North American river otter

The North American river otter (Lontra canadensis), also known as the northern river otter and river otter, is a semiaquatic mammal that only lives on the North American continent, along its waterways and coasts. An adult North American river otter can weigh between 5.0 and 14 kg (11.0 and 30.9 lb). The river otter is protected and insulated by a thick, water-repellent coat of fur.

North American river otter
A pair at the San Francisco Zoo in 2005
CITES Appendix II (CITES)[1]
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Mustelidae
Genus: Lontra
Species:
L. canadensis
Binomial name
Lontra canadensis
(Schreber, 1777)
Subspecies

See text

Synonyms

Lutra canadensis

The North American river otter, a member of the subfamily Lutrinae in the weasel family (Mustelidae), is equally versatile in the water and on land. It establishes a burrow close to the water's edge in river, lake, swamp, coastal shoreline, tidal flat, or estuary ecosystems. The den typically has many tunnel openings, one of which generally allows the otter to enter and exit the body of water. Female North American river otters give birth in these burrows, producing litters of one to six young.[2]

North American river otters, like most predators, prey upon the most readily accessible species. Fish is a favored food among the otters, but they also consume various amphibians (such as salamanders and frogs[3]), freshwater clams, mussels, snails, small turtles and crayfish. The most common fish consumed are perch, suckers, and catfish.[4] Occasional reports also show the river otter eating other small animals, such as mice, squirrels, birds, and even dogs that they've attacked and drowned.[5][6][7]

The range of the North American river otter has been significantly reduced by habitat loss, beginning with the European colonization of the Americas. In some regions, though, their population is controlled to allow the trapping and harvesting of otters for their fur. North American river otters are very susceptible to the effects of environmental pollution, which is a likely factor in the continued decline of their numbers. A number of reintroduction projects have been initiated to help halt the reduction in the overall population.[8]

Taxonomy and evolution

The North American river otter was first described by German naturalist Johann Christian Daniel von Schreber in 1777.[9] The mammal was identified as a species of otter and has a variety of common names, including North American river otter, northern river otter, common otter and, simply, river otter.[9] Other documented common names are American otter, Canada otter, Canadian otter, fish otter, land otter, nearctic river otter, and Prince of Wales otter.[10] The North American river otter was first classified in the genus Lutra. Lutra was the early European name (from Latin), and the specific epithet canadensis[9] means "of Canada".[11]

In a new classification, the species is called Lontra canadensis, where the genus Lontra includes all the New World river otters.[12]

Molecular biological techniques have been used to determine when the river otter and the giant otter (Pteronura brasiliensis) of South America diverged. These analyses suggest they diverged in the Miocene epoch 23.03 to 5.33 million years ago (Mya), which is "much earlier" than indicated in the fossil record.[13] Fossils of a giant river otter dating back 3.5 Mya (during the Pliocene)[14] have been found in the US Midwest; however, fossils of the modern river otter did not appear in North America until about 1.9 Mya.[15][16] The New World river otters originated from the Old World river otters following a migration across the Bering Land Bridge, which existed off and on between 1.8 million and 10,000 years ago. The otters migrated to North America and southwards again across the Panamanian Land Bridge, which formed 3 Mya.[10]

Subspecies

Listed alphabetically:[17]

  • L. c. canadensis (Schreber, 1777) – (eastern Canada, U.S., Newfoundland)
  • L. c. kodiacensis (Goldman, 1935) – (Kodiak Island, Alaska)
  • L. c. lataxina (Cuvier, 1823) – (U.S.)
  • L. c. mira (Goldman, 1935) – (Alaska, British Columbia)
  • L. c. pacifica (J. A. Allen, 1898) – (Alaska, Canada, northern U.S., south to central California, northern Nevada, and northeastern Utah)
  • L. c. periclyzomae (Elliot, 1905) – (Queen Charlotte Islands, British Columbia)
  • L. c. sonora (Rhoads, 1898) – (U.S., Mexico)

Description
Skull of a North American river otter
The North American river otter's streamlined shape allows it to glide through the water.

The North American river otter has long whiskers that are used to detect prey in dark waters.[18] An average adult male weighs about 11.3 kilograms (25 lb) against the female's average of 8.3 kilograms (18 lb). Its body length ranges from 66 to 107 centimetres (26 to 42 in).[19] About one-third of the animal's total length consists of a long, tapered tail.[16] Tail lengths range from 30 to 50 centimetres (12 to 20 in).[19] Large male North American river otters can exceed a weight of 15 kilograms (33 lb).[20] It differs from the European otter by its longer neck, narrower visage, the smaller space between the ears and its shorter tail.[21]

Its whiskers (vibrissae) are long and thick, enhancing their senses.[16]

Males and female river otters show different non-sexual physical characteristics,[22] with males typically being larger.[16]

North American river otters can live 21–25 years in captivity,[22][16] or 8–13 years in the wild.[22][16]

Form and function
The North American river otter's sensitive whiskers allow it to detect prey in murky water
Foot of North American river otter close up.

North American river otters characteristically approach within a few feet of a boat or a person on shore because they're near-sighted, a consequence of vision adapted for underwater sight. North American river otters have transparent nictitating membranes to protect their eyes while swimming.[10][23][24]

Most mustelids, including otters, have 36 specialized teeth, including sharp canines and carnassials that inflict lethal bites to prey. Also, North American river otters have large molars used for crushing hard objects, such as the shells of molluscs.[25] Additional premolars may be present.[16] The dental formula is 3.1.4.13.1.3.2.

North American river otter have small ears

Behavior

North American river otters only settle in areas that consist of vegetation, rock piles, and sufficient coverage.[26]

Movement
Sliding across ice is an efficient means of travel. Note the long, tapered tail.
Tracks in the snow

Playing

North American river otters are renowned for their sense of play. Otter play mostly consists of wrestling with conspecifics. Chasing is also a common game. North American river otters rely upon play to learn survival skills such as fighting and hunting.[25]: 52–54 

Hunting
Raft of L. c. pacifica surfacing to eat fish

A highly active predator, the North American river otter has adapted to hunting in water, and eats aquatic and semiaquatic animals. The vulnerability and seasonal availability of prey animals mainly governs its food habits and prey choices.[27] This availability is influenced by the following factors: detectability and mobility of the prey, habitat availability for the various prey species, environmental factors, such as water depth and temperature, and seasonal changes in prey supply and distribution in correspondence with otter foraging habitat.[28][29]

The diet of the North American river otter can be deduced by analyzing either stool obtained in the field,[30] or gut contents removed from trapped otters.[31] Fish are the primary component of the North American river otter's diet throughout the year.[32] Every study done on the food habits of the North American river otter has identified varying fish species as being the primary component of its diet. For instance, an Alberta, Canada study involved the collection and analysis of 1,191 samples of North American river otter scat (feces) collected during each season.[33] Fish remnants were found in 91.9% of the samples. Moreover, a western Oregon study revealed fish remains were present in 80% of the 103 digestive tracts examined.[32] Crustaceans (crayfish), where regionally available, are the second-most important prey for otters. Crustaceans may even be consumed more than fish. For example, a study conducted in a central California marshland indicated crayfish formed nearly 100% of the river otter's diet at certain times of the year.[34] However, North American river otters, as foragers, will immediately take advantage of other prey when readily obtainable.[35] Other prey consumed by North American river otters includes fruits,[36] reptiles, amphibians, birds (most especially moulting ducks which render the birds flightless and thus makes them easier to capture), aquatic insects, small mammals, and mollusks.[27] North American river otters are not scavengers; they avoid consuming carrion (dead animals).[29][37] North American river otters do not generally handle prey of a large size relative to themselves but there are occasions where they've been observed ambushing and killing adult common snapping turtles while the large turtles (which are roughly equal in average body weight to a North American river otter) are hibernating.[38] Remains of the much larger North American beaver have been found in North American river otter scat in some regions, although most otter dietary studies in areas where otters and beaver live near each other do not show them to be regular predators of beavers (despite the claims of fur-trappers that otters frequently hunt beavers) and perhaps only young beaver kits may be attacked.[37][39]

Ecological impacts on prey species

When left unchecked, though, otters can be significant predators under certain circumstances (e.g. in hatcheries or other fish culture facilities). Likewise, the potential predatory impact of otters may be considerable whenever fish are physically confined (most commonly in smaller ponds offering sparse cover or other escape options). To protect fish in these cases, the otters may need to be removed or relocated. Even in larger bodies of water, river otters may take disproportional advantage of any seasonal concentrations of fish when and where only very limited areas of suitable spawning, low-flow, or over-wintering habitat may exist. Even fast-swimming fish like trout slow down in extremely cold water, leaving them vulnerable. As such, careful consideration of any threatened, endangered, or fish species of special interest is warranted prior to reintroduction of otters to a watershed.[25]

Social behavior
A pair of captive North American river otters at Phillips Park Zoo in Aurora, IL.

Male North American river otters disperse from such family groups more often than females. When females leave, they tend to move much further away (60–90 km or 37–56 mi) than males (up to 30 km or 19 mi), which tend to move shorter distances. Male North American river otters do not seem to be territorial, and newly dispersing males may join established male groups.[40]

Reproduction and life cycle

Delayed implantation distinguishes the species from the European otter, which does not do this.[41][42]

In early spring, expectant mothers begin to look for a den where they can give birth. The female otters do not dig their own dens; instead, they rely on other animals, such as beavers, to provide suitable environments to raise their offspring. When the mothers have established their domains, they give birth to several kits.[19] Litter size can reach five, but usually ranges from one to three.[16] Each otter pup weighs approximately five ounces.[19]

The mothers raise their young without aid from adult males. When the pups are about two months old and their coats grow in, their mother introduces them to the water. North American river otters are natural swimmers and, with parental supervision, they acquire the skills necessary to swim.[19] The North American river otters may leave the den by eight weeks and are capable of sustaining themselves upon the arrival of fall, but they usually stay with their families, which sometimes include the father, until the following spring. Prior to the arrival of the next litter, the North American river otter yearlings venture out in search of their own home ranges.[43]

Geographic range
The species inhabits coastal areas, such as marshland

The North American river otter is found throughout North America, inhabiting inland waterways and coastal areas in Canada, the Pacific Northwest, the Atlantic states, and the Gulf of Mexico. North American river otters also currently inhabit coastal regions throughout the United States and Canada. North American river otters also inhabit the forested regions of the Pacific coast in North America. The species is also present throughout Alaska, including the Aleutian Islands, and the north slope of the Brooks Range.

However, urbanization and pollution instigated reductions in range area.[1] They are now absent or rare in Arizona, Kansas, Nebraska, New Mexico, North Dakota, Ohio, Oklahoma, South Dakota, Tennessee, and West Virginia. Reintroduction projects have expanded their distribution in recent years, especially in the Midwestern United States. Since their reintroduction to Kentucky in the early 90s, they have recovered to the point that a trapping season was started in 2006, and the species is now found in all major waterways.[44] In 2010, the Colorado Department of Wildlife reported the species, reintroduced in the 1980s, was "thriving" and recommended its protection status be reconsidered.[45] In late 2012, a river otter nicknamed Sutro Sam took up residence around the former site of the Sutro Baths in San Francisco, the first river otter sighting in that city in more than half a century.[46] In Canada, North American river otters occupy all provinces and territories, except until recently Prince Edward Island.[1] Otters have recently re-established themselves on Prince Edward Island.[47]

Historical records indicate North American river otters were once populous throughout most major drainages in the continental United States and Canada prior to European settlement. North America's largest North American river otter populations were found in areas with an abundance and diversity of aquatic habitats, such as coastal marshes, the Great Lakes region, and glaciated areas of New England. In addition, riverine habitats in interior regions supported smaller, but practical, otter populations.[1] The North American river otter existed on all parts of the Pacific Coast, including the seashore and inland streams and lakes.[48] However, large populations never occurred in areas of Southern California such as the chaparral and oak woodlands and Mojave Desert seasonal waterway regions, or in the xeric shrubland regions in New Mexico, Texas, Nevada, and Colorado. In Mexico, the North American river otters lived in the Rio Grande and Colorado River Deltas.[22]

Habitat
A North American river otter in the San Anselmo Creek

Although commonly called a "river otter", the North American river otter is found in a wide variety of aquatic habitats, both freshwater and coastal marine, including lakes, rivers, inland wetlands, coastal shorelines, marshes, and estuaries. It can tolerate a great range of temperature and elevations. A North American river otter's main requirements are a steady food supply and easy access to a body of water. However, it is sensitive to pollution, and will disappear from tainted areas.[22]

Like other otters, the North American river otter lives in a holt, or den, constructed in the burrows of other animals, or in natural hollows, such as under a log or in river banks. An entrance, which may be under water or above ground, leads to a nest chamber lined with leaves, grass, moss, bark, and hair.[22]

North American river otters swimming in San Francisco Bay stop to sun themselves on rocks at Richmond, CA Marina

Population localization

Aquatic life ties North American river otters almost exclusively to permanent watersheds.[16]

Diet

Fish

North American river otters consume an extensive assortment of fish species ranging in size from 2 to 50 centimeters (0.79 to 19.69 in) that impart sufficient caloric intake for a minute amount of energy expenditure.[29] North American river otters generally feed on prey that is in larger supply and easier to catch. As a result, slow-swimming fish are consumed more often than game fishes when both are equally available.[31][35] Slow-moving species include suckers (Catostomidae), catfish, sunfish and bass (Centrarchidae), daces, carp, and shiners (Cyprinidae).[28] For instance, Catostomidae are the primary dietary component of North American river otters in Colorado's Upper Colorado River Basin.[49] Likewise, the common carp (Cyprinus carpio) is a preferred fish species for the North American river otter in other regions of Colorado.[49] Fish species frequently found in the diets of the North American river otters include: Catostomidae, which consists of suckers (Catostomus spp.) and redhorses (Moxostoma spp.); Cyprinidae, made up of carp (Cyprinus spp.), chubs (Semotilus spp.), daces (Rhinichthys spp.), shiners (Notropis and Richardsonius spp.), and squawfishes (Ptychocheilus spp.); and Ictaluridae, which consists of bullheads and catfish (Ictalurus spp.).[25] Other fish an integral part of the North American river otters' diets are those that are often plentiful and found in large schools: sunfish (Lepomis spp.); darters (Etheostoma spp.); and perches (Perca spp.).[25][31][32] Bottom-dwelling species, which have the tendency to remain immobile until a predator is very close, are susceptible to North American river otters. These include mudminnows (Umbra limi) and sculpins (Cottus spp.).[25][31][32] Game fish, such as trout (Salmonidae) and pike (Esocidae), are not a significant component of their diets.[29][31] They are less likely to be prey for the North American river otters since they are fast-swimming and can find good escape cover.[29] However, river otters will prey on trout, pike, walleye (Sander vitreus vitreus), salmon (Oncorhynchus spp.), and other game fish during spawning.[32] Otters have been found to consume invasive Asian carp.[50]

Adult North American river otters are capable of consuming 1 to 1.5 kilograms (2.2 to 3.3 lb) of fish per day.[35] A study conducted on captive otters revealed they preferred larger fish, ranging from 15 to 17 centimeters (5.9 to 6.7 in), more than smaller fish, ranging from 8 to 10 centimeters (3.1 to 3.9 in), and they had difficulty catching fish species less than 10 centimeters (3.9 in) or larger than 17 centimeters (6.7 in).[27] Otters are known to take larger fish on land to eat, whereas smaller fish are consumed in the water.[35]

North American river otter eating a white sucker (Catostomus commersonii) at the Seedskadee National Wildlife Refuge (Wyoming)

Crustaceans

North American river otters may prefer to feed on crustaceans, especially crayfish (Cambarus, Pacifasticus, and others) and crabs[51] more than fish where they are locally and seasonally plentiful.[28] In Georgia, crayfish accounted for two-thirds of the prey in the summer diet, and their remnants were present in 98% of the summer spraint. In the winter, crayfish made up one-third of the North American river otter's diet.[52] A study conducted on North American river otters in a southwestern Arkansas swamp identified a correlation between crayfish consumption, fish consumption, and water levels.[53]

During the winter and spring, when the water levels were higher, North American river otters had a greater tendency to prey upon crayfish (73% of scats had crayfish remains) rather than fish.[53] However, when water levels are lower, crayfish will seek out shelter while fish become more highly concentrated and susceptible to predation. Therefore, fish are more vulnerable to being preyed upon by otters because the crayfish have become more difficult to obtain.[28]

Reptiles and amphibians

Amphibians, where regionally accessible, have been found in the North American river otter's diet during the spring and summer months, as indicated in many of the food habit studies.[31][33] The most common amphibians recognized were frogs (Rana and Hyla).[32] Specific species of reptiles and amphibians prey include: boreal chorus frogs (Pseudacris maculata); Canadian toads (Bufo hemiophrys); wood frogs (Rana sylvatica);[33] bullfrogs (Rana catesbeiana); green frogs (Rana clamitans);[35] northwestern salamanders (Ambystoma gracile); California giant salamander (Dicamptodon ensatus); rough-skinned newt (Taricha granulosa);[32] and garter snakes (Thamnophis).[25][32]

Amphibians and reptiles are more obtainable by the North American river otter during the spring and summer as a result of breeding activity, appropriate temperatures, and water supply for the prey.[53]

Birds

Waterfowl, rails, and some colonial nesting birds are preyed upon by North American river otters in various areas.[31][49] Susceptibility of these species is greatest during the summer (when waterfowl broods are vulnerable) and autumn.[31] The North American river otters have also been known to catch and consume moulting American wigeon (Mareca americana) and green-winged teal (Anas crecca).[33] Other species of birds found within their diets include: northern pintail (Anas acuta); mallard (Anas platyrhynchos); canvasback (Aythya valisineria); ruddy duck (Oxyura jamaicensis); and the American coot (Fulica americana).[32]

Although they consume birds, North American river otters do not feed on bird eggs.[27]

Insects

Aquatic invertebrates have been recognized as an integral part of the North American river otter's diet.[25][33][35][49] Otters consume more aquatic insects in the summer as the populations increase and specific life stages heighten their susceptibility.[33] Most aquatic invertebrates preyed upon by the otters are from the families Odonata (dragonfly nymphs), Plecoptera (stonefly nymphs), and Coleoptera (adult beetles).[33][49] Invertebrates discovered within scats or digestive tracts could most likely be a secondary food item, first being consumed by the fish that are subsequently preyed upon by the North American river otters.[30][32]

Mammals

Mammals are rarely consumed by North American river otters, and are not a major dietary component.[29][30] Mammals preyed upon by North American river otters are characteristically small or are a type species found in riparian zones.[49] The few occurrences of mammals found in the North American river otter's diet include: muskrats (Ondatra zibethicus); meadow voles (Microtus pennsylvanicus); eastern cottontails (Sylvilagus floridanus); and snowshoe hares (Lepus americanus).[33][35][54]

Records of North American otters preying upon North American beavers (Castor canadensis) vary; it has been reported in the southern boreal forest of Manitoba.[55] Trappers in Alberta, Canada commonly assert North American river otters are major predators of North American beavers.[33] A 1994 river otter study reported findings of beaver remains in 27 of 1,191 scats analyzed.[33] However, many other studies did not report any findings of North American beaver remains in the scat sampled.[53][56]

Threats
North American river otters are hunted and trapped for their valuable fur

Cases where otters have been ambushed and consumed by grizzly bears (Ursus arctos horribilis) and polar bears (Ursus maritimus) have also been reportedly witnessed near the Arctic region.[57][58][36][59]

Threats to North American river otter populations in North America vary regionally. North American river otter inhabitation is affected by type, distribution, and density of aquatic habitats and characteristics of human activities. Preceding the settlement of North America by Europeans, North American river otters were prevalent among aquatic habitats throughout most of the continent. Trapping, loss or degradation of aquatic habitats through filling of wetlands, and development of coal, oil, gas, tanning, timber, and other industries, resulted in local extinctions or population declines in North American river otter populations in many areas. In 1980, an examination conducted on U.S. river otter populations determined they were locally extinct in 11 states, and lost significant population in 9 other states. The most severe population declines occurred in interior regions where fewer aquatic habitats supported fewer otter populations. Although the distribution became reduced in some regions of southern Canada, the only province-wide extinction occurred on Prince Edward Island.[1]

During the 1970s, improvements in natural resource management techniques emerged, along with increased concerns about North American river otter population declines in North America. Consequently, many wildlife management agencies developed strategies to restore or enhance otter populations, including the use of reintroduction projects. Since 1976, over 4,000 otters have been reintroduced in 21 U.S. states. All Canadian provinces except Prince Edward Island and 29 U.S. states have viable populations that sustain annual harvests. While current harvest strategies do not pose a threat to maintaining otter populations, harvest may limit expansion of otter populations in some areas.[1]

Oil spills present a localized threat to otter populations, especially in coastal areas. Water pollution and other diminution of aquatic and wetland habitats may limit distribution and pose long-term threats if the enforcement of water quality standards is not upheld. Acid drainage from coal mines is a persistent water quality issue in some areas, as it eliminates otter prey. This dilemma prevents, and consequently inhibits, recolonization or growth of North American river otter populations. Recently, long-term genetic consequences of reintroduction projects on remnant North American river otter populations has been discussed. Similarly, many perceived threats to North American river otters, such as pollution and habitat alterations, have not been rigorously evaluated. Little effort has gone into assessing the threat of disease to wild North American river otter populations, so it is poorly understood and documented.[1]

Conservation status
Improvements in water quality and reintroduction projects have been particularly valuable in restoring otter populations in many areas of North America

Lontra canadensis is listed in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). They have been virtually eliminated through many parts of their range, especially around heavily populated areas in the midwestern and eastern United States.[60] Appendix II lists species that are not necessarily threatened with extinction currently, but may become so unless trade is closely controlled.[61]

The North American river otter is considered a species of least concern according to the IUCN Red List, as it is not currently declining at a rate sufficient for a threat category. By the early 1900s, North American river otter populations had declined throughout large portions of their historic range in North America. However, improvements in water quality (through enactment of clean water regulations) and furbearer management techniques have permitted river otters to regain portions of their range in many areas. Reintroduction projects have been particularly valuable in restoring populations in many areas of the United States. However, North American river otters remain rare or absent in the southwestern United States. Water quality and development inhibit recovery of populations in some areas. The species is widely distributed throughout its range. In many places, the populations have re-established themselves because of conservation initiatives. Reintroduction of river otters may present a problem in that it may contaminate the genetic structure of the native population.[1]

References
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Further reading
  • Kruuk, H. (2006). Otters: ecology, behaviour and conservation. Oxford: Oxford University Press. ISBN 0-19-856586-0. Recent monograph on otters in general, with many references to the river otter.
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