Sunday 8 July 2012

Weasels at Sea: Sea and Giant Otters

Sea otter mother with pup
Most otters are considered to be 'semi-aquatic' animals. That is, they spend most of their time swimming in rivers or lakes, where they catch the great majority of their food, but they return to dens on the bank in order to sleep and raise their young. However, there is one exception: an otter that is fully aquatic, and is, in fact, the only truly aquatic mammal to have feet, rather than flippers - all the others are seals, whales, dolphins, and the like. This is the sea otter (Enhydra lutris).

Most other otter species will enter salt water on occasion, especially if they live on small islands where fresh water is scarce. The marine otter is unusual in that it habitually hunts at sea, and only occasionally enters rivers, but it still creates dens along the shoreline, and so is considered only semi-aquatic. Generally when otters do swim in the sea, they avoid water that's any deeper than most rivers, and even marine otters won't venture more than about 150 metres (500 feet) offshore. Not so the sea otter.

Sea otters live along the coasts of the northern Pacific. In the south, they reach as far as California, and once even reached the west coast of Mexico, but the majority are now found off the southern coast of Alaska, and there are also sea otters along the coasts of far eastern Siberia and the extreme north of Japan. They are not close relatives of the other American otters (Lontra spp.), but are instead later arrivals on that continent, closer in origin to the various Old World species.

At up to 1.2 metres (4 feet) in length, not counting the tail, sea otters are much larger than most of their riverine kin. Indeed, they look even larger than they are, because of their pelt of long, thick fur, quite unlike the sleeker coat of their cousins. This coat is vital to their survival, because, unlike other marine mammals, they don't have a thick layer of blubber, and the coat helps trap air bubbles that keep them insulated from the cold. This is why oil spills are so deadly to sea otters - it's not so much that there's poisonous goop on the sea (although that clearly doesn't help), it's that their fur becomes matted and soiled, and the animal rapidly dies from hypothermia. When the Exxon Valdez sank in 1989, the resulting spill may have taken almost 5% of the worldwide population of sea otters with it.

Sea otters have a similar, brown, colour to other otters, with paler fur on their head, but less so on their chest and underparts as otters normally do. Their tail is unusually short for an otter, and doesn't have the usual tapering shape, although it is muscular, and can be used to scull through the water. Anal scent glands, a key feature of most members of the weasel family, are entirely absent in sea otters. When you spend your entire life at sea, there is, after all, simply nothing to scent mark.

Indeed, sea otters can survive perfectly well without ever once setting foot on dry land. They sleep at sea, lying on their backs, often among kelp beds, whose long fronds stop them from drifting too far. They are capable of moving about on land when they want to, but look uncomfortable and ungainly while doing so. This is partly due to the adaptations of their hind limbs to swimming. The forepaws are fairly normal for otters, but the hind-legs are short (even for an otter), with unusually long feet so heavily webbed that they look rather like the flippers of sea-lions. This enables them to swim well, and they can move at over 5 mph (9 kph) underwater, but it does make them a little awkward on land.

Even more so than clawless otters, sea otters rarely eat fish. Instead, their preferred prey are abalone, crabs, clams, and sea urchins, although they will eat other invertebrates if they're hungry. Their cheek teeth are well adapted to this diet, being large and broad, suited to crushing hard shells. Famously, however, sea otters do not rely entirely on their teeth to get at the soft flesh inside shellfish. Instead, in a rare example of non-primate tool use, they pick up rocks from the sea bottom, and float on their backs with the stone held on their chests, before using it as an anvil to smash open shells held in their forepaws. This seems to be an instinctive behaviour, not the sort of reasoned tool use found in primates, but even so, there are some variations in the exact methods used by different populations. They obtain most of the water they need from their food but, unusually, are also capable of drinking sea water, and have particularly efficient kidneys, not unlike those of desert-dwelling animals.

Although sea otters hunt in deeper water than other otters do, and further from the shore, the food that they need simply isn't found in the truly open sea. Sea otters have an unusually high metabolic rate, presumably to help stave off the cold, and they need to eat a huge amount to maintain it - up to a third of their own body weight daily. As a result, they spend a lot of their time hunting, and have no good reason to travel far out from sea; they are rarely found more than 1,000 metres (0.6 miles) from the coast.

They generally dive for about a minute at a time, although they're apparently capable of holding their breath for up to five minutes if they have to, and have lungs two or three times larger than those of comparably sized land mammals.Even so, that limits the depth of water in which they can forage, and they seem to prefer depths of no more than about 40 metres (130 feet), and often as little as 10, although they can go much deeper if need be.

Sea otters are diurnal, although they tend to take a nap around mid-day. They are generally solitary, although large groups can sometimes be found snoozing in kelp beds. Males and females live apart for most of the year, co-opting separate lengths of coastline. When the males are ready to mate - which can happen at any time of year - they move into female territories, and establish their own patch of water, driving off other males that come near. Although they can sometimes form brief pair-bonds, these don't last more than a week, and the males are likely to mate with any female that approaches.

Mating is quite vigorous, involving a lot of splashing about in the water. It lasts for up to half an hour, for most of which time the male is holding the female's snout in his jaws, with the result that she usually has a bloody nose by the time it's all over and she decides to leave. Pregnancy can last for anything between four and twelve months, evidently involving a degree of delayed implantation. The variability in length of pregnancy means that while mating can take place at any time of year, births are more common at some times than others - the summer in Alaska, and early spring in California.

Otters are not born able to swim, which is why even marines otter are only semi-aquatic, sheltering their young in dry coastal caves until they can take to the water. Obviously, this presents something of a problem for sea otters, which have no dens at all, and avoid the dry land. Instead, they give birth at sea - something that even seals can't manage. The mother then carries her offspring about on her chest for six months or more; while they begin to swim at around two months, it's apparently something it takes them some time to learn to do well.

The necessity of carrying the young about imposes a limitation not experienced by other otters, since simple dynamics means that the mother can realistically only carry one pup at a time. Therefore, while most other otters commonly have litters of two to four young, sea otters normally give birth to single offspring (and have only two teats, instead of four). Twin births do occur, but they aren't much more common than in humans, and, while there are reports of females rescuing abandoned pups from other mothers, and raising them as their own, more commonly, one of the twins will not survive.

The demands of raising young mean that sea otters don't normally breed every year. Males take around four years to reach sexual maturity, while females give birth for the first time at three and a half years, or slightly later if food resources are scarce.

Sea otters do have one great disadvantage as a species, and that is that their fur is perhaps the most valuable of any living animal. That's partly because, in order to maintain the animal's body heat at sea, the underfur is perhaps the densest of any mammal, with up to 100,000 hairs per square centimetre of skin (650,000 per square inch). This luxuriantly thick fur has imperilled sea otters through widespread hunting. There have been attempts to control sea otter hunting as far back as 1799, but it was only in 1911 that the trade was banned altogether. By this time, only one or two thousand of the animals survived, in just a few scattered pockets along their former range.

Since that time, sea otter numbers have significantly recovered, and there have been attempts to re-introduce them to some areas where they had previously gone extinct. These attempts have had variable levels of success, and there are no longer any sea otters off the coasts of Oregon or Mexico, and very few off Washington or British Columbia. For a while, it was thought that conservation had come too late to save the southern subspecies, which once lived off California and Baja California, and was believed to have gone extinct in 1920. In 1938, however, it became clear that a small population, probably less than a hundred individuals, had survived near Big Sur. The descendants of that solitary group now number around 3,000, and are found more widely along the southern California coast, although they are still a shadow of the pre-fur trade population.

Although populations remain significantly higher than at their 1911 low, they have been dramatically declining over the last 30 years or so. Oil spills have been a major factor, but, in California, a number of infectious diseases seem to be sweeping through the populations, including some that may have been initially contracted from domestic cat faeces. The low genetic diversity of the southern subspecies, thanks to its previous near-extinction, doubtless also plays a role here. In Alaska, sea otters have suffered from knock-on effects of the decline of local seal and sea-lion populations - as their preferred prey have disappeared, killer whales have switched to eating the otters instead, leading to precipitous declines in their population as well.

Giant otter
There is one last species of otter to cover. As its name implies, the giant otter (Pteronura brasiliensis) is the largest of all otter species. Indeed, at up 1.4 metres (over four and a half feet), excluding the tail, it's the largest of all the members of the weasel family; almost half again the size of a wolverine. However, this honour of 'largest mustelid' is perhaps, somewhat arguable, since it is only slightly longer than the sea otter, and, because they have a much more slender build, giant otters are actually quite a bit lighter than sea otters. Even so, a fully grown male can reach 32 kg (70 pounds), and females about 80% of that.

Giant otters once lived throughout the Amazon and Orinoco river basins, and into lands further south. However, their large size makes their pelt valuable, and they have long since vanished from Argentina and Uruguay, and are no longer common elsewhere. Illegal hunting continues, although more recently, the major threats to the species have come from loss of their forest habitat, due to logging and mining, and from infection with diseases such as canine distemper, contracted from local livestock. They also face persecution by local fishermen, in the mistaken belief that they eat the same food that humans want to catch. Like sea otters, giant otters are formally considered to be an endangered species.

Also like sea otters, giant otters are not closely related to the other otters of the Americas. In their case, however, they are older, not younger, the last remnant of an early migration of otters to the New World, around a million years before the ancestors of the North American river otter and its southern kin took the same route.

Apart from their large size, they have a generally similar body plan to other riverine otters. For instance, they have a slender body, short limbs, and a long, tapering, muscular tail. Their head and feet are proportionally somewhat larger than in most other otters, and the toes are very well webbed. The coat is sleek and dark brown, sometimes almost black, with distinct yellowish-white blotches on the throat and chin, that may sometimes merge into a single patch.

They live in slow moving rivers and lakes surrounded by dense forest, especially where there is clear water. During the rainy season, they may swim further afield, ranging across the floodplains of lowland jungle in search of food. They clear large patches of forest, up to 50 metres (160 feet) across, and dig dens with chambers large enough to comfortably sleep in at night. They feed primarily on fish, although they do also take other aquatic prey from time to time, and have even been reported to attack small anacondas.

They are perhaps the most sociable of otters, and among the most sociable of all members of the weasel family. Giant otters live in relatively persistent family groups, typically consisting of a mated pair, one or two sub-adults, and the young from the latest litter. That normally results in a group of five to eight individuals, although some groups with up to twenty members have been recorded. The group jointly defends their territory from outsiders, although actual fighting seems to be rare, as different groups generally stay well clear of one another.

They give birth to litters of up to five young at the beginning of the dry season, roughly from August to October; at around 65 to 70 days, pregnancy lasts only marginally longer than it does in smaller otters, although there is some evidence of delayed implantation in captive individuals. The young are much smaller than those of sea otters at birth, but they grow rapidly, and are sexually mature at two years. Giant otters in captivity have reached the age of nineteen, although they probably don't live so long in the wild.

Remarkably, one study observed an elderly and apparently post-menopausal matriarch, no longer able to hunt effectively, being supported by younger members of her family. Such behaviour is unusual among wild animals, and in this case, might potentially be due to the matriarch being able to provide assistance to the group. Perhaps that was due to her greater knowledge of the area, something also observed in elephants.

With that, I have finished my survey of the otters of the world, but not of the weasel family as a whole. In terms of number of species, the mustelines, martens, and otters are the three largest groups within the family, but there remain a number of smaller subfamilies, and, in August I'll begin my survey of those by looking at the badgers.

[Images by Mike Baird and Eric Gaba, from Wikimedia Commons. Cladogram adapted from Koepfli et al. 2008.]

1 comment:

  1. There is one thing sea and giant otters have in common. Their hind paws look like flippers of a sea lion.