Eurasian otters, also known as European otters or Eurasian river otters, are the most widely distributed species of the weasel family native to Europe, Asia, and Northwest Africa. Though these Palaearctic mammals sleep for about half their lives, they are naturally energetic and playful, gliding on riverside mud for fun and plunging to depths of waterbodies to procure bottom-dwelling creatures.
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They usually appear silvery underwater because of the air bubbles trapped in their fur. These otters require clean rivers with abundant vegetation to hide their dens in the cavities of riverbanks.
Length: Body – 55 to 95 cm (21.6 to 37.4 inches); Tail – 32 to 56cm (12.5 to 22 inches)
Weight: 7 to 12 kg (15.4 to 26.4 lbs)
Body and Coloration:
These otters have long, streamlined bodies that are typically dark brown dorsally and creamy white ventrally. Their entire body is covered with two layers of fur: a thick, waterproof outer layer and a warm inner layer. Eurasian otters can also be recognized by their small eyes, rounded ears, dark snouts, webbed feet, and thick, tapering tails. They are distinguished from their North American counterparts by their broader heads, shorter necks, longer distance between the ears, and longer tails.
They are abundant in Europe and Asia (Eurasia), ranging from Ireland in the west to Russia and China in the east and further southwards to Morocco, Algeria, and Tunisia in North Africa. In Europe, they are distributed along the coast of Norway, western Spain, and Portugal, southern Italy, and across Great Britain and Ireland. While in Asia, these otters are concentrated in the mountainous creeks of Syria, western Nepal, the Himalayan foothills, and the Western Ghats of India.
They once became extinct in Belgium, Netherlands, and Switzerland but are now reviving in number due to cross-border migration from countries like Germany and conscious conservation efforts.
These otters are semi-aquatic, foraging in fresh and brackish water bodies like ponds, lakes, streams, rivers, fjords, swamps, marshes, and coastal regions. They prefer to rest in dry terrestrial habitats like banks of waterways, earth tunnels or holts, boulder piles, and tree roots. Sometimes, they are also found in the sea, especially off Scotland’s Shetland and western coasts.
They are carnivores, feeding primarily on small, bottom-dwelling fishes (piscivores) like eels, perch, carps, and salmonoids in Mediterranean and temperate freshwaters. However, in winter and colder environments, they consume crustaceans, amphibians, waterbirds like ducks and greylag geese, and small mammals. In the coastal regions, they consume a wider variety of prey like rockling, eelpout, pollack, butterfish, octopus, and crabs.
They remain mostly solitary, aggregating only during the mating season and for a short time after the birth of their pups. However, they are also found in loose-knit groups of up to six when not mating.
They are primarily nocturnal, actively foraging around dusk and at night when surrounding temperatures are low, and rest in their dens on the land during the day. However, in some regions like the Scottish Isles, they are diurnal, foraging in the sea during the day and later searching for freshwater pools to release salt from their pelt.
They are playful in nature, sliding down snow and muddy riverbanks on their bellies. Sometimes, they are found diving into waterbodies and remaining underwater for about two minutes in search of pebbles to play with. The adult otters often chase, slide, and play with the juveniles to train them to master hunting techniques.
They communicate using a range of vocalizations, including twittering or chattering, whistling, and high-pitched squeaking. These calls are helpful when fighting a conspecific, communicating with pups, conveying alarm, or during courtship.
As a means of communication, they mark their territorial boundaries using the musky odor from the scent gland (at the base of their tail) that also helps them identify the age, identity, and sexual receptivity of other individuals in their vicinity. They also prioritize their territories and track other otters by marking the ground with their feces called ‘spraints.’ These spraints have a sweet and musky smell, and more than 100 different scent components have been identified from these droppings.
Male otters are highly aggressive towards other males, often targeting their groins and fracturing the baculum (penis bone) during fighting. Both sexes ferociously combat the other species of Mustelids, like American minks, weasels, pine martens, and badgers.
They build den sites called ‘holts’ in existing cavities around riverbanks, tree roots, and rocky clefts. Sometimes, they are also found to dig extensive underground tunnels and line them with leaves and other plant parts.
The length of an otter’s territory averages 18 km but could extend up to 40 km, depending upon the availability of food and water in their range.
They usually live around 3 to 5 years in the wild, with the oldest wild specimen being 16 years. In captivity, they can survive as long as 22 years.
Reproduction and Life Cycle
Eurasian otters are believed to be either polygynous, a single male otter (dog) mating with multiple females (bitches), or polygynandrous, in which both sexes have multiple partners. They are non-seasonal breeders, capable of breeding at any time of the year, as the females have a continuous estrus cycle. These females reach sexual maturity at around 18 to 24 months and usually breed for the first time when they are about three years old.
After a short phase of courtship involving chasing, playing, rolling, and diving, the pairs generally mate in water and sometimes on land between February, March, and July. A 60 to 70-day gestation is followed by the birth of 1 to 4 pups or kits weighing around 90 to 120 gm each. These pups are weaned at about three months but remain dependent on their mothers till they are about 14 months old. They are then dragged to the water by their mothers at around 16 months and thus quickly adapt to the aquatic environment.
The natural predators of these otters include birds of prey, crocodiles, lynx, wolves, and sea eagles.
These otters are well-adapted to live in water due to their webbed feet, outer waterproof guard hair, and warm underfur. They close their ears and nose during underwater foraging to prevent the water from entering the auditory and nasal passages.
Their eyes are placed high on their heads, enabling them to scan the coastline from a distance while submerged.
The presence of large lungs and their ability to slow down their heartbeat help these otters stay underwater for up to four minutes.
They have extremely sensitive whiskers or vibrissae on their snout that help them detect the movements of prey underwater.
Their bones are osteosclerotic, tending to increase in density to reduce buoyancy in water. This feature helps them feed at the bottom of the water bodies, where the prey is mainly concentrated.
Eurasian otters are globally listed as ‘Near Threatened’ (NT) in the IUCN Red Data List and Appendix I of CITES. In Pakistan, India, Bangladesh, Myanmar, and Thailand, this species has been listed as Endangered (EN), whereas it is marked ‘Critically Endangered’ (CR) in Mongolia.
Between the 1950s and 1960s, these otters had almost disappeared from many European countries, including the UK, mainly due to contamination of waterways with organic chemicals like polychlorinated biphenyls and organochlorine (as runoff from agricultural fields).
Another major threat to the otter population is the increasing occurrences of oil spills in oceans. In Alaska, thousands of otters were killed in the Exxon Valdez spill (1989), while they were badly impacted by the Braer disaster (1993) in Shetland. Other potential threats to their numbers include accidental trapping in fish nets, construction of dams, and other human interventions to their habitats.
Despite being pushed to the edge of extinction by the late 1960s, the Eurasian otter population has now revived in most parts of Europe, including the UK, where around 11,000 individuals are reported to exist today, mainly due to strict legal regulations and concerted conservation efforts.