Sunday 23 April 2017

Pinnipeds: Ribbon and Ringed Seals

Ribbon seal (male)
Although there is, perhaps, more variety amongst seals than one might at first expect, when it comes to colouration, there isn't all that much to distinguish the different species. The majority are brown or grey, often with black or pale splotches of some kind. I've already described one exception, the harp seal, but arguably the most distinctive of all seal coat patterns belongs to its close relative, the ribbon seal (Histriophoca fasciata).

Male ribbon seals are black, or very dark brown, with clear, wide bands of pure white fur around their necks, shoulders, and just above the hips. Females are medium-brown with light tan stripes, so the pattern is less striking, but it's still present, and in the same shape. Neither sex is born like this; even once baby ribbon seals shed their pure white fur at around a month of age, they are initially plain in colour, only fully developing the stripes by the time they are two years old.

Ribbon seals are native to the far northern Pacific, and most of them spend the winter in either the Bering Sea between Siberia and Alaska or the Sea of Okhotsk between Siberia and Japan. During this time of the year, they tend to restrict themselves to the edges of the pack ice, avoiding both dry land and the more solid ice further north. However, they can range much further afield, especially once the ice begins to retreat during the summer, when they swim many miles out into the open ocean, reaching far beyond the Aleutian Islands in the south and up into the Chukchi and East Siberian Seas on the northern side of the Bering Straits. But even then, when there is little floating ice for them to use, they tend to stay away from land, and they aren't, for example, found east of the Alaskan Peninsula.

Perhaps because of this remote location, compared with some other seals, ribbon seals have not been extensively studied. (For example, one their main breeding sites lies in territory disputed by Japan and Russia, making flying over it politically awkward). On the other hand, this area is also less affected by climate change than some other parts of the Arctic, and the population estimates we do have suggest that there has been no significant decline in their quite considerable numbers over the last few decades.

Cod of various kinds seem to form the bulk of their diet, but this includes both bottom-dwelling species and those more typically found near the surface. Given their propensity for travelling far from land during the summer and autumn, it is unsurprising that their diet changes over the course of the year, and surface-dwelling prey would likely be more important at such times. Indeed, like most seals, they'll eat more or less what they can find, and both squid and shrimp are known to supplement their fish-based diet.

Ribbon seals can dive to at least 500 metres (1,600 feet) when foraging for food, deeper than is typical for other seals living in the same waters, although by no means record-breaking for seals across the world. They give birth in March and April, on the edge of the ice sheets. Males spend a lot of time grunting and roaring below the water at this time of year, and presumably this has something to do with either attracting mates or deterring rivals, but we know little else about how they breed. It is likely, however, that females are sexually promiscuous, since they don't seem to gather in sufficiently dense colonies for males to establish the harems seen in some other seal species.

While ribbon seals spend the winter at the margins of free-floating pack ice, other seals living in the same general area have different preferences. Harbour seals are more commonly found along rocky shores, while bearded seals are found further north, where the ice is thicker. Ringed seals (Pusa hispida), however, prefer fast-ice, which is to say ice attached to the land.

Ringed seal
Ringed seals are among the smaller species of seal, weighing only around 50 to 90 kg (110 to 200 lbs), a little over half the weight of a typical harbour seal. They have dark backs and silver underbellies, with fully-grown adults sporting irregularly-shaped pale rings scattered across the upper parts of their bodies. Like ribbon seals, they are found in the Bering Sea and Sea of Okhotsk, although typically much closer to the land, and not reaching the more remote Aleutian and Kuril Islands. However, they are also found across the Arctic, reaching the coasts of Labrador, Greenland, and Lapland, as well as across northern Canada and Russia.

Occasional wandering individuals have been sighted as far south as California and Portugal, but a more permanent population, usually described as a distinct subspecies from the Arctic and Pacific sorts, lives in the Baltic. Although one would think that this population would be isolated from those further north, since ringed seals are not commonly seen along all but the most northerly parts of the Norwegian coast, genetic studies have shown that they must interbreed far more often than one would suppose - to the point that their status as a separate subspecies has been questioned.

During the spring and winter, ringed seals feed on cod and herring, but during the summer, as the ice melts and they find themselves forced out into the open water, they switch to small crustaceans as these become more widely available. Although they can dive as deep as 300 metres (1,000 feet), in practice, they feed near the surface, rarely going deeper than 100 metres (330 feet), and usually not even half that.

While all true seals have claws on their fore-flippers, which can act as pitons while pulling themselves across ice, those on ringed seals are larger than usual because they also need them for digging. Specifically, after they haul themselves out in March or April, pregnant seals dig artificial caves in thick icy hummocks or piles of snow. They give birth within these lairs, which help protect them from land-based predators that might wander out onto the ice, as well as providing some shelter from harsh winds.

While this is going on, males begin to establish territories near to the birthing lairs, and protecting breathing holes in the ice (the digging claws also help here, too, of course). They can be quite aggressive while this is going on, and, while we still don't have detailed studies of their mating behaviour, this, taken together with population studies and the relative size of their genitals suggests that, unlike ribbon or harp seals, older and more experienced males dominate and protect a small number of nearby females.

At some point towards the end of the Last Ice Age, perhaps around 11,000 years ago, a section of the eastern end of the Baltic Sea became cut off from the main body of water. A number of ringed seals were trapped behind this new land barrier, and they still live there today, in what is now the freshwater Lake Ladoga, near St. Petersburg. At just over two-thirds the size of Lake Erie, this is the largest lake in Europe, and it is able to support a population of around 5,000 freshwater seals.

Slightly smaller, and with narrower heads, than the ringed seals elsewhere, these animals have been separated from their kin for long enough that they now form a distinct subspecies. During the late spring moult, when ringed seals are normally hauling themselves out onto northerly ice floes before they break up, those in Lake Ladoga are gathering in groups of up to fifty along the frozen shoreline - which is as sociable as ringed seals ever get.

Still more recently, however, around 3,000 BC, a new river broke through to Lake Ladoga from the north. This was the Vuoksi River, and some of the Ladoga ringed seals swam up it, eventually reaching its source at Lake Saimaa in what is now Finland. This is only around a quarter the size of Lake Ladoga, yet the isolated population here also survived long enough to form its own subspecies, slightly larger, and with shorter snouts, than the Lake Ladoga sort.

Living, like the Lake Ladoga seals, entirely in freshwater, Lake Saimaa seals feed on smelt, perch, and ruffe. While there are thought to have been several thousand of them at one time, today no more than around 350 individuals survive, with less than half of those breeding adults. In the past, pollution from mercury and organohalides was the major threat, but this has largely been cleaned up in recent years. Indeed, while the population has begun to recover a little lately, the subspecies is still considered "endangered", threatened by accidental capture in fishing nets as well as by use of the lake for cross-country skiing and the like.

While their genetic diversity is higher than one might expect, given their tiny population, it is still barely on the edge of what is viable, not least because individual groups stick to the same spots on the lake shore year after year, limiting interbreeding even among those that remain. Since, like all ringed seals, they rely on those artificial ice caves to rear their young, a warming world is also a particular threat for a subspecies that doesn't have the option of heading further north.

Fresh water is not really the sort of habitat in which one expects to find seals. But, as it turns out, the Lake Ladoga seals and their Lake Saimaa relatives are not the only examples. Even further back in time, close relatives of the modern ringed seal found themselves trapped even further from the sea... and it is to the descendants of those that I will turn next.

[Photos by Labunski Liz, U.S. Fish and Wildlife Service, and Lee Cooper of the National Science Foundation, in the public domain. Cladogram adapted from Higdon et al. 2007 and Fulton and Strobeck, 2009.]

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