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The living species of pinniped are placed within three families, although one or two extinct ones are also recognised. Most pinnipeds belong to the Phocidae, or "seal family". While the term "seal" is often used in modern English to refer to pinnipeds in general, the seal family proper consists of animals especially well-adapted to living in the water. They lack visible ears, and their hind limbs are arranged so to be efficient for propulsion underwater, but almost useless on land, forcing them to haul themselves about using their front flippers only.
Almost all the other species belong to the Otariidae, which can loosely be described as the "sea lion family". More accurately, most species in this family are referred to as "fur seals" in English, with there being no clear scientific distinction between what counts as a fur seal and a true sea lion; the names have been applied rather haphazardly. These do have visible ears, and they can walk - or at least waddle - on all fours while on land. Underwater, they propel themselves with their front flippers, using the hind ones for steering, essentially the opposite of how true seals swim.
I have previously discussed the differences between these two families in more detail, as part of a series looking at the "true", phocid, seals. What I didn't talk about in that post, except in passing, is the one species of living pinniped that belongs to neither family.
The walrus (Odobenus rosmarus) is an unusual and distinctive animal. The origin of the common name is most likely an Old Norse word literally translating as "whale-horse". It remains similar in other Germanic languages, as well as those that have borrowed the word from English, although the Romance and Slavic languages have instead tended to borrow the Sami word "morsa". The second half of the scientific name is also Scandinavian in origin, although rather more abused to make it look Latin, and means something like "red whale".
The walrus, perhaps unsurprisingly, appears in the first officially recognised catalogue of scientific names, published in 1758. It helps that it's not just an obviously distinctive animal, but that the list in question was compiled by Carl Linnaeus who, being Swedish, was clearly going to be familiar with them.
At the time, he classified it as one of four known (to him) species of "seal", along with the common harbour seal, the elephant seal, and what we would now call the northern fur seal. Four years later, French zoologist Mathurin Brisson decided it was distinctive enough to be split off from the seals and given its own genus name. This name, Odobenus, means "tooth walker", since he had heard that they sometimes pulled themselves about on land with their teeth. Their further elevation to family status came surprisingly late, in 1880, but only because Linnaeus later changed his mind about it being a seal, and placed it with the manatees; later zoologists seem to have agreed with him, presumably not taking a close look at one or the other animal in the process.
The average walrus is about 3 metres (10 feet) in length, but they are much heavier than one would expect from this figure alone. This is due to an extremely powerful build, perhaps matched only by elephant seals, that means their shoulder girth is almost the same as their body length. A typical female weighs around a ton, while the males are half as much again.
While they are more closely related to sea lions than to true seals, walruses are anatomically intermediate between the two in at least some respects. Like sea lions, they are able to rotate their hind limbs so that they can use them to walk while on land, but, like seals, these hind limbs are their main source of propulsion underwater. However, they do use their front flippers to assist in swimming, paddling as well as using them to steer. Like seals, this is also assisted by muscular undulations of the main body, something that's particularly useful on land because of their great bulk.
The skin of walruses is unusually thick, typically around 4 cm (1½ inches). It is thickest on the neck, especially that of males, where it can reach up to 10 cm (4 inches). This is not counting the insulating blubber beneath, which all pinnipeds possess, and which reaches another 15 cm (6 inches). Walruses do have fur, which is brownish, but it is so sparse that, as with animals like elephants, it isn't immediately obvious at a distance. Adult males have numerous warty nodules around their necks, which may help protect them while fighting, but are more likely to just be something that the females find attractive.
The faces of walruses are distinctive because they are so shortened. They have especially large and thick whiskers, and comparatively small eyes for a pinniped. These are positioned so as to give them partial binocular vision, but it's likely that their vision is relatively poor, albeit with some evidence that they may have some limited colour vision. Like seals, their ears are not visible, opening directly onto the skin without the usual flaps of skin and cartilage that most other mammals possess.
They do have a tail, but it's so short as to be almost invisible, and is hidden by a flap of skin.
Perhaps the most distinctive feature of walruses, however, is their teeth. Since they are carnivorans, their ancestors had the same basic arrangement of teeth that dogs, bears, and the like possess. In their case, however, it has been highly modified, with many of the teeth disappearing altogether. Typically, carnivorans have three sets of incisor teeth in each jaw, but walruses have just a single pair in the upper jaw only, and these are small and rounded.
The lower jaw has four teeth on each side, all of them relatively flat and oval in outline. The first and largest of these is the canine tooth, while the premolars behind them becoming gradually shorter towards the back of the mouth. Like most other pinnipeds, there are typically no molar teeth at all, and there is no hint of the carnassial teeth that are so significant in land-dwelling carnivorans. (I say "typically" here because there is some variation, with some individuals having additional, tiny and rudimentary teeth that raise the total number listed here).
The upper jaw has three sets of premolar teeth, like the lower one, but is most notable for the greatly enlarged canine teeth. These are true "tusks" in the sense that they project outside of the jaw, and they grow continuously throughout life. They can reach a metre (3' 3") in length, and weigh up to 5 kg, and curve slightly backwards to a blunt tip. When they first appear, these have a coating of enamel like any normal teeth, but this soon wears away to leave only the dentin/ivory beneath.
Walruses live only in cold, coastal environments of tha far north. There are two recognised subspecies, of which the more widespread is found from Hudson Bay and the southeastern Canadian Islands, eastward past Greenland to Novaya Zemlya and the Franz Josef islands off the north coast of western Siberia. The second, which is slightly larger on average, lives from the Laptev Sea off eastern Siberia, round to the Chukchi and Bering Seas, including the western and northern coasts of Alaska. Despite that this means many members of both subspecies live in the Arctic Ocean, they are commonly referred to as "Atlantic" and "Pacific" walruses, respectively.
The driving factor here is the walrus's preferred habitat of shallow seas with regular ice floes on which to haul out. While they will use rocky shores if ice isn't available, they do prefer the latter, and may migrate northwards with it during the summer if it doesn't force them into deep water. Ice that's too thick, on the other hand, is also a problem, since they can't break through ice much more than 20 cm (8 inches) thick.
The shallow seas are important because walruses feed on animals that live on the sea floor. While they can dive as deep as 600 m (2,000 feet) if they absolutely have to, 80 m (260 feet) is much more comfortable for them, so a sea deeper than that is less than ideal. Even these dives can last over seven minutes, and they don't seem as exhausting as one might expect, partly due to an ability to store oxygen in their muscles that develops rapidly after birth.
Molluscs are the primary food of walruses, with some studies showing that clams alone provide 81% of their calorie intake. Most of their other food consists of snails and sea worms, with occasional soft-shelled crabs. They search for such food by probing the bottom with their long and sensitive whiskers, sometimes clearing away the mud with their front flippers, then sucking it in using their lips and tongue. On rare occasions, some have even been known to eat seabirds or small seals, although this is probably a case of desperation.
The old belief that they scrape clams off the seabed using their tusks is a myth, perhaps arising because they do sometimes prop themselves up with their tusks while feeding.
There have been few studies conducted on the social lives of walruses, and what there have been suggest that there isn't much of it to look at. Walruses aren't antisocial and will gather together on ice floes or rocky shorelines, but neither do they seem particularly sociable, in the sense of really interacting with one another outside of the breeding season. They do not, for instance, make much in the way of noise to communicate, and they could hardly leave scent marks, living where they do. But, without detailed studies to show otherwise, it's always possible that there's plenty we're missing.
The exception, however, is during the winter breeding season. At this time of year, the males compete with each other to impregnate as many females as possible, and may establish a temporary dominance hierarchy to do so, much as other pinnipeds do. This is largely achieved by puffing themselves up to look impressive and then shouting at each other. These are loud, booming or bell-like sounds, which they can make thanks to the presence of two large sacs on either side of their windpipe, which are absent in the females. If this fails, they will get into physical combat with their tusks, but this is comparatively uncommon.
Possibly attracted by the calls, the females gather around and mating takes place in the water. This may be aided by the fact that the male has an unusually long, 60 cm (24 inch), bone in his penis to provide added support.
Pregnancy lasts a full 15 months, and almost always results in the birth of a single young. While still in the womb, the baby develops the same white fluffy coat that newborn seals have, but this is shed before they are born, so that they initially look like miniature, tuskless adults. This means that, unlike seals, baby walruses are able to swim from the moment they are born. In another exception to the general lack of vocal communication, the mother regularly barks to her infant to bond with it and ensure it does not get lost.
Initially weighing about 60 kg (130 lbs), the young walrus grows rapidly, but stays with its mother for up to two years before striking out on its own. As a result, females give birth, at most, once every two years, half the rate we see in other pinnipeds, and usually less often than that. While walrus milk is not quite as nutritious as true seal milk, perhaps partly because of this slower development, it is still richer than that of most land mammals. Unusually, baby walruses can suckle while their mother is swimming at sea, something that would be pointless in seals, which are not born able to enter the water.
Historically, walruses were widely hunted for their meat and tusks, leading to dramatic declines in the Atlantic population. This has greatly declined since the 1990s, although it is allowed at very low levels in a few indigenous communities. Today, the worldwide population is thought to be over 100,000 and is more threatened by melting sea ice and the development of shallow waters for oil and gas drilling. As a result, while it isn't formally considered an endangered species, it isn't a wholly safe one, either.
[Photo by Joel Garlich-Miller, of the U.S. Fish and Wildlife Service. In the public domain.]
Synapsida takes a break next week, but will be back in August.
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