Sunday, 26 March 2017

Pinnipeds: Grey and Harp Seals

Grey seal
There are four different species of seal that live off the coasts of northern continental Europe. Perhaps the best known is the harbour seal, or "common seal", which is also the most widespread of all seal species, being found across both the North Atlantic and the North Pacific. When the scientific naming of species was introduced in 1758, it was the only such species recognised from the area, but it only took a few decades for proper scientific descriptions of the others to follow, recognising that they were distinct from their "common" cousin.

The last of the four to be split off was the animal we now know as the grey seal (Halichoerus grypus), in 1791. While that's still fairly early as such things go, the fact that it was the last of the local seals to be formally identified as something different from the regular sort is likely down to the fact that it does look very similar to the harbour seal. Although, in fairness, it was first named by an entomologist, so clearly you don't have to be a mammal specialist to tell them apart.

Grey seals are typically grey with dark blotches, but this is very similar to one of the patterns also seen in harbour seals, and, moreover, some grey seals are, in fact, silver, brown, or even near-black in colour. They are, however, much larger than harbour seals; the largest of male harbour seals reach about 190 cm (6 feet) and 140 kg (310 lbs), while a great many female grey seals are larger than that, and the males reach up to 230 cm (7' 6") and 310 kg (680 lbs). The extra weight of the fully-grown males compared with those of harbour seals is also due to much heavier and more muscular neck, shoulders, and chests, which, together with a longer, broader, nose, also makes it easier to tell the sexes apart even without knowing their size.

Grey seals are also found in broadly similar climes to harbour seals. They are the only other species of seal to live around the coasts of the British Isles, and, like harbour seals, are also found along the southern shores of the English Channel, around the North Sea, and along the coasts of Norway. They are, however, also found throughout the Baltic and its associated gulfs, travelling much further east and north than harbour seals do in that locality. However, while grey seals in general are widespread and common, those in the largely enclosed Baltic have fared poorly; the local population dropped by over 90% during the 20th century, due at first to hunting, and later to organochloride pollution.

On a more positive note, in 1980, grey seals colonised the Wadden Sea off the Netherlands, where they had been absent since the 16th century, and the new population there seems to be successful. Elsewhere, they live off Iceland, and a separate population (sometimes regarded as a distinct subspecies) lives off northeastern America, from Newfoundland to Connecticut, with the occasional lost traveller even reaching as far as the shores of New Jersey.

Like harbour seals, they prefer shallow, coastal seas near to rocky shores or sandy beaches. They spend much of their time foraging for food on the sea bottom, but, while they can stay underwater for up to half an hour if they really have to, most feeding dives last no more than ten minutes. Exactly what they're eating depends on where they are, and what time of year it is, but bottom-feeding fish are the main component of the diet. Off Nova Scotia, for example, over two thirds of their diet consists of sand lances, with the balance made up primarily of cod and flatfish.

Indeed, grey seals have been implicated in the slow recovery of cod stocks off Canada following their collapse in the 1980s, since the seals eat smaller fish before they have a chance to reach the size that fisheries are interested in. (That there are now apparently fewer sharks to eat the seals may not help). Surprisingly, they have also been reported to attack much larger prey on occasion, including porpoises and even young harbour seals, although these are clearly not major food items.

Grey seals, like harbour seals, are pretty antisocial outside of the breeding season, and they tend to stick to known patches of the coast, travelling short distances to places where the water is the right depth for them to feed, although they are seemingly capable of navigating over long stretches of open sea if they have to. Things change during the winter breeding season, when large numbers of females gather on offshore islands to give birth - often returning to the exact same spot year after year. They stay there, not bothering to feed, for up to three weeks while they raise their young and wait for the males to turn up.

While harbour seals are born already able to swim, and with the adult coat already grown, grey seals are more typical in that their newborn young have yellowish-white fluffy coats, and basically lie around doing very little until they are weaned. They take their first swim at about a month of age, by which time their baby fur has gone and the ability of their blood and muscles to store oxygen has drastically improved. It is commonly said that mothers invest more time and effort in raising male pups, which are larger and need to grow rapidly, but not all studies confirm this, so the reality may be more complicated.

Once the pups are out of the way, it's time for the males to show up and get down to the serious business of mating. Males do compete with one another violently, which explains the need for those muscular chests and shoulders, but, unusually for land-breeding seals, they don't seem to try and defend specific territories from rivals. Instead, their tactic seems to be to simply hang around as many females as they can in order to have sex as often as possible.

The apparent success of smaller males sneaking up on females when the larger ones aren't looking, and of pairs mating in the water where they can't be seen, implies that this tactic doesn't work as well as one might think. It has been suggested that this may be a hold over from an evolutionary past in which they mated on floating sea ice where territorial defence would have been more difficult. Even today, some grey seals do breed and give birth on ice anchored to the shore, rather than on actual rocks.

For grey seals, breeding on ice in this manner is unusual, if not exactly rare. But there are other species of seal for which it is very much the norm. To find them, though, we generally have to head further north (or very much further south). One such species is the harp seal (Pagophilus groenlandicus), for which the first half of the scientific name literally means "ice-lover".

Harp seal
Harp seals are about the same size as harbour seals, although unlike them, there is relatively little difference in size between the males and females. They have silvery fur, which, in adults, is marked by wide black bands merging over the shoulders, and said, from above, to form the shape of a harp. The alternative name "saddleback seal" refers, of course, to the same pattern.

Whatever name you call them by, these seals inhabit the subarctic waters of the North Atlantic and Arctic Oceans. They are typically found along the southern edge of the pack ice, migrating as it moves over the course of a year. While wanderers are sometimes found further south, in Europe this means that, for the most part, they are seen only near Norway, Iceland, and the islands of Svalbard. To the east they are found off western Siberia and the islands of the Russian Arctic, and to the west off Greenland, and North American coasts from Maine to Hudson Bay.

Even when they are not following the ice, harp seals are out in the open sea. They could perfectly well survive without ever visiting dry land at all, and some of them probably do. They can travel over thousands of miles in a 12-month period, often following the migration of their favoured food of small fish, such as capelin. Younger seals, in particular, also eat a large quantity of small crustaceans, such as shrimp, while larger adults supplement their diet with occasional herrings, or even cod.

Harp seals frequently dive to depths of 200 metres (650 feet), and can reach twice that if pushed. As with other seals, they have high levels of haemoglobin and key enzymes in their blood, along with oxygen stores in their muscles, allowing them to hold their breath for over a quarter of an hour at a time, and slow their heart rate down to almost half of its normal level as they do so. The structure of their retinas suggests that they can see more acutely underwater than they can in air, and, while they cannot see colour, they have excellent night vision, and are particularly sensitive to what we would perceive as green light. Similarly, their hearing seems to be better underwater, and they make more calls to one another while diving than they do while on the surface.

During the February to March birthing season, expectant mothers haul themselves out onto ice floes in large numbers. The resulting pups have pure white coats that have historically made them key targets for human hunters. With the more recent decline in the hunting of baby seals, populations have recovered significantly, although there is reason to suppose that that may change again as the amount of Arctic ice begins to shrink.

Young harp seals are only fed by their mother for the first nine to twelve days of their life, being immediately abandoned thereafter. During this time, they can put on over 2 kg (4 lbs 6 oz.) body weight per day... after which they rapidly slim down, since they can't actually swim and feed themselves until they're around seven weeks old. While that's going on, the females are busy selecting their mates. The males are less violent than those of grey seals, but otherwise rely on broadly the same tactics of mating with as many females as possible instead of bothering to defend particular territories. Their genitals grow more rapidly than the rest of their bodies after they pass through puberty, making penis size a good indicator of overall physical fitness. Having said that, they vary enough in length that it's likely that different females have different preferences, presumably reflecting varying mating strategies on the part of the males (that is, guarding partners versus sneaking about).

Of course, harp seals are by no means the only seals to inhabit the ice floes of the Arctic, and next time I'll turn to some more of those species.

[Photos by Andreas Trepte and the Virginia State Parks staff, from Wikimedia Commons. Cladogram adapted from Higdon et al. 2007 and Fulton and Strobeck, 2009.]

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