River Otter(Lutra canadensis Schreber)From: Saunders, D. A. 1988. Adirondack Mammals. State University of New York, College of Environmental Science and Forestry. 216pp.
With the demise of the wolverine (Gulo luscus Linnaeus) in the Adirondacks in the late 1700’s or early 1800’s, the river otter became the region’s largest member of the weasel family. Adults weigh 5.4-13.6 kg (12-30 lb) and are 89-130 cm (35-51.2 in) in length. Females are slightly smaller than males. The round, tapered tail accounts for approximately one-third of the total length. The cylindrical body is long with short legs. The five, clawed toes of each foot are webbed with furred soles. The head is broad and flat with short, rounded ears and small eyes. The ears and nostrils are valvular and can be closed underwater. The short muzzle ends in a large nose pad. The glossy guard hairs overlay a short, dense, oily underfur. The general color is brown, darkest on the upper parts but paler and grayish below.Range and Habitat: The range is most of North America from the tree line south to Mexico except for parts of the arid Southwest in the U.S. Water pollution and, formerly, the unregulated trapping of this valuable furbearer, eliminated many populations throughout this broad region. The river otter is a common resident throughout the Adirondack Park in lakes, beaver ponds, rivers, streams and their tributaries to elevations of at least 681 m (2234 ft).
River otters do not dig their own dens but may modify the unused burrows of other mammals. Otters also dwell in abandoned beaver lodges, hollow logs, under tangles of roots, and within rocky ledges. Dens are usually near water and often have inconspicuous underwater entrances.
Food and Feeding Behavior: River otters are carnivorous. They eat fish, amphibians, reptiles, and aquatic invertebrates. Occasionally, river otters may kill and eat birds and mammals, especially young beavers, and muskrats. Fish and crayfish make up a substantial part of the diet. Bullheads, suckers, and other species of slow-moving rough fish rather than game fish represent the majority of the fish in the diet. River otters overtake or outmaneuver fish underwater, and carry them to the shore, or the surface of the ice before eating them.
Activity and Movement: The river otter is active during all months of the year, leaving the den at night or twilight but often continuing its foraging or other activities until mid-morning or even later in the day. Much activity occurs in the water. The webbed feet, strong legs, and long, muscular body and tail adapt the otter to a semi-aquatic lifestyle. The furred soles of the feet enable an otter to climb over slippery rocks, logs, and ice. Once in the water, an otter is a tireless and superb swimmer, propelling itself with the tail, which also serves as a rudder, and powerful thrusts of the hind limbs. A river otter may swim at speeds up to 9.7 km/hr (6 mph), and remain submerged for several minutes at a time, diving 13.7 m (45 ft) or covering 402 m (1320 ft) while submerged. Although river otters tend to move along waterways, they may travel overland to reach a different watershed. On land, they lope or bound. When moving over snow or ice, otters alternate bounding with gliding. Grooming is a frequent activity and entails rolling in the snow, mud or vegetation, often at the same location known as a landing or a hauling-out site. River otters engage in activities collectively designated as play, for example, repeated touching, shoving, tossing, and manipulating of prey, stones or other objects, or tobogganing down mud or snow slides.
Reproduction: River otters mate in late winter or early spring, usually March or April, shortly after the female bears her annual litter. Mating may take place on land, but is more likely to occur in the water. Most embryonic development is during the last 50-61 days of the 9.5-12.5 month gestation period. The average litter size is 2 or 3 (range 1-6). Newborn otters are blind, fully furred, and weigh approximately 130 g (4.6 oz). Their eyes open by 35 days. The young begin to venture outside the natal den about 10 days later. At this age the female begins to teach her young to swim by carrying, dragging, or enticing them with food into the water. She may even permit them to ride on her back for short distances as part of the training. The female also teaches her offspring to hunt by letting them attack and retrieve fish and crayfish she releases near them. Weaning is complete by the time the young are 91-120 days old. They remain with the female until she bears her next litter the following spring.
Young females may breed when one year old, although most do not mate until two years of age. Males are capable of breeding as two-year olds, but are more likely to be successful when older. River otters may live 20 years in captivity; few survive more than 10 years in the wild.
Predators: The river otter has few natural predators because of its strength, sharp teeth, aggressiveness when attacked, and aquatic habits. Snapping turtles, coyotes, bobcats, and foxes may occasionally prey upon young otters, and rarely, adults.
- Social system - Comparatively little information exists about the mating system and social organization of the river otter. The female-young or family group comprises the basic social unit. After the young are weaned, a male, presumably the female’s mate, or non-breeding female may accompany the family group temporarily. Most adults live solitary lives except during the breeding season. Adult males may mate with more than one female, and may compete aggressively for a female in estrous. Home ranges overlap considerably, but avoidance appears to be a key factor in the social organization of neighboring adults. Home range size and shape vary with food abundance, age, sex, and habitat configuration. The home range of an adult is 4.8-16.1 km (3-10 mi) at any given season, and over the lifetime of an individual may encompass 32-50 km (19.9-31.1 mi) or more of shoreline. A study of the river otters in Idaho indicated a density of one individual for each 2-3 km (1.2-1.9 mi) of “straight-line” waterway or between 1.7 and 3.7 otters per 10 km (6.2 mi) of streams.
- Communication - Vocal, tactile, and chemical signals are the chief means of regulating otter social behavior. River otters use chirping and humming sounds as contact calls. They hiss, purr, and whistle in other contexts. In aggressive and sexual encounters, river otters growl and squall. Products from anal glands coat droppings or spraints, as otter scats are known. River otters defecate in conspicuous places within the home range. These toilets and other objects such as scrapes or grass tufts that otters twist together and spray with scent from the anal glands, are likely clues of occupancy, individual identity, sexual status and perhaps, other information.
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Liers, E.E. 1951. Notes on the river otter (Lutra canadensis). Journal of Mammalogy, 39:483-439.
Melquist, W.E. and M.G. Hornocker. 1983. Ecology of the river otters in Idaho. Wildlife Monographs, 83:1-60.