Sunday 10 June 2012

Weasels in Warm Rivers: Otters of the Tropics and the South

Hairy-nosed otter
The body plan of otters is evidently a successful one in evolutionary terms. While the two species of "common otter" are the only ones inhabiting the rivers of the northern temperate zone, there are at least ten species in other parts of the world, and most of them look remarkably similar. They all have long, sinuous, bodies with short legs and webbed feet. Their tails, quite unlike the slender or stumpy appendages of other members of the weasel family, are powerful and muscular, and somewhat flattened to better help propel them through the water when swimming. Their fur is dense and, in most cases, short and sleek.

Furthermore, they are all more or less the same colour. Otters, of all species, are brown over most of their body, and they usually have paler underparts with a particularly noticeable patch of pale fur on the chest, which may extend up onto the chin. Compared with the variety of weasels, or even martens, the visible differences between most species of otter can be subtle, and it's hard tell them some of them apart.

The closest living relative of the familiar Eurasian otter is the hairy-nosed otter (Lutra sumatrana) of South East Asia and western Indonesia; the two species probably diverged during the early Ice Ages. It is also a highly endangered species, and, combined with its remote jungle location, that means we know relatively little about it. This story of threats to otters is, sadly, one I'll come back to a lot in this post. Of all the members of the weasel family, the otters are the ones most at risk, probably because their riverine habitat makes them particularly susceptible to pollution, and may also make them easier to hunt for their valuable fur.

Certainly, the main threat to the continued existence of the hairy-nosed otter comes from hunting, as much for its meat as for its fur. It's a protected species in every country where it is found, and all international trade in the fur is banned, but that has done little to prevent illegal poaching. So bad has the situation been for this species, that there was a real fear in the 1990s that it might have gone extinct, although the information we had was so sparse it was difficult to tell. In 1998, living animals were discovered in Phru To Daeng in Thailand, and it has since also been reported from a nature reserve in the Mekong Delta of Vietnam, from Tonle Sap lake in Cambodia, and from Sumatra. They're probably also found in peninsular Malaysia and northern Borneo, although recent reports are hard to come by. All of these surviving populations are small, however, and seem to be getting smaller; it's been estimated that there are only half as many hairy-nosed otters alive today as there were just 30 years ago.

Physically, hairy-nosed otters look very similar to their more widespread Eurasian cousins. They're slightly darker, and the fur on the chest and underparts is not as noticeably pale, apart from a distinct patch of white on the chin. As their common name suggests, they have a hairy nose, rather than the wet hairless kind that most otters, like dogs, possess. They inhabit a range of tropical freshwater environments, although apparently with a preference for peat swamp forests. We know that they like eating fish... and that's about it.

There are, however, two other species of otter that live in South East Asia. Neither is quite so badly off as the hairy-nosed otters, but that's all relative - both species are considered seriously threatened, with small, and declining, populations, but they are so far holding on just well enough to avoid being formally classified as an endangered species.

Smooth-coated otter
Smooth-coated otters (Lutrogale perspicillata) are the largest otter in the region, being up to four feet in length, counting the tail. Their fur is exceptionally sleek, and often has a slightly reddish tinge, but they otherwise have the normal otter coat pattern. The tail is slightly more flattened than in other otters, presumably aiding in their swimming ability. They don't just live in South East Asia, but are also found throughout much of India (where they are best known), and up into Pakistan, Nepal, and Afghanistan, as well as in Sumatra and Java in Indonesia. One subspecies has been reported from as far west as southern Iraq, but, for obvious reasons, there have been few detailed studies of the local wildlife there in recent years. Indeed, since the last time anyone really looked for them, Saddam Hussein began draining the otters' swampland habitat as part of his campaign against the Marsh Arabs. It's unlikely that otters were high on anyone's priority list at the time, but, in retrospect, it's hardly promising.

Swamps in general seem to be a preferred habitat of smooth-coated otters, including man-made rice paddies, although they also inhabit smaller rivers far inland. Like many otters, they can hunt along the seashore, although they need at least some fresh water nearby. They typically eat fish, as you'd expect, but, in areas where there is a significant dry season, they probably also hunt in nearby woodlands, and they don't appear to be very picky about their food. One report even describes an otter attacking and killing a monitor lizard noticeably larger than itself, dragging it tail-first from a fallen tree and then overpowering and drowning it after a lengthy struggle. Whether or not it actually ate the lizard is unclear; it may have been a mother pre-emptively defending her young.

They are fairly sociable animals, with the young staying with their parents for a few years, so that families may consist of up to a dozen individuals, able to 'herd' fish together to make it easier to catch them - in India, fishermen apparently use them to drive fish into their nets.

It's long been recognised that smooth-coated otters are clearly distinct from Eurasian otters, but it was something of a surprise when genetic analysis showed that they actually appear to be a species of "clawless otter". It's surprising because, like most otters, they have large, strong claws on their webbed feet, with which they often dig their own burrows. As the name suggests, that's not true of the animals we now know to be their closest relatives.

Asian small-clawed otter
Asian small-clawed otters (Aonyx cinerea) are the world's smallest otters, and live throughout South East Asia and western Indonesia, in southern China, and parts of India. They are popular exhibits in zoos, because they readily breed in captivity and are playful and vocal animals, sure to keep visitors entertained. Their claws are tiny, no more than rudimentary stubs, and their feet are only partially webbed. As a result, their paws are far more sensitive and dextrous than those of more typical otters.

They can live happily alongside other species of otter, without either depleting the other's food supply. That's because, while small-clawed otters will catch fish if they can, it isn't their preferred food. Instead, they use their agile paws to dig around in the mud and under stones, catching crabs and molluscs rather than fish. Their teeth are also modified for this diet, with large, broad carnassials and upper molars, better able to crush hard shells. They have also been reported to leave some shellfish on river banks to dry out, so that they partially open, making it easier to get at the flesh inside.

Small-clawed otters are gregarious, living in groups of a dozen or so individuals, although they are sexually monogamous, breeding up to twice a year. Because so many are bred in captivity, we have a good knowledge of their reproduction. Like most otters, pregnancy lasts around two months, and they give birth to litters of about four pups, although up to seven have been reported (they only have four teats, however, which presumably makes larger litters a little problematic). The young are born blind, as they are in many carnivores, and covered in a silvery-grey fur, but they are active and able to swim after as little as seven weeks. Both parents care for the young, with the father concentrating on maintaining the grass-lined den, while the mother trains and looks after the children.

Members of the weasel and marten subfamilies have had little success in colonising Africa. There is a single species of weasel found in the extreme northeastern corner of the continent, where it meets Asia, but that's all. Otters, on the other hand, have been more successful, with two, or perhaps three, species alive today. The most widespread is the African clawless otter (Aonyx capensis), which is found almost everywhere south of the Sahara, apart from the deserts of Namibia. It seems to prefer shallow, slow-moving water, as might be found in ponds or sluggish rivers, and can live along the coast, so long as there is fresh water nearby to drink. Aside from that, their main requirement seems to be a reasonable amount of dense vegetation in which to hide, although accounts differ as to whether or not they can dig their own burrows, or simply appropriate crevices beneath boulders or trees.

Although they are much larger than the Asian small-clawed otters, in other respects, their relationship is clear. If anything, their adaptation to eating crabs and lobsters is even more extreme, for their front feet have no claws or webbing at all, giving them mobile, tactile fingers not unlike those of primates. Their hind feet have very limited webbing, and tiny claws on three out of the five toes, which they apparently need for grooming. Compared with other otters, they tend to be slightly darker in colour, and have especially clear pale silvery markings on the chest and cheeks. One of the largest of all otter species, adult males can weigh up to 21 kg (46 lb), five times the weight of their dainty Asian kin.

They are less sociable than the Asian species, living mostly solitary lives. However, they have been reported to have a clan-based social structure, with related males living apart but jointly defending a single, shared territory from outsiders. Being found so widely across Africa, and seemingly relatively tolerant of human activity, African clawless otters are numerous and at no apparent risk as a species, although they are hunted in some parts of their range.

There is some debate as to whether they represent a single species, or two. Although there has not been any detailed study to confirm the idea, some researchers point to significant differences between the otters found in the Congo and those elsewhere in Africa. They argue that Congo clawless otters are darker, with shorter fur, more prominent claws (which, admittedly, isn't saying much), and smaller teeth than their kin elsewhere. They also apparently prefer mountain streams to lowland ponds, and may eat more fish, or even more land-dwelling animals, than other "clawless" otters. So far, it's a minority opinion, but not one without plausible evidence.

Spotted-necked otter
The fact that the clawless otters of Africa feed mainly on crustaceans leaves the field open for a fish-eating species, just as it does in Asia. That role is taken by the spotted-necked otter (Hydrictis maculicollis), which is almost as widespread as the African clawless species, although, naturally, it too avoids arid environments. It is so similar in appearance and habits to Eurasian otters that many sources consider it to belong to the same genus. However, while it's exact place in the otter family tree is unclear, the best current evidence does seem to indicate that it belongs to neither the Eurasian nor the "clawless" branches.

Spotted necked otters are similar in size to Eurasian otters, and with similarly webbed and clawed feet. They have dark fur, and, unlike other otters, rather than having a broad patch of silvery grey fur on their chests, they have a number of yellowish spots that become whiter as they age. Even by the standards of otters, they rarely travel far from rivers - perhaps no more than ten metres (30 feet) - and they are one of the few otter species that won't venture into the sea, even if they live near the coast. Their favoured prey seems to be a type of cichlid fish, but they'll eat whatever they can catch, including shellfish if there are no clawless otters around to get there first. They catch prey with their mouths, like Eurasian otters, and not with their fore-feet as the clawless species do.

They are fairly sociable, travelling in single sex bands of around six to eight adults, although they don't seem to cooperate much in catching food. After breeding, the two sexes go their separate ways again, the males play no part in raising their young. They seem to be mostly nocturnal, and sometimes dig their own burrows in river banks and other suitable locations. Unlike the African clawless otters, their population is apparently decreasing, but not rapidly enough to be of any real concern for the time being.

Clawless otters never crossed over to the New World. In fact, the only otters that did seem to have crossed the Beringia land bridge before the first clawless otters ever evolved, and they have mostly retained the appearance of standard, fish-eating, otters elsewhere. The only otter in Canada and the US is the North American river otter, which, as we've already seen, is remarkably similar to the Eurasian otter. However, as with so many other animals, some of its relatives did travel further south, and, today, three other species are recognised from its genus.

Neotropical otter
Heading south, the first such species we come to is the Neotropical otter (Lontra longicaudis). It is the most widespread of the three, being found from Mexico, through Central America, and across South America as far as northern Argentina. Most live east of the Andes, but there is also a population in Ecuador. Neotropical river otters are very much an inland species, and only inhabit the sort of clear, fast-flowing streams that are found in highlands far from the coast.

They look very similar to their North American cousins, although perhaps slightly paler, with a hint of grey in their fur. They mainly eat fish, although, in the absence of clawless otters, they will certainly take river crabs if that's what's going. They are among the more solitary and antisocial of otters, avoiding one another outside the breeding season, although they don't actually fight for control of territory. The habitat they live in is often at risk of flooding, so they tend to den quite high up on river banks, and have been reported to use caves and the hollow tops of drowned trees.

During the 1970s, neotropical river otters suffered badly from persecution and hunting for their fur, and their populations dramatically declined. Hunting is now banned, but there is no clear evidence on how well they've recovered. It's probably fair to say that they are not endangered, and they seem to be recovering in Uruguay at least, and they may be holding on in Brazil. The same is not necessarily true elsewhere, with Mexican populations a particular concern.

Although similar in appearance to river otters, aside from being somewhat smaller and more uniform in colour, marine otters (Lontra felina) have a very different habitat. They live along the coasts of Peru and Chile, and only rarely venture into river mouths. Instead, they hunt offshore, in rocky and shallow seas where they have some chance of avoiding the worst of the weather. They eat a fairly equal mix of fish and crabs, dragging the larger ones ashore, but eating the smaller ones where they find them.

Marine otter
Most of their day is spent resting on the shoreline, but they can venture up to 150 metres (500 feet) offshore during the day, diving below the waves for up to a minute at a time in search of prey. In other respects, they hunt much as river otters do, and they don't travel long distances out to sea, or along the coast. They sleep in caves or rocky crevices which (unless they are mothers with young) may be submerged for parts of the day when they are not resident. They are generally solitary animals, although males seem at least moderately tolerant of potential rivals.

Since 1996, marine otters have been internationally recognised as an endangered species, suffering from pollution, increased human settlement along the coast, and illegal hunting. There may be less than a thousand individuals left, primarily in southern Chile, where humans are least numerous, and their overall population is believed to be declining rapidly.

Lastly, we come to the southern river otter (Lontra provocax) of Patagonia, inhabiting cool forested rivers and occasional shorelines at the southern end of the continent. While clearly preferring freshwater today, there is some evidence that they retreated to the coast during the height of the last Ice Age, enabling them to survive without having to head north. They are of moderate size and of undistinguished appearance for an otter, hunting at night for either fish or crustaceans, whichever may be the more available. They are generally solitary, although males and females at least tolerate one another's presence outside the breeding season. They can dig their own burrows, although they are also willing to make do with hollow logs or rocky crevices.

In most respects, their habitat is similar to that of the common otters, since they live in the temperate zone of the southern hemisphere. As a result, there is a distinct winter breeding season, with young being born in the spring - which is to say, September or October

Although it is illegal to hunt them, this has had little effect, given their habitation of an area remote from much in the way of law enforcement, and they have been officially considered an endangered species since 2000. They may not be quite so badly off as some other endangered species, but the continued fragmentation of their habitat, and the value placed on their pelts does not bode well for their future. There has also been concern that introduction of salmon to their native rivers has not helped the species, because the salmon may out-compete local fish, and are themselves too fast for the otters to catch.

[Image of Asian small-clawed otter by the author. Other pictures by "IOSF1957", Lip Kee Yap, Derek Keats, Carla Antonini, and "Sakura 1994", from Wikimedia Commons. Cladogram adapted from Koepfli et al. 2008].

1 comment:

  1. Being a mustelid. These animals are capable of eating even monitor lizards of all types and small crocodilians and snapping turtles. The'reptiles'not standing much of a chance even being twice'the'size. The otters canine and carnasial teeth. Would quickly attack the lizard and gnaw head off, the lizard may run around headless and confused till it collapses and otter quickly dismembers and eats it, komodo is lucky honey badgers, otters and weasels don't exist there, any size komodo dragon would quickly become food for mustelids. I wouldn't even be close. A 20 pound Honey badger