Sunday 10 September 2017

Pinnipeds: Ross and Weddell Seals

Weddell seal
The majority of 'true' seal species live in the Northern Hemisphere, with just five found south of the equator. While they were once more widespread, today, all five of these live in the extreme south, with the most widespread - the southern elephant seal - reaching Tasmania, southern New Zealand, and Patagonia. The remaining four, which are all relatively closely related to one another, do not range so far north and are exclusively Antarctic and sub-Antarctic.

The Weddell seal (Leptonychotes weddellii) is a typical example. With fully-gorwn males up to 3 metres (10 feet) in length, and weighing about half a ton, they are noticeably smaller than elephant seals, but still pretty large by the standards of seals in the north. They were first discovered during the expeditions of navigator James Weddell, who, in the 1820s, sailed further south than anyone had previously travelled, into the sea that now also bears his name. Since the seals are named for him, rather than for the body of water where they were first found, it's perhaps unsurprising that they are not unique to that sea, and are equally common right round the frozen continent. During the winter, they can travel as far north as South Georgia and other islands of the extreme South Atlantic, but they don't normally reach (for example) the Cape Horn at the southern tip of South America.

So they certainly like the cold. Indeed, they are never found far from the ice, whether it be free-floating pack ice at sea or sheets of "fast ice" anchored to the land at one edge. They feed primarily on fish, but also eat some squid, octopus, and krill. Able to feed both in the mid-water column and on shallow sea-floors, the exact details of their diet seem to vary significantly from year to year. Most sources cite Antarctic cod as a particular favourite, as a result of studies of their stomach contents conducted in the 1980s. However, this seems to be largely a component of their summer diet, and at different times of the year, they may switch to icefish or humped rockcod, among others. There may even be some variation from year to year, presumably depending on temporary changes in fish abundance.

Diving Weddell seals have been clocked at around 3 mph (4.7 km/h), but, while they are said to be able to dive down to a kilometre (3,300 feet) and for well over an hour, if they have to, most dives are short and relatively shallow. Deeper dives seem to be a response to weather conditions that concentrate the tastiest fish close to the seafloor, and are clearly something that they plan in advance - rather tan just continuing to dive until they find something. It takes several years for them to develop their full diving potential, although doing so apparently takes enough of a toll on them that older seals do show age-related muscle wasting even in the wild.

Studies on the microstructure of Weddell seal's eyes show them to be adapted to the extreme low-light conditions of deep water, at the expense of both colour vision and visual acuity. While you might think that regular diving would at least mean that the seals would not be troubled by air-breathing parasites in their fur, this isn't so, since they can be afflicted by lice that can drag down tiny pockets of air with them using scales on their body surface, and that can survive the pressures of the deep sea.

Most Weddell seals never stray far from the Antarctic coast, but even those that do travel to more northerly islands in winter head south in the spring, where females haul themselves out onto the ice floes to give birth. Perhaps because there's relatively little that might want to eat them there, newborn Weddell seal pups are not so white as those of ice-breeding seals in the Northern Hemisphere, although they are a sort of pale creamy colour.

They are weaned by six or seven weeks, which is relatively late for seals. Even so, Weddell seal milk is highly nutritious, with about 5,000 Calories per kg, and the mother makes so much of it that she has lost over half of her weight by the time she leaves her pup, causing her to dive deeper than usual to find enough food to bulk herself back up again. Remarkably, given the effort that this entails even under normal circumstances, some mothers have been seen nursing two pups simultaneously. This is usually due to fostering of unrelated pups, but, in rare instances, Weddell seals can actually give birth to twins - the only seal species known to do so.

As in many other seals, pups can identify their mothers by the unique sound of their voices, and, at least by two weeks of age, may have sufficiently distinct voices themselves for the recognition to work both ways. Indeed, adults of both sexes are unusually vocal for seals, and use a wide range of different calls, often underwater. Populations hundreds of miles apart have distinctive "accents", presumably developed over long periods of separation, although such differences are much less between colonies close enough to interbreed regularly, since most seals return to the same sites year after year.

Pups leave their mothers shortly after weaning, and swim out to sea, initially sticking close to the Antarctic coastline rather than braving the open waters. Females are ready to try for pups of their own by about six years of age.

Unlike the females, the males don't bother to haul out much onto the ice in the breeding season (although they have to do so for the moult, later in the year), instead relying on regular visits to breathing holes. While there is some evidence that at least some of them do maintain territories during the breeding season, they don't gather obvious harems of females, which remain relatively scattered and anti-social at this time. That they can do this is perhaps largely due to the fact that mating takes place entirely underwater, so there's no need to sit about on land being protective; just wait until a female in heat gets hungry enough to dive. This in turn means that most males do get to father at least one pup in a given year, rather than a small number of dominant bulls monopolising all the females.

Ross seal
Weddell seals are very common across much of Antarctica, and some estimates suggest that there may be as many as a million of them worldwide. The least common of the Antarctic seals, however, is the Ross seal (Ommatophoca rossii), and, as a result, it is probably also the least studied. It's the smallest of the Antarctic seals, although still larger than most of those found in the Northern Hemisphere, and is a fairly bland dark brownish-grey in colour, with a particularly distinct pale underbelly. While they do occasionally become lost and turn up further north, for the most part they don't even reach the sub-Antarctic islands, sending the winter far out at sea, and the summer close to the Antarctic coast.

Anatomically, they have unusually large eyes and small teeth even less adapted for slicing meat than those of most other seals. These are likely both related to their feeding habits, which tend to focus on nocturnal dives for squid, although they do eat a reasonable amount of fish, too. They don't appear to be bottom-feeders, typically making moderate dives to no more than 200 metres (650 feet) for up to ten minutes at a time, although a maximum depth of 792 metres (2,600 feet) has been recorded. On the other hand, when they do dive, it's rarely to less than 100 metres, presumably because they don't feed on surface-dwellingfish.

Since they spend so much of the year far from land, it's hard to know how common they are, and it could be that most estimates of their population are on the low side, simply because they're so hard to find. On the positive side, it could also be that they rely less on ice than other Antarctic seals, and will be less affected by climate change.

Having said which, they certainly do haul out on ice to breed (it's just not obvious whether or not they absolutely have to), and the little we know of their reproduction indicates that they give birth in October and November, and mate over the two months following. Like Weddell seals they probably mate underwater, but weaning is quicker, taking only around four weeks. They seem to be fully grown at around nine years, but can presumably breed long before they reach their maximum size.

By focussing on night-time squid at moderate depths, Ross seals likely avoid too much competition with the deeper diving, more diurnally active and fish-eating, Weddell seals. The remaining two species of Antarctic seal, however, have even more distinctive diets, and it is with those that I will conclude my survey of the world's seals.

[Photos by Christopher Michel and by Mike Cameron of NOAA, from Wikimedia Commons. Cladogram adapted from Higdon et al. 2007 and Fulton and Strobeck, 2009.]

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