Weasels in Cool Rivers: The Common Otters

Eurasian otter

The otters are, arguably, the least weasel-like of all the members of the weasel family. Although the relationship has been clear for a long time - the first scientific description of an otter, by Linnaeus in 1758, actually placed it in the genus Mustela, to which stoats and weasels belong - so have the differences. As early as 1771, Danish zoologist Morten Thrane Brünnich split them off from their fellow mustelids, erecting the new genus Lutra to describe them.

By 1838, at least three genera of otter had been described, and the group were raised to 'subfamily' status to distinguish them from their terrestrial kin. Other subfamilies were erected later, largely to distinguish the badgers, but the otters have remained as a clearly distinct group within the overall weasel family.

Genetic studies in the last four years have given us a much clearer picture of how the various kinds of mustelid are related to one another, and it turns out to be more complicated than we thought. For example, martens turn out to have been around longer than otters have, which was something of a surprise. The otters, however, survived the revision unscathed, confirmed as a genuine evolutionary group, albeit one that is younger than we might have guessed. Their closest relatives within the family turn out to be the mustelines, the group that, among others, includes the semi-aquatic mink.

Nonetheless, otters have apparently been in the water for longer than mink have, and they show greater adaptations for their riverine environment. It was perhaps inevitable that some weasel-like animal would take to the water, because their long, slender bodies and short limbs are ideal for swimming, even if their original purpose was probably to chase prey into burrows. However, a number of evolutionary changes were clearly needed as otters switched to an increasingly aquatic lifestyle.

Otters, for example, have longer, more muscular, tails than other members of the weasel family, and these are often flattened to form a flexible rudder. While mink have partially webbed toes, most otter species have fully webbed feet, better suited to propelling themselves through the water. They are generally better swimmers than mink, too, using all four feet, along with the tail, where mink primarily use their forefeet. As a consequence, however, they are less agile on land, although they are certainly able to run for long distances over dry ground to travel between neighbouring bodies of water. They are are also capable of closing their nostrils and ears underwater, and can stay submerged for four or five minutes when necessary, although most dives are less than half that long.

Otters also have short, very dense, fur that helps insulate them even when wet. Their eyes are modified to enable them to see better underwater, although the necessity of staying on dry land while sleeping or raising young, means that they also need good eyesight above water, and so their eyes are less modified than among some more fully aquatic mammals. That's less of an issue than it might be, partly because river water can be murky, making it difficult to see very far anyway. To help compensate, otters have highly tactile paws and sensitive whiskers enabling them to hunt even when they can't see what they're doing.

We might suppose that otters' sense of smell might be somewhat reduced, compared with other mustelids, because it isn't much use underwater. The evidence for this is somewhat ambiguous; the parts of the brain that normally interpret smells are smaller than in their relatives, but the nasal structures themselves are not. Whatever the relative merits of otters' ability to smell when compared with other weasels, however, it is undeniably impressive compared with that of humans.

That's because, while smell may not be much use for tracking prey along a river, it is still useful for scent marking along the bank, for example to mark out territory, or advertise sexual status. One of the distinctive features of the weasel family is the presence of well developed anal glands producing a pungent aroma that they use to signal one another. In otters, these glands are reduced, but they are still present, and the animals have other scent glands as well. Many species leave scent marks along their territory, often repeatedly using the same latrine sites, so that long-lasting dung piles develop. The lush plant growth that often develops on these heaps of natural fertiliser makes them even more obvious to passing otters curious to learn the latest news.

Otters are also unusually sociable for mustelids. Most other members of the weasel family are solitary animals, prepared to put up with the presence of members of the opposite sex, but barely interacting even with them outside of the breeding season. Although that's true of some otters, too, many do group together, at least at certain times of the year. This probably allows them to use cooperative tactics to hunt schools of fish, 'herding' them together so that the individual otters can catch more than they would on their own.

The animal referred to in Europe as the "common otter" is more correctly called the Eurasian otter (Lutra lutra). Another alternative name is "European otter", but that's not terribly accurate, since, while it is native to most countries in Europe, it is also found much further east. In fact, Eurasian otters are found across southern Russia and into Korea, and further south, they are found in Turkey, and in a narrow band across the Middle East and along the southern slopes of the Himalayas, as well as across much of southern and central China. They also inhabit the Moroccan and Algerian coasts, Sri Lanka and southern India, and even the islands of Sumatra and Taiwan. Across this vast range, there are at least seven subspecies, and probably more.

There might be Eurasian otters in Japan, too, but that's unclear for two reasons. For one thing, at least some Japanese otters might represent a separate species, although that is somewhat controversial. More seriously, there doesn't seem much evidence that any otters have been seen alive in Japan since 1986. It's just possible some survive on Shikoku, but even this is far from clear.

However many species or subspecies they might represent, physically, Eurasian otters are typical members of their group. They are about two feet long as adults, with another foot for the tail, although males are noticeably larger than females. They have sleek dark brown to near-black fur, with paler underparts, and off-white chins and throats. The tail is large and muscular, and the feet are fully webbed with strong claws. The forefeet are highly dextrous, and they often use them to hold and manipulate prey that they have caught with their jaws.

Eurasian otters inhabit every freshwater habitat available, from streams to rivers, lakes, and marshes. They are also found along the coast, especially where they live on smaller islands, and can forage in brackish or salt water, so long as it's shallow enough. They mainly catch fish, and, where they hunt along the coast, they eat little else. Further inland, however, there are plenty of other opportunities, and they increasingly supplement their regular diet with amphibians and crayfish. In colder climates, their rivers may freeze over, and while they can punch holes in thin ice, or dig hibernating frogs out of their holes, they often switch to eating rabbits, rodents and small birds in the depths of winter. In general, they seem to focus on different food than mink do, allowing both species to live alongside one another without problems.

Male Eurasian otters are fairly antisocial, and will fight one another for access to females. The females, while not truly sociable, as some other otter species are, are nonetheless fairly tolerant of one another, and are often observed together. They dig burrows in which to sleep, often with submerged entrances, although the main chamber itself is lined with dry, comfortable leaves. Although each otter typically has its own burrow, they will also sleep in any available crevice from time to time. They are generally either nocturnal or crepuscular, and try to avoid coming out in the day.

In relatively comfortable climates, such as that of England, they seem happy to breed at all times of the year, but further north, in places such as Sweden, they restrict themselves to the late winter, ensuring the young are born in the spring. Pregnancy lasts a couple of months, and results in the birth of two to five young, which will be ready to swim at around two months of age. The mother initially shows them how to catch fish, allowing them to practice for themselves on some she has caught for them. They are weaned at four months, and leave home after about a year. It has been said that the Eurasian otter is the most playful of all mustelid species, and individuals of all ages seem to take great joy in sliding down mud or snow banks whenever they get the chance.

North American river otter

In America and Canada, however, the term "common otter" refers instead to the North American river otter (Lontra canadensis). In most respects, it is remarkably similar to its Eurasian counterpart, living in the same sorts of environments, although it is, perhaps, better able to tolerate long, cold, winters and frequently swims for long distances under the ice. It is, on average, slightly larger than the Eurasian otter, and the paler fur on the throat and chin is less distinct, although still clearly noticeable. Other than that, it looks pretty much the same, and it also lives on the same kind of food.

As their scientific name indicates, North American river otters are found throughout Canada, at least outside of the high Arctic, and they are also common in Alaska. However, like their Eurasian kin, they are clearly able to tolerate a wide range of climates, and they are also found around the Great Lakes, down the west coast and through the Rockies as far as California and New Mexico, and across the eastern US as far as Florida and Louisiana, only being absent in and around the Appalachians and the Great Plains.

They are perhaps more sociable than Eurasian otters, and gather together in single-sex groups at times of the year when fish shoals are particularly common. The male groups, in particular, break up in the breeding season, as they begin to fight one another, although female groups may be longer lasting. North American otters do not construct their own burrows, instead stealing them from other animals, such as beavers, but they use them in much the same way as Eurasian otters use theirs, and are active at the same times of day.

Uniquely among otters, North American otters do not give birth for ten months or more after mating. The actual pregnancy lasts around two months, as it does for other otters, but, like many weasels and stoats, the embryo remains in suspended animation in the womb for eight months before starting to develop. In this way, given the timing of the breeding season, the young are born between January and April, when food is at its most abundant across most of the animals' range - there is some indication that this doesn't happen in Florida, where pregnancy may follow the more typical otter two-month pattern.

Like many other mustelids, otters have been hunted for their fur, because of its appearance and waterproof properties. They have also been persecuted, at times, by river fishermen under the (generally mistaken) impression that they endanger local fish stocks. As a result, the species on both continents are no longer so widespread as they once were, and the North American otter is really only absent from the central US because of past human activity.

Having said that, a rather greater threat to otters comes from pollution, to which they are particularly vulnerable because of their riverine lifestyle. Being carnivores, not only is their food supply reduced by pollution, but, when they do eat, any contaminants in the food end up being concentrated within the otter's own body, making them particularly susceptible to high levels of pollutants. Polychlorinated biphenyls and mercury have been cited as particular concerns, although those living on the coast will doubtless also be affected by oil spills.

Despite these threats, otter populations have been recovering on both continents in recent years. The North American species is currently considered safe, especially in Canada. The increased potential for human interaction with North American otters has even led to a reported incident in which an eleven year old child suffered a vicious and unprovoked attack by an enraged otter in California. Such attacks appear to be extremely rare, however, and one can't help wondering (as the doctors evidently did) whether the otter in question had rabies.

In Europe, otters have fared less well, and the Eurasian otter is no longer found in Switzerland or the Netherlands, or across much of Germany and northern Italy. On the other hand, populations have recently rebounded in many areas, and, despite the ongoing threat of pollution, the species is not considered under any immediate threat. Indeed, populations in Ireland, which one might reasonably expect to be somewhat insular, are surprisingly healthy and genetically diverse.

Outside of Europe, the story is less encouraging. I've already mentioned Japan, and the population in Thailand also appears to be extinct. Here, poaching seems, along with habitat loss, to be at least as much of an issue as pollution, and the otters of China and southern Asia appear to be under significant threat. Perhaps, one day, the older name of "European otter" will become alarmingly accurate.

[Pictures by Bernard Landgraf and "Kawausosu" from Wikimedia Commons. Cladogram adapted from Koepfli et al. 2008].