans. Although in many areas, the legal harvest of otters serves to remove animals that are part of an annual surplus above what the habitat will support, other human activities can be detrimental to otter populations. Because otters are fairly easily trapped, accidental trapping, primarily in beaver sets, can be a problem when otters are being established and their numbers are still low. In Nebraska, accidental trapping has been the largest known mortality factor for reintroduced animals.
Other detrimental human actions are more indirect and include habitat destruction and the introduction of pesticides and pollutants into the food chain. Through biomagnification, pesticides and pollutants become more concentrated in the tissues of otters and other organisms at the top of the food chain.
Mercury, DDT, dieldrin and polychlorinated biphenyls have all been identified in river otter tissues. Little is known regarding toxic levels of these residues to otters or their effects on reproduction and long-term survival. These contaminants are known to cause a variety of problems in other wildlife and humans.
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