Otter Biology and Species Information

Family, Appearance and Lifestyle     

The European otter belongs to the weasel family, which includes stoats, badgers, polecats and mink, and has a geographical distribution that extends from Ireland to Japan and from North Africa to the north of Russia and Finland. The otter is one of Britain’s largest carnivores and is well adapted to its aquatic lifestyle having a streamlined body, long flattened tail, short legs and partially webbed feet. Male otters weigh up to 30% more than females. The characteristic "spiky" appearance of the coat when wet is due to the clumping of the guard hairs over an under-layer of dense, insulating fur.

Range, Habitat, Territory and Home

Otters hunt in both fresh and salt water and can be found by rivers, canals, marshes, streams, ditches, lakes, estuaries and coastal areas if conditions are right. Otters occupy linear ranges, the length of which depends on the food supply and habitat quality, but they can be as long as 80kms. Up to thirty resting sites may be used within a home range and these include holts (underground dens), couches (above ground in reedbeds, tall vegetation and scrub) and breeding sites which are generally more secure from disturbance and flooding and sometimes a long way from water.

Activity and Social System

Otters are largely nocturnal, spending most of their time during the day on land in their holts or couches. They are generally solitary and a male or dog otter will have a territory, probably linear in shape, which encompasses the territories of two or three females.

Food

Otters are carnivorous, feeding predominantly on fish but with birds, amphibians, molluscs, crustaceans and small mammals, including rabbits, also playing a part in their diets. Foraging occurs at largely night in freshwater when prey species are resting but coastal otters are more likely to forage during the day as many sea fish are nocturnal.

Breeding – mating, season, numbers

Breeding occurs throughout the year with between one and four cubs per litter. The cubs remain dependant on their mother for up to their first year and so bitches tend to breed every other year. It is unusual for females to breed before their third year and, as the average life expectancy is only four years, the successful raising of cubs is essential.

Threats

Historically, hunting was the greatest threat to otters but agricultural intensification and the associated introduction of organochlorine insecticides has led to chronic, and sometimes lethal, effects on populations. Both hunting and these chemicals are now banned or restricted and so otter populations are recovering. Significant numbers are killed on roads and in fishing nets and traps. Many may die of starvation in the later winter and early spring when food is short and conditions are harsh and about 4% die in fights with other otters.

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