Sunday, 26 February 2017

Pinnipeds: Harbour and Spotted Seals

Harbour seal
When the modern system of scientific names for organisms was devised, and the first recognised catalogue of such things was published in 1758, Carl Linnaeus named four species of seal. Two of these are, for anatomical reasons explained in my previous post, no longer considered members of the "true" seal family. One of the others was the elephant seal, which Linnaeus had presumably only heard of by reputation. The remaining one, however, was likely an animal he was much more familiar with, given that they are found around the coasts of his native Sweden.

In fact, when Linnaeus described what was essentially a typical "seal-like" animal, he would have been thinking of what we now know to constitute, like the "elephant seal", a number of different species. The one that was likely the most familiar to him, however, is the one that retains the scientific name that he gave it, and which is known in many parts of the world simply as "the common seal". In more recent times, the alternative name of harbour seal (Phoca vitulina) has become more widely used, and it's this that I'll use to describe the animal to which, taxonomically speaking, all other seals are in some sense compared.

Harbour seals are one of two species of seal commonly found around the coasts of the British Isles, and also inhabit the south coast of the English Channel, all the coasts of the North Sea, and round the Atlantic/Arctic coasts of Scandinavia as far east as the southern shores of the Barents Sea in Russia. They often venture through the straits between Denmark and Sweden, and so are also seen in the Baltic, even reaching the Polish coast. At the opposite extreme, they have apparently been seen as far south as Portugal, although that certainly isn't somewhere they regularly live.

This, however, represents only one of four or five subspecies of harbour seal. A second one lives on the far side of the Atlantic, ranging from Virginia in the south to Baffin Island and south-western Greenland in the north. One or other of these two subspecies also lives in Iceland, but it's unclear which, and it may even be both, at different ends of the island. Harbour seals also inhabit Hudson Bay; these are sometimes regarded as a separate subspecies from their western Atlantic kin, and sometimes not. But that's not all, because harbour seals are also found in the Pacific, in a coastal arc with one end at Baja California in Mexico and the other at Hokkaido in Japan. There are certainly two subspecies here, with the boundary line between them likely somewhere around the Alaska Peninsula.

This is quite a wide area, and its notable that the climates of say, Mexico and Greenland, aren't exactly what you'd call "similar". There are clearly some limits, with the wide geographic gap between the Pacific and Atlantic populations likely being explained by them having been separated during the Ice Ages, when the Arctic Sea became too cold. But even so, a particular sort of weather isn't too far up their list of requirements.

What harbour seals do like, however, are shallow coastal waters, and they don't venture too far out to sea. This is partly because they like to rest on rocky coasts and mudflats, with a preference for sheltered bays, and, yes, harbours - although thick sea ice will do if that's all that's available. They haul themselves out of the water once every day or two during the summer, but less often during the winter. Even during the summer, however, they seem to avoid the hottest parts of the day, which presumably make them uncomfortable.

Given the wide area over which they live, it is difficult to make many generalisations about what they eat, beyond the fact that it's mainly fish. Off Cape Cod, for example, their most common food is sandlance, while in Oregon, it's sculpin. It even changes throughout the year, with Scottish harbour seals eating herring in winter and sandeels in summer. What these fish have in common, beyond the fact that they are particularly abundant in those particular areas, is that they tend to be found close to the sea bottom. For example, harbour seals feeding on herring do so mainly at night, when the fish shelter near the sea bed, rather than during the day when they are higher up, and more spread out.

It's for this reason that they prefer shallow seas; although they can dive up to a remarkable 800 metres depth, 20 metres seems to be a more typical foraging depth, and they rarely travel more than about 45km (30 miles) from their favoured resting sites when they do so. Even so, especially if they're hunting at night, it's relatively dark down there, and blindfolded harbour seals have been shown to be able to use their acutely sensitive whiskers to detect and follow subtle ripples in the water that might be made by swimming fish.

Despite the fact that they tend to rest in large aggregations of hundreds, or even thousands, of individuals along the coast, harbour seals are actually fairly antisocial animals. They hunt alone, and, even when males are not competing with one another during the breeding season, most of the sounds they make seem to be warnings to each other to stay away and leave them alone. It may also be notable that the beaches used by harbour seals tend to be single sex, with mothers abandoning their young at just three or four weeks of age.

The time of year that harbour seals are born varies with latitude. In Mexico, births occur in March and April, but they steadily become later the further north you go, with Alaskan and Canadian populations waiting until June or July. Oddly, however, while a similar pattern occurs along the Atlantic coast of the Americas, and probably also in eastern Asia, it isn't seen in Europe - it's not at all clear why this should be.

As soon as mothers have managed to divest themselves of their young, the mating season begins in earnest, although those females too young to have given birth do manage to get a head start, mating slightly earlier, while their older sisters are still raising their children. Males compete with one another, partly by growling and roaring at one another underwater as well as by vocal and visual displays on land; it's possible that these also attract potential mates. Compared with many other seal species, a high proportion of the males get to mate in any given year, with larger and more dominant individuals failing to attract much of a harem.

Mating itself occurs underwater, and may be preceded by bouts of aquatic sex play shortly before the mothers give birth. As is common for seals, the resulting embryo doesn't attach itself to the uterine wall for a full two months. Even then, growth is initially slow to non-existent, so even though pregnancy lasts a full eleven months (as it has to, given the relative timing of the birthing and breeding seasons), actual development only lasts about eight.

When most people think of baby seals, they envisage white fluffy pups with large, dark eyes. Newborn harbour seals, however, while they have the large eyes, they already have the same sleek, patterned coat as their parents. It's not that they don't grow the white fur that we normally expect it's just that they shed it even before they are born (although there are apparently occasional exceptions). This has the advantage that, unlike most other seal species, the young are born already able to swim without getting hypothermia. In fact, pups often follow their mothers into the water almost immediately, presumably benefiting from the increased protection. Even so, they take up to a month to get the hang of hunting after being abandoned, surviving off fat reserves in the meantime.

Spotted seal
The closest living relative of the harbour seal is the spotted seal (Phoca largha).  Physically, the two species are very similar, both being medium-sized seals with no particularly distinguishing features , and where the males are only slightly larger than the females. The spotted seal has a grey or silvery coat marked with black blotches... but harbour seals can be very similar, although in their case the pattern is sometimes reversed, with pale spots on a dark background, and browner shades are also more common. This makes it difficult to tell the two apart from a distance, even for experts, so that the population of spotted seals is difficult to know for certain (it appears to be high, though, and the species is not considered threatened).

In terms of behaviour, however, the two species are rather more different. The spotted seal lives only in the North Pacific, primarily along the Asian coast, where it reaches as far south as Korea and southern Japan. In the north, however, it is found on both sides of the Bering Sea, where it inhabits the western and northern coasts of Alaska, as well as the southern shores of the East Siberian Sea. These are, for the most part, cold climates, and where the harbour seal prefers sheltered rocky coasts, spotted seals for the most part rest on ice floes, keeping them safe from most land-based predators other than polar bears.

Obviously, this doesn't really work at the southern end of their range, where offshore rocks have to suffice, and not even over much of the northern parts during summer. Indeed, during summer, spotted seals, unlike their harbour cousins, simply head out to sea, spending little time near land at all, sometimes not coming ashore for a full month at a time. As a result, they can be found as far as 200km (125 miles) offshore, much further out than harbour seals are.

Nonetheless, they too, primarily hunt in shallow seas, looking for fish swimming near the bottom. They eat pretty much whichever appropriately-sized fish they happen to find enough of, although, given their smaller geographic range than harbour seals, there is less variability here with cod, smelt, and sand lances forming a large part of their diet. They presumably hunt in the same way, although it's worth noting that, in addition to their ability to sense water movements with their whiskers, they also have a good sense of hearing and at least some colour vision.

Mating behaviour seems to be similar to that of harbour seals, although they prefer to give birth on thick sheets of sea ice where possible, rather than on dry land. They appear to be mostly monogamous, with the pair, which are otherwise fairly antisocial, interacting and making growling calls to one another more frequently as the breeding season approaches. Unlike harbour seals, young spotted seals are born with a coat of creamy-white fur, which is lost when they are weaned at three to four weeks.

Apart from giving birth, the other time of the year that seals of both species have to spend a lot of time out of the water is the annual moult. This occurs between May and June for spotted seals, but is slightly more variable for harbour seals, depending on the local climate. As with all true seals, the moult begins on the head, and slowly works its way down the body as the weeks pass. During the moult proper, as old hair is shed and the new begins to appear, the seals slow their metabolism, perhaps to conserve energy at a time when swimming for food might be troublesome.

But, while Linnaeus might have recognised only one species of seal off the coasts of Europe, these are now considered to represent multiple different species, and, next time, I'll begin by looking at the most widespread of those others.

[Photos by Charlesjsharp and jomilo75, from Wikimedia Commons.]


  1. Linnean names of seals:
    Phoca vitulina "sea-calf"
    Phoca ursina "sea-bear"
    Phoca leonina "sea-lion" (It's the elephant seal, probably confusing it with Patagonia sea-lion, Otaria)

    1. In French, "veau marin" is still a common name for the harbour seal today - although "focque commun" is at least as widely used, so far as I can tell.

    2. Jk Revell, Phoque commun is the correct spelling ;)