How to identify small carnivores (mustelids)

Most predators in Britain are mustelids. Apart from the badger and otter, they are small or medium-sized with long, thin bodies, short legs and scent glands that produce a strong, often pungent smell.

Many are fairly common, but they are all solitary and not easy to see. Here's how to tell them apart and interpret some of their behaviour.

• Mustelid males are generally bigger than females, so there is a considerable size overlap between species.
• Weasels are the smallest, designed to pursue rodents down burrows. Males eat more rabbits than females. Tails are short and may not be visible in the field.
• Stoats have a conspicuous black tip to the tail and are often seen near rabbit warrens.
• Polecats have distinctive face masks and white ear markings. They are spreading from Wales into much of southern England, with introduced populations in northern England and Scotland.
• Feral ferrets can be distinguished from polecats by their paler face markings. They are only common on islands with no foxes (the Isle of Man and some Scottish islands), though some populations occur on the mainland.
• Mink are dark chocolate-brown. They have a white patch on the chin and several on the chest and belly.
• Pine martens are cat-like with bounding movements and dark brown fur, a cream or orange chest or throat patch, large, rounded ears and a distinct tail. Rare outside Scotland.
• By comparison, otters are larger, with a flattened head and long, tapering tail, whose thick base leaves a V-shaped wake when swimming.

• Stoats and weasels turn white in winter as an adaptation and response to long periods of snow cover. While white stoats can be seen, white weasels do not occur in Britain.
• Autumn temperature determines whether a stoat’s winter coat is brown or white.
• Stoats in full ermine (a white coat with a black tip to the tail) are most common in northern Scotland, but full or partial ermines can be seen anywhere.
• Females are more likely to turn ermine than males.

• Stoats, mink and pine martens breed once a year and have delayed implantation – the fertilised egg is retained in the uterus and implanted later.
• Weasels breed once or twice a year; polecats and otters once. None have delayed implantation.
• Male stoats mate with females that are still in the nest and less than three weeks old – they are usually not his offspring. The female kits then disperse with their fertilised eggs and give birth the following spring. Male stoats are not sexually mature until 10-11 months old.

• Though all of these mustelids are mainly nocturnal, they are often seen during the day.
• Male stoats can be seen during the day in spring when looking for mates, while female stoats are active on summer days when feeding young.
• Mink can be seen hunting along riverbanks during the day.
• Otters hunt on coasts and in fresh water during the day, even in urban rivers.

• Stoats and weasels were once thought to ‘dance’ – leaping and running in circles – to hypnotise small birds and mammals.
• in fact, the cause is a parasitic worm Skrjabingylus nasicola, which lives in the nasal sinuses and leads to swellings in the skull that put pressure on the brain, causing this frenzied behaviour.  

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