Spot the difference

Richard Collins on fellow members of the weasel family, the otter and the mink.

A CALLER this week wanted to know how to distinguish a mink from an otter. Was it possible, she asked, to do so if the animal is in the water, with most of its body hidden from view? It’s a frequently asked question.

These two members of the weasel family have broadly similar lifestyles. Both are elusive and usually seen fleetingly. Mink generally stay hidden but they have an inquisitive streak; curiosity can get the better of them. Some can be quite brazen and, on balance, you are more likely to encounter a mink than an otter. Otters occasionally cavort in daylight but mostly they dislike making public appearances.

In general, a mink’s fur is darker than an otter’s but colour is not much help with identification. Irish mink are descended from ones which escaped, or were released, from fur farms in the 1950s and 60s. Animals were bred for their pelts and exotic colour strains were much in demand. Although natural selection has ensured that their descendants are the customary dark brown, lighter-coloured mink are seen from time to time. Both mink and otters have whitish throat patches but those of mink are very variable.

The disparity in size between the two animals is an obvious distinguishing feature; a mink will weigh less than 2kg, whereas a big male otter might tip the scales at 16kg. However, it is notoriously difficult to estimate the size of an animal in the water. There might be a moorhen or a duck close by for comparison but, with a predator around, these birds generally flee. I have seen even mute swans panic at the sight of an otter. Nor are reports from members of the public much help; people invariably exaggerate the size of a creature with a fearsome reputation. A rat seen in a suburban garden is invariably said to be ‘huge’. Whether people really imagine that an animal is large, or embellish their reports for dramatic effect, is an interesting psychological question.

The difference in their swimming profiles is probably the surest way to tell minks from otters. A mink’s back and tail are above the water when it swims, whereas an otter rides low and only its head can be seen. The mink is semi-aquatic, with only partially webbed feet. It will hunt and forage on land and individuals have been recorded up to 2km from a river or lake. Location, therefore, sometimes provides a clue to a creature’s identity; a long skinny animal, which is definitely not a stoat or a pine marten and is seen well away from water, is almost certainly a mink. Such wandering is driven by hunger. Small animals lose body heat more rapidly than large ones. About 90% of a mink’s food intake goes to heating itself and a mink must forage and hunt through the night to get enough. It’s no surprise, therefore, that small predators are more ferocious than large ones. The tiny stoat is the only Irish animal to surpass the mink at killing.

The mink does its fishing from the bank, spotting a fish and plunging in to catch it. It has to operate from land because it can’t stay underwater for more than 20 seconds. Limited angling skills mean that only small fish are taken. To supplement its diet, the little opportunist eats virtually anything edible that it can find.

Otters are truly aquatic and have fully-webbed feet. They dive silently from the surface and can stay submerged for up to a minute, long enough to find and outmanoeuvre a fish. Being larger, an otter’s food requirements are proportionally lower than a mink’s and it doesn’t have to hunt through every waking hour to get enough to eat. This is not a jack of all trades but a specialist fisherman and only occasionally an opportunist. An otter was accused recently of killing flamingos at Fota Wildlife Park, but the evidence is circumstantial.

Although it’s a threat to ground-nesting birds, rendering the scoter virtually extinct as an Irish breeding duck, the mink has not been as destructive as was once feared. In any case, it’s so well-established here now, that there’s no hope of eradicating it. We have lost the battle with this American invader and the little killer is now a full member of the Irish fauna.