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.
That which follows is a summary of European otter natural history. The detailed article for this species will follow in due course.
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 cf. 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. Thick double-layered coat; short (12mm / 0.5 in.) underfur provides insulation while longer (25mm / 1 in.), coarse guard hairs provide waterproofing. 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 typically dark brown, appearing almost black in some individuals when wet. Large brown/black nose, small eyes, small rounded ears that only just protrude beyond fur, and webbed feet. Head broad and flat with long whiskers. Short legs give 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 to 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. May hunt or travel through ditches with only few cm of water. Also hunts 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. Salt impairs guard hair waterproofing, so source of freshwater essential even for coastal otters to ensure coat maintenance.
Longevity: Typically live 3-5 yrs. Oldest wild specimen estimated at 16 yrs. In captivity, female at Zürich Zoo lived to 18 yrs 2 mo. Captive otters show signs of senility ~10 yrs.
Sexing: With experience, sometimes possible at distance on mature specimens (e.g. females have smaller, thinner skull with less prominent forehead cf. males) but unreliable for animals <3yrs old. Most reliable way is via distance between anus and penis/vulva (much larger in males). Male is “dog”, female is “bitch”.
Activity: Primarily nocturnal, although some local (e.g. urban and particularly coastal) populations are commonly active during daytime. Perfectly adapted to water, swimming proficiently. Swims low in water with V-shaped bow wave and leaves bubble trail while submerged. Can sustain swimming speed of ~2kmph (1.2 mph) for up to 8 hrs and dive for up to 45 seconds. Will travel across land, particularly when moving between ponds and dispersing, with distinct humpback gait. May sleep above ground in specially constructed “couches” or use holts throughout home range as “rest over” sights while travelling.
Territory/Home Range: Varies considerably with habitat from ~1 sq-km (250 acres) in productive marshland to 70km (43.5 mi.) of sparse riverbank in Shetland. Males range further vs. females; ~39km (24 mi.) vs. ~20km (12.4 mi.) riverbank. Ranges of both sexes overlap. Journeys of 13km (8.1 mi.) between watersheds recorded. Radio-tracking suggests males more likely to cover full extent of territory while females concentrate time in most productive/secure area(s). Mark ranges with droppings (spraint), frequency varying with habitat quality, season and, particularly, fish abundance.
Create den site called a “holt”; commonly a cavity in riverbank, among tree roots or in rocky crevice/cleft. Typically use natural cavities, but may dig extensive tunnel network (e.g. in peat). In urban areas, drain pipes and culverts may be used. Entrance to holt may be underwater; chambers often lined with leaves. Females more easily disturbed (use more secure den sites) cf. males. May create “slide” outside holt entrance where vegetation worn away allowing easy/rapid access to water.
Diet & Feeding: Carnivorous; primarily taking fish, which can account for 70-95% of diet – most are relatively small, bottom-living species such as eels, perch and salmonoids. Carp may be locally important, which puts them in conflict with fisheries. Crayfish and amphibians taken according to season and local abundance while small waterbirds and smaller mammals (e.g. water voles and rabbits) may also be predated, particularly in winter. Coastal otters take 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 ~70%. Males require ~12% body weight per day in winter.
Reproduction: Aseasonal breeders; cubs reported in every month in most habitats with no clear peak. Strong tendency for May-August births in NW Scotland, Shetland and Netherlands. Courtship involves chasing and play over ~1 wk, with copulation taking place in water. Typically 2-3 cubs, ~13cm (5 in.) long, born following ~62 day gestation. Cubs blind and furred at birth; eyes open ~30 days and first “enforced” swim ~12 wks, joining mother on fishing trips ~4 mo. Weaned ~14 wks and independent ~10 mo. Males mature at ~7.5kg (16.5 lbs.) (~14 mo.); females earlier at ~4.6kg (10 lbs.) (~10 mo.), although reports of pregnancies as young as 8 mo and females capable of producing litter per year. Parental care by female only.
Behaviour and Sociality: Typically solitary, but small family groups may persist where resources allow (e.g. coastal and large lakes) – often family group split when cubs reach 7-12 months. Will stand on hind legs to look around. Very playful and will dive for pebbles with which to play. Wide application of scent for communication; encodes age, sex and reproductive status. Several soft vocalisations described, including squeaking/whistle contact call, “chittering” threat call and chirrup by very young cubs. Vocal during courtship.
Can be highly aggressive to others of either sex with fatal fights documented. When males fight, they target groin and can fracture baculum (“penis bone”) – males commonly carry wounds. Males may kill cubs and both sexes show aggression to other mustelids, esp. American mink. Spend considerable time grooming, particularly in coastal populations.
Predators: Predation rare in UK, bar Golden eagles taking cubs in Scotland. In Europe, lynx, wolves and sea eagles occasionally take adults and cubs. One report of badger fighting with otter.
Threats: Protected in England, Wales and Scotland under s5 and s6 of Wildlife and Countryside Act (illegal to kill, trap, sell or disturb breeding holts). Legal protection across Europe under Bern Convention and a species requiring special protection according to EU Habitats Directive. Invariably highest mortality from traffic (one study found 60% of violent deaths in UK were road casualties). May drown in lobster pots or killed by fishery owners. Population has rebounded concomitant with water quality improvements and pesticide bans in 1970s. Remain vulnerable to habitat loss and highly susceptible to heavy metal and pesticide poisoning, particularly organochlorine and PCBs – these endocrine disruptors appear to cause reproductive deformities in males.
Atlas of the Mammals of Great Britain & Northern Ireland - by Multiple Contributors
Pelagic Publishing -- 2020 -- ISBN: 978-1784272043
Britain’s Mammals 2018: The Mammal Society’s Guide to their Population & Conservation Status - by Multiple Contributors
The Mammal Society -- 2020 -- ISBN: 978-0993567339
Collins Field Guide: Mammals of Britain and Europe - by David MacDonald and Priscilla Barrett
HarperCollins Publishers -- 1993 -- ISBN: 978-0002197793
Otters - by Paul Channin
Whittet Books -- 2019 -- ISBN: 978-1873580844
Otters in Britain - by Liz Laidler
David & Charles -- 1982 -- ISBN: 0-715308069-9
Otters of the World - by Paul & Grace Yoxon
Whittles Publishing -- 2014 -- ISBN: 978-1849951296
Otters: Ecology, behaviour and conservation - by Hans Kruuk
Oxford University Press -- 2008 -- ISBN: 978-0198565864
RSPB Spotlight: Otters - by Nicola Chester
Bloomsbury Natural History -- 2014 -- ISBN: 978-1472903860
The Otter - by Gordon Woodroffe
The Mammal Society -- 2007 -- ISBN: 978-0906282632
The Otter - by James Williams
Merlin Unwin Books -- 2010 -- ISBN: 978-1906122225
UK Mammals: Species Status & Population Trends - by The Tracking Mammals Partnership
JNCC/TMP -- 2005 -- ISBN: 978-1861075680