The following information presented is a full break down on the North American River Otter. All information was collected from various sources for use on this web site. It is current and up to date information as of
8/1/04. If you find that something was left out or have a question about
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Mustelids also include skunks, badgers, weasels, minks, and sea otters. River otters make up the subfamily Lutrinae, of which there are 13 recognized species in four genera.
D. Genus, species----Lutra canadensis
There are eight species in the genus Lutra. Species can be distinguished by the shape and size of the hairy patch on the nose pad.
E. Fossil records
The first aquatic otter appears in the fossil record 30 million years ago. Another species, Paralutra jaegeri, lived 25 million years ago and may have been an ancestral form of modern river otters.
River Otter Habitat and Distribution
Historically, North American river otters were widely distributed throughout Alaska, Canada, and the contiguous United States. Due to fur trapping and hunting, by the nineteenth and twentieth centuries river otter populations were greatly reduced throughout the lower 48 states except in the northwest, the upper great lakes region, New York, New England, and the states along the Atlantic and Gulf coasts.
Today the North American river otter is found in reduced numbers throughout Alaska, Canada, and the contiguous United States. Other related river otter species are found in South America, Europe, and Asia.
River otters depend on wetlands or other aquatic environments such as lakes, rivers, streams, and ocean bays. River otters live in burrows under roots and logs or in abandoned dens of foxes, badgers, or rabbits. A river otter's den may have one entrance below water and one above water. River otters can also be found in hollow tree trunks.
Although highly aquatic, river otters may travel several miles over land to reach another stream or lake.
A river otter's home territory is usually about 5 to 48 miles along a river or stream bank. Males have larger territories than females. The territories of different otters may overlap each other.
A river otter marks its territory with feces and urine, then sprays its droppings with a strong-smelling liquid from scent glands near its tail. Since different river otters use the same sites to mark their territories, the droppings seem to be a visual indicator of where an otter should sniff to determine which particular river otters have been in the area. They also rub their musky scent on logs, stones, or mounds of grass throughout their home areas.
Male river otters average 3 to 4 ft. in length including the tail, which accounts for about one third of total body length. The largest river otter ever recorded measured 5 ft. An adult male may weigh 10 to 25 lbs.
Female river otters measure over 3 ft. An adult female may weigh 12 to 18 lbs.
B. Body shape--
A river otter is long, muscular, and streamlined.
Like its relative the weasel, the river otter has an upward flex in its spine which causes a hump at the hips. The spine is so flexible that an otter can bend itself nose to tail tip forwards, sideways, and backwards to form a complete circle.
A river otter is brown and has a silvery sheen on its belly.
A river otter's forelimbs have paws with well-developed webs that extend at least to the base of the last bone on each digit. The webs are covered with fur, but the toes and sole pads are furless.
Front paws have well developed, non-retractive claws that are about 1/2 an inch long. The digits are very sensitive to touch.
River otters use their dexterous paws for feeling for prey in muddy waters, enlarging a den in a riverbank, handling small objects, and swimming.
E. Hind limbs--
Hind paws are webbed between each of the five toes. Toes are tipped with sharp, non-retractable claws.
Hind paws have small calloused pads to help give the river otter traction on slippery surfaces. The webs are covered with fur, but the toes and sole pads are furless.
River otters can stand upright on their hind paws.
The river otter's muzzle is broad, flat, and short, with a blunt snout.
The nose is broad, black, and rounded. A river otter closes its nostrils while under water.
River otters have long stiff vibrissae (whiskers) on their muzzle. Vibrissae are 2 to 4 inches long.
Vibrissae are used to locate prey and to explore the size of holes or burrows.
Vibrissae are also used to detect underwater movement or obstructions by changes in water current.
River otters have 36 sharp teeth, which they use both for catching food and
Otters use their incisors to tear small pieces of flesh.
Canine teeth are used to grasp slippery fish or break the tough skin of a fish.
Well-developed premolars are used for crushing bones.
A river otter's brown eyes are located near the front of the skull.
A river otter's ears are small, rounded, and set back on either side of its head. Ears are about 3/4 of an inch long. River otters close their ears under water.
A river otter's flexible, muscular tail is flattened and thick at the base, tapering to a point.
The powerful tail acts as a rudder when the river otter is swimming, and as a brace when the river otter stands on its hind paws.
The river otter's fur is short and dense.
River otters have two kinds of hair: long strong guard hairs, which form the outer layer, and finer, more dense underfur.
The guard hairs form a waterproof barrier that covers and protects the underfur. The waterproof guard hairs on the back are about 1 inch long.
The underfur traps a layer of air to provide insulation. Underfur on the back is about 3/4 of an inch long.
There are about 156,000 hairs per square inch on a river otter.
Fur insulates an otter by preventing water from penetrating the otter's skin.
A river otter's hearing is very acute. Hearing is more important than smell for detecting danger on land.
River otters may be able to see better under water than above water. They can change the shape of their lens to a more spherical shape to compensate for the refractive qualities of water.
Paws are sensitive to touch in locating food in mud or beneath rocks. Vibrissae detect vibrations of prey's movement in the water.
Little is known about a river otter's sense of taste.
A river otter's sense of smell is of the greatest importance in locating food on land. It also alerts them to the whereabouts of other river otters, home, and danger.
Adaptations for an Aquatic Environment
While swimming slowly or at the surface, river otters "dog paddle" using all four legs. River otters swim as well on their backs as they do on their bellies.
Webbing increases the foot area for extra push in swimming.
On the surface a river otter can swim about 6 miles per hour. Below the surface, the river otter can swim at 3 to 4 miles per hour.
When swimming, the river otter uses its body and tail for steering.
Records indicate that one river otter was caught in a fish net that was set 60 feet deep. This is likely an exceptional depth. The average diving depth is not known.
All aquatic mammals have special physiological adaptations used during a dive. These
adaptations enable a river otter to conserve oxygen while it is under water.
River otters, like other mammals, have a slower heartbeat while diving.
When diving, blood is shunted away from tissues tolerant of low oxygen levels toward the heart, lungs, and brain, where oxygen is needed.
River otters can hold their breath six to eight minutes before resurfacing. They have been known to swim 1/4 of a mile underwater before resurfacing.
While swimming under frozen ponds and lakes where air is not available, river otters can breathe bubbles of air trapped against the ice. They can also exhale a breath underwater and immediately rebreathe the bubble that formed below the surface of the ice.
Fur insulates an otter by preventing water from penetrating to the otter's skin. See Physical Characteristics under H
A high intake is necessary to maintain a high metabolic rate, which helps an otter keep warm. River otters eat about15% to 20% of their body weight daily. See Diet and Eating Habits under B.
A. Daily activity cycle--
River otters may be either diurnal (daytime) or nocturnal (nighttime) but generally more active at night.
Much of a river otter's day is spent grooming its fur to spread oils from skin glands throughout its fur for waterproofing.
B. Social structure--
River otters are basically solitary.
The basic family group consists of a female and her young. Such families break up before the female gives birth again. Male river otters may or may not take part in the family group.
There is no strong pair bond between a male and female except for several months during the breeding season.
C. Social behavior--
Some river otter behavior could be described as "playful", but what we interpret as "play" is adaptive behavior that reinforces social bonds and encourages young river otters to practice their hunting and fighting techniques.
River otters slide down muddy or snowy banks headfirst with their forepaws at their sides. Upon reaching the bottom, they climb up again and repeat the slide.
Juveniles chase one another in the water and tackle each other on shore.
A river otter's running stride leaves tracks from 1 to 2 feet apart. Tracks can be distinctive, especially in snow, where its body may leave a trough up to 1 foot wide as the otter pushes itself along or "sleds". Foot tracks are 3 inches wide.
Diet and Eating Habit
A. Food preferences and resources--
River otters are active predators whose diet consists mainly of fishes. In the water, river otters also prey on frogs, snakes, turtles, salamanders, crabs, crayfish, water
beetles, aquatic insects, larvae, mussels, snails, worms, and ducks. On land, river otters prey on mice, small rabbits, and ground-nesting birds and their eggs.
River otters also eat some grasses, tubers, pond weeds, algae, shoots, and blueberries.
If food is scarce in the winter, river otters prey on muskrats. Occasionally they eat young beavers.
River otters do not store food for the future. They do not kill more then they eat.
B. Food intake--
River otters eat 15% to 20% of their body weight daily, about 3 lbs. of food. They usually don't eat much more then 1 lb. at a time.
C. Methods of collecting and eating food--
River otters hunt mainly at night, usually in the water.
A mother river otter will hunt with her young. She may also join in the hunt with other females and, in the the breeding season, with males.
A river otter catches a fish by waiting just above the water surface. When it locates a fish, it glides into the water and after a brief chase, catches the fish with its teeth.
In shallow water, a river otter drives fish into inlets by slapping the water with its tail. It may corner fish to make them easy prey.
At times several river otters hunt cooperatively to drive fish together.
River otters eat small fish 3 to 5 inches long. Occasionally they catch larger fish of 4 to 5 lbs., but not often. The larger fish are stronger, faster, and harder to catch. Smaller fish are more abundant and easier to catch.
River otters usually bring their fish out on a rock, tear off the scales, and eat the fish headfirst. They may consume their food in the water.
River otters grab ducks by the legs and pull them under water. River otters also take ducks wounded or killed by hunters before the ducks are retrieved.
River otters regurgitate fish bones and scales.
A. Sexual maturity--
Female river otters reach sexual maturity at about two years of age.
Mature female river otters usually go into estrus ( in heat ) in the first few months of the year.
Males are not usually sexually mature until five to seven years of age.
B. Mating activity--
The peak breeding season is March and April, when river otters travel more often and leave many scent markings to mark their territory.
River otters become territorial during the breeding season. Males will sometimes fight, competing for mates, if mutual avoidance does not work.
One male will mate with several females.
Young females coming into their first estrus are usually the first to breed. The older females usually rebreed within a day or so after giving birth.
Mating activity usually takes place in the water.
The male, swimming up behind the female, seizes her by the neck with his teeth and bends his body down around and under her tail.
When breeding takes place on land, the male curls up and wraps around the female.
Mating lasts 15 to 20 minutes.
Birth and Care of Young
The total gestation period is approximately 9 1/2 to 10 months. The maximum total gestation period recorded was 12 1/2 months and the minimum 9 1/2 months.
Gestation includes a period during which the embryo remains undeveloped. This holding period is called delayed implantation.
The fertilized egg divides into a hollow ball of cells one layer thick ( a blastula ), stops growing and lies freely in the uterus for an undetermined length of time. The blastula then implants in the uterine wall and continues to develop.
Delayed implantation assures that the pup is born during the best time of the year for survival and allows the female to get into good physical condition and to use her energy for nursing her newborn pups.
The actual embryonic development is about two months.
B. Birth seasons--
River otter pups are born between February and April.
C. Frequency of birth--
A female river otter may bear a litter each year.
The average litter of river otters is two to four. On rare occasions there will be five.
Female river otters choose a snug den for the birth of their young. They use muskrat houses and woodchuck burrows close to the water. A mother will also pup in cavities under a stream bank, in hollow logs, or in openings under a tangle of roots along river banks.
The mother chooses a den where there is an abundant food supply nearby to minimize the time pups are left alone while she goes hunting.
The female river otter drives the male away from the den at birthing time.
The male may travel away or may stay near a female's den. After six to eight weeks the male river otter may rejoin a female and her pups.
The female is rebred by any male within the area shortly after birth when she ventures from the den in search of food.
E. Pup at birth--
A river otter pup is 7 to 8 inches long and weighs 4 to 5 oz.
River otter pups are born toothless, blind, and helpless.
A pup's black, silky fur is not waterproof.
River otter pups spend most of their time either suckling or sleeping.
F. Care of young--
Mother river otters nurse their pups. Otters' milk is more than 24% fat and 11% protein.
A female river otter has four nipples on her abdomen just forward of the hind paws. When hungry, a pup crawls up on its belly in search of her nipples, twittering softly until it locates them. Suckling vigorously, the pup wags its tail from side to side and kneads its mother's belly with its front paws.
Pups are weaned at about three to four months.
A mother river otter warms the pup by curling completely around it, putting her head in the center of this circle.
If the young are weaned and the mother is killed, the male river otter may raise and protect the pups.
G. Pup development--
Young river otters grow rapidly. At one week of age they weigh about 6oz. and by ten days their weight increases to about 1 lb.
At five weeks the pups' eyes open and they begin to interact with one another.
Pups begin to acquire the waterproof coat of an adult and shed their baby fur at five weeks.
At two months the pup and family leave the den to explore their home range.
Pups begin learning to swim at about two months.
The mother encourages her pup into the water by swimming around and calling out to it. Once in the water, the pup holds onto the mother's fur.
The mother swims underwater, forcing the pup to swim. The mother may also grab the pup by the scruff of the neck and carry or throw it into the water.
When first placed in the water pups have difficulty staying upright, but after a few minutes they can swim on their own.
Young river otters learn how to hunt by mimicking their parents. At first they catch only slow-moving prey like snails and worms but soon catch fish.
Young river otters remain with their mother throughout their first winter. Pups are independent before the mother gives birth again.
Pups reach adult size after two years.
River otters are easily recognized by their staccato, "chuckle".
When anxious, river otters utter a hissing sound or a yelp.
When threatened, the sound is a deep, nasal growl or a piercing scream.
A twittering sound is used as a companionship call.
The pups in a den peep like chicks.
The begging and alarm calls of pups and the mating calls of adults resemble soft whistles.
A one-syllable chirp occurs as a contact call that carries quite a distance.
B. Scent markings--
River otters use urine and a musky scent from scent glands to communicate with other river otters.
Scent markings may provide otters with important information such as
individual identity of the animal that produced the scent, its age, sex, breeding condition, and territory.
The scent of a river otter's spray is very strong and can be detected near the deposition site for several weeks.
Longevity and Causes of Death
Available information indicates that a river otter has a potential lifespan of about 15 years.
Natural enemies are few. Healthy adult river otters are the top predators in their food chain.
Bobcats, lynxes, coyotes, wolves, and large reptiles may prey upon sick, injured, or old river otters.
River otter pups are occasionally prey to great horned owls.
C. Human interaction--
Hunting and trapping--
Trade in river otter fur was once widespread. It has declined considerably due to the reduction in river otter numbers and legal action regulating hunting. River otters are still hunted for their fur in 38 states and provinces in the United States and Canada.
River otters are sometimes trapped accidentally in traps set for other species, such as beavers.
The river otter's primary food source, fish, has declined through straightening streams, lowering water levels, shore construction, and activities that promote silting and clouding of streams.
Many river otters are hit by automobiles when crossing roads that divide bodies of water or river otter hunting territories.
River otters ingest poison from fish contaminated by toxins. They've been found with
toxic levels of the insecticide DDT and environmental pollutants such as PCBs and cesium. Such toxins enter the food chain through agriculture and industry.
D. Disease and parasitism--
As in any animal population, a variety of diseases and parasites can be responsible for river otter deaths.
A. Endangered Species Act--
North American river otters have been listed as endangered or threatened in several states but have
not been listed for protection under the United States Endangered Species Act.
The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) is an international treaty developed in 1973 to
regulate trade in all wildlife species. The North American otter is protected by this treaty.