Despite being relatively rarely seen, many of us could probably identify an otter and can draw to mind the "Cockney costermonger" in Kenneth Grahame's classic 1980 children's novel, The Wind in the Willows, who's distraught when his young son Portly goes missing. Globally, however, the otters form a family (the Lutrinae) within the Mustelidae (the badger and weasel family) composed of 13 species found throughout the Americas and Eurasia. Europe is home to only a single species: the European or Eurasian otter (Lutra lutra), which, despite its name, can be found as far south as north Africa.
First classified as Mustela lutra by Linnaeus in his 1758 opus, translating roughly as "weasel otter", the Eurasian otter is now placed in the Lutra genus, Lutra being the Latin for "otter", with two other species. The word "otter" appears to be of Germanic origin, stemming from otor and, ultimately, perhaps the Greek hydōr, meaning "water".
Despite having suffered dramatic historical declines, to the point of virtual extinction by the late 1960s, otters have enjoyed a resurgence and can now be found on all British river systems, even stretches running through urban areas. Indeed, the 2018 edition of Britain's Mammals suggests otters have experienced a population increase of 49% since 1995, having both increased in number and expanded in range significantly since 1970. The current UK population is estimated at around 11,000 individuals, based on habitat density data.
European otter at a glance
Size: Typically 60-90cm (2-3 ft.), with males larger than females; latter reaching only ~70cm. On average males ~70cm (2.3 ft.) and females 65cm. Tail 35-47cm (1.1-1.5 ft.), again longer in males than females. Stands ~30cm (12 in.) at shoulder and weighs 6-17kg (13-37 lbs.); typically ~9kg (20 lbs.) and 7kg (15.4 lbs.) for male and female, respectively. Possess a thick double-layered coat consisting of short (12mm / 0.5 in.) underfur that provides insulation and longer (25mm / 1 in.), coarse guard hairs for waterproofing. The coat is dense, with ~70,000 hairs/sq-cm (~455,000 sq-in.).
Colour/Appearance: A large semi-aquatic mustelid with long, thick tapering tail. Fur is typically dark brown, appearing almost black in some individuals when wet. They have a large brown/black nose, small eyes, small rounded ears that only just protrude beyond fur, and webbed feet. The head is broad and flat with long whiskers. Short legs give the typical low-slung mustelid appearance.
Distribution: Widespread throughout Britain, although patchily distributed in central and south-east England and parts of southern and central Scotland. Widespread in Ireland, rare on Isle of Wight, and absent from Isle of Man. More widely, found east through Russia into China, and south into north Africa and India.
Habitat: Typically a freshwater species inhabiting rivers, canals, marshes, and larger still water bodies (i.e., lakes, ponds and gravel pits) with sufficient cover. They may hunt or travel through ditches with only few centimetres of water. They will also hunt in urban areas with garden ponds, and on fishery sites. Occasionally found in seawater, particularly along rocky coastlines off Shetland and west coast of Scotland, although salt impairs their guard hair waterproofing and a source of freshwater is essential to ensure coat maintenance.
Longevity: Otters typically live 3-5 yrs, with the oldest wild specimen estimated at 16 yrs. In captivity, a female at Zürich Zoo lived to 18 yrs 2 months. Captive otters appear to show signs of senility at around 10 yrs old.
Sexing: With experience it is sometimes possible to sex mature specimens at distance (e.g., females have a smaller, thinner skull with less prominent forehead cf. males), but this is unreliable for animals less than 3 yrs old. The most reliable way is using the distance between the anus and penis/vulva, which is much larger in males. Males are referred to as dogs, females as bitches, and young are cubs.
Activity: Primarily nocturnal, although some local (e.g., urban and particularly coastal) populations are commonly active during the daytime. Perfectly adapted to water, swimming proficiently. Swims low in water with v-shaped bow wave, leaving a bubble trail while submerged. Otters can sustain swimming speeds of around 2 kmph (1.2 mph) for up to 8 hrs and dive for up to 45 seconds (possibly longer). Otters will travel across land, particularly when moving between ponds and dispersing, moving with a distinct humpback gait. they may sleep above ground in specially constructed "couches", or use dens constructed throughout their home range as "rest over" sites while travelling.
Territory/Home Range: Varies considerably with habitat from ca. 1 sq-km (250 acres) in productive marshland to 70 sq-km (43.5 mi.) of sparse riverbank in Shetland. Males range further than females; e.g., an average of ca. 39 km (24 mi.) vs. 20 km (12.4 mi.) of riverbank, respectively, in one study. Ranges of both sexes overlap. Journeys of 13 km (8.1 mi.) between watersheds have been recorded and radio-tracking suggests males are more likely to cover full extent of their territory, while females concentrate time in the most productive/secure area(s). Ranges are marked with droppings (called spraint), the frequency varying with the habitat quality, season, and (particularly) fish abundance.
An otter will create a den site called a holt, which is usually a cavity in riverbank, among tree roots, or in rocky crevice/cleft. Natural cavities appear selected where available, but animals may dig an extensive tunnel network (e.g., in peat). In urban areas, drain pipes and culverts may be used. Entrance to the holt may be underwater and chambers are often lined with leaves. Females appear more easily disturbed than males and may thus use more secure den sites. May create a "slide" outside the holt entrance, where vegetation worn away allowing easy/rapid access to the water.
Diet & Feeding: Otters are carnivorous, primarily piscivorous. Indeed, fish can account for 70-95% of diet, mostly relatively small, bottom-living species such as eels, perch, and salmonids. Carp may be locally important, which can put them in conflict with fisheries and amateur fish keepers alike. Crayfish and amphibians taken according to season and local abundance, while small water birds (e.g., up to about the size of a mallard) and mammals (e.g., water voles and rabbits) may also be predated, particularly in winter. There are several anecdotal reports of otters exiting rabbit warrens, the implication being they were hunting there, but only one report of direct predation – on 1st March 2023, Alan Kennedy photographed a female otter with three dependent cubs on South Uist, Scotland, catching and dragging a rabbit (Oryctolagus cuniculus) into a pool where it drowned it. Coastal otters take a wider variety of prey, including eelpout, rockling, butterfish, pollack, crabs, and octopus. May blow bubbles while hunting underwater to collect scent. Diet overlaps with American mink (Neovison vison) by ca. 70%. Males require ca. 12% body weight per day of food during winter.
Reproduction: Aseasonal breeders, with cubs reported in every month and no clear peak in births in most habitat, although there appears to be a strong tendency for May-August births in NW Scotland, Shetland, and the Netherlands. Courtship involves the pair chasing and playing for about a week, with copulation taking place in water. Typically 2-3 cubs are born, each about 13 cm (5 in.) long, following a gestation of ca. 62 days. Cubs are blind and furred at birth; eyes open around 30 days old and their first "enforced" swim is about 12 wks old, joining their mother on fishing trips by 4 months old. Cubs are weaned around 14 wks and independent at about 10 months old. Males mature at ca. 7.5 kg (16.5 lbs.) at 14 months old, and females slightly earlier at ca. 4.6 kg (10 lbs.) at about 10 months, although some reports pregnant animals as young as 8 months. Females produce a single litter per year and parental care is provided entirely by the female.
Behaviour and Sociality: Typically a solitary species, the family group of mum and cubs ;breaking down as the cubs reach 7-12 months old, but small family groups may persist where resources allow, such as productive coastal habitats and large lakes. A very playful species that will dive for pebbles with which to play and stand on hind legs to survey the landscape. There is a wide application of scent for communication; it encodes at least age, sex, and reproductive status. Several soft vocalisations have been described, including a squeaking/whistle contact call, a "chittering" threat call, and a chirrup by very young cubs. Vocalisation seems an important part of courtship. Can be highly aggressive to others of either sex, with fatal fights documented. When males fight, they target the groin and can fracture the baculum ("penis bone") – males commonly carry wounds. Males may also kill cubs, and both sexes show aggression to other mustelids, especially American mink. Otters spend a considerable amount of time grooming, particularly in those living in coastal populations.
Predators: Predation appears rare in UK, with few predators able to tackle an adult otter. In Scotland, golden eagles (Aquila chrysaetos) may take cubs, and elsewhere sea eagles (Haliaeetus albicilla) also have the potential to predate adults and cubs, although data are lacking. Reports of eagles chasing and swooping at adults likely reflect birds trying to steal fish from the otter, rather than predation attempts. Coastal otters around Scotland may be vulnerable to killer whales (Orcinus orca), although I know of no confirmed reports of them being killed by orcas in Britain. In Europe, there are reports of lynx (Lynx lynx), wolves (Canis lupus), and sea eagles occasionally taking adults and cubs. One report of badger fighting with otter.
Threats: Protected in England, Wales, and Scotland under schedules 5 and 6 of the Wildlife and Countryside Act, making it illegal to kill, trap, or sell otters, or disturb their breeding holts. Legal protection extends across Europe under the Bern Convention, and the EU Habitats Directive list European otters as a species requiring special protection. Invariably, the highest mortality is from traffic, and one study found 60% of violent otter deaths in UK were road casualties. Otters may also drown in lobster pots, or get illegally killed by fishery owners, but there are no statistics. The population in Britain rebounded in response to water quality improvements and pesticide bans in 1970s, although more recent declines in the quality of our freshwater sources owing to increased pollution may have impacted this. Otters remain vulnerable to habitat loss and are highly susceptible to heavy metal and pesticide poisoning, particularly organochlorine and PCBs – these endocrine disruptors appear to cause reproductive deformities in males.
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