Sunday, 18 June 2017

Pinnipeds: Hooded and Bearded Seals

Hooded seal pup
Most of the species of seal found in the waters of the North Atlantic are roughly the size of harbour seals, with full-grown males measuring about 160 cm (5' 3") in length, and weighing around 120 kg (265 lbs); females, of course, are somewhat smaller. Of the three species that are significantly larger than this, the biggest of all are the hooded seals (Cystophora cristata). While not a patch on the largest seals of the Pacific, with males up to 270 cm (8'9") and weighting around 300 kg (660 lbs), they're still pretty hefty.

Hooded seals live relatively far north in the Atlantic, spending most of their time in cold waters northward from Nova Scotia to the coasts of Greenland and Iceland, and some of the remote islands that lie to the north of Europe. They are known to have four, relatively small geographic areas in which they do their breeding - the Gulf of Saint Lawrence, off the north coast of Newfoundland, the Davis Strait between Greenland and Baffin Island, and around Jan Mayen island east of Greenland. Although these appear to be quite distinct, there is no evidence of any significant genetic difference between the populations, and hence, no recognised subspecies of the animal.

This may be in part because the seals do travel quite considerable distances out to sea during the non-breeding season, regularly being away form home for over a month at a time. While most do seem to return to the place of their birth, there is surely plenty of opportunity for intermixing. In fact, there have been occasions when hooded seals have been spotted much further south than we'd normally expect; they can reach Florida, Portugal, and the Beaufort Sea north of Alaska without too much difficulty. In 1990, one female was apparently spotted in, of all places, San Diego... which would suggest that she was, really, really lost.

Unlike other local seals, a large portion of their diet consists not of fish, but of squid, although, especially in the summer months, they also favour cod, redfish, and halibut. While cod is quite a common food item for seals, being large, hooded seals tend to go for older, and larger, cod than their neighbours do, which, particularly following the collapse of Canadian cod stocks, does tend to bring them into conflict with human fishermen, to the seals' general detriment.

Cod aside, squid, redfish, and halibut all have one thing in common that isn't shared by more typical seal foodstuffs such as sandlance - they're all deep water creatures. Most northern seals are relatively shallow-water animals, commonly feeding at around 300 metres (1,000 feet) depth, although at least some species can dive much deeper than that on occasion. While hooded seals can, and do, feed at such depths, they regularly dive to depths of up to 600 metres (2,000 feet). In the open ocean, where they can spend as much as 90% of their time below the waves, sometimes holding their breath for over 52 minutes at a time, they have been recorded feeding at depths of over a kilometre (3,300 feet).

This (as we'll see in later posts) isn't quite a record for seals, but it has to be said that it's still a very long way down. While hooded seals have about the same number of red blood cells as you'd expect for their blood volume, those cells are both much larger, and more densely packed with haemoglobin than those of more typical animals, significantly increasing their ability to hold oxygen. Even so, during their longest dives, oxygen levels in the blood can fall so low that most other animals would simply drop dead from asphyxiation; while other factors also contribute to this, it's noteworthy that the brain cells of hooded seals are (for unknown reasons) unusually good at functioning almost entirely without oxygen, managing it for over an hour, rather than the five minutes or so that we'd normally expect.

But their remarkable diving ability is not the feature that hooded seals are most famous for. That, of course, is their nose.

The nose of male hooded seals is large and bulbous, and they can inflate it make it appear even larger. Not only that, but the barrier down the middle of the nose, between the left and right nasal cavities is thin and flexible, allowing the seal to blow air into it from one side and pop it out of the other nostril like a large red balloon. Shaking this bladder about and making appropriate noises is apparently quite intimidating to other males, although they can resort to violence on those occasions where one of them fails to back down.

The purpose of this is, as so often, to compete for mates. Once they've done this, however, the male hangs around a single female, not forming the harems more typical of some other seals. This may make it look as if they're monogamous, but the fact is, as soon as they've had their way with the female, the male immediately heads off to look for another one. That they pack close together on the ice means that he won't have to go far to do so, and despite the relatively short March to April breeding season, a successful male will typically have several partners - and all without having to exhaust himself driving off too many rivals. The females, on the other hand, are reasonably faithful, only mating with one partner, so that the males don't need to develop the unusually large testicles found in more promiscuous species.

As is typical for seals, pregnancy lasts almost exactly a year, so that the mother only needs to haul itself out once to give birth, and then mate again shortly thereafter. However, even by the standards of seals, the gap between birth and mating is remarkably short. This is because the young pup is fully weaned and essentially independent from its mother by the age of just four days. This is, so far as we know, the absolute shortest suckling time of any mammal species anywhere.

During those four days, the pup practically doubles in size, putting on something like 6 to 7 kg (about 14 lbs) of weight every day. The total weight gain by the end of the period is actually about the same as in other similarly sized seals... it just that the period in question is typically at least three times longer than that, and often much more. To achieve this feat, they drink something like 10 litres (21 US pints) of milk a day, and the female, unsurprisingly, loses an awful lot of weight herself in the process. The milk itself is also astonishingly nutritious, again probably holding the record for any mammal species, with 61% fat content and about 6 Calories in every gram.

Unfortunately for the pups, four days is not long enough for them to fully develop the hunting skills that they need to feed themselves once their mother had gone off to mate, and they do initially lose quite a lot of weight once she does. However, unlike most seals, they shed their fluffy white coat while still in the womb, being born with waterproof fur that's dark grey above and creamy coloured beneath, and also with a layer of insulating blubber that most newborn seals lack. So they're into the water pretty quickly, diving to 100 metres (330 feet) depth by the time they're just three weeks old. It takes a while for their young bodies to fully develop the remarkable breath-holding abilities of the adults, but they're pretty much there by their first autumn.

Bearded seal
The hooded seal was first formally named and described by Johann Erxleben in his 1777 book Systema Regni Animali. The same book also named two other seal species for the first time. One was the harp seal, and the other the bearded seal (Erignathus barbatus). Named for its thick and heavy whiskers, this is typically a plain brown in colour, rather than the grey-with-black-blotches of the hooded seal, but it, too, is one of the larger seals of the area.

Like hooded seals, the bearded sort do sometimes wander far afield, but for the most part they inhabit waters even further to the north. In the Atlantic, they reach as far south as Labrador and the north coast of Iceland, but they are also found throughout the margins of the Arctic Ocean, and in the North Pacific as far south as the northern tip of Japan. They often follow the margins of the pack ice as they shift throughout the year, preferring isolated ice floes to more stable sheets or anything attached to the land.

Despite this, bearded seals tend to live relatively close to the shore, rather than out in the deep ocean. This is likely related to their feeding habits. While they are capable of diving almost as deeply as other, typical, seal species, they rarely do so, mostly feeding at depths of less than 100 metres (330 feet), and staying submerged for only ten minutes or so at a time. Here, they consume cod, sculpins, and other small to medium fish, along with a high proportion of crabs. It's likely that they are opportunistic in what they catch, since it seems to vary by locality, with food items such as clams and whelks being common in some areas, but only rarely eaten in others.

While, like any seal, they can deliver quite a nasty bite if they want to, their underwater feeding is primarily done by simply pursing their lips and sucking food into their mouths. As to finding it in the first place, this may partly be the function of that "beard" of heavy whiskers. These have multiple fine nerve endings that suggest that, even compared with the whiskers of other animals, they are unusually touch sensitive, something that may help when searching the sea bottom for food.

Relatively little is known of mating habits of bearded seals. There is some evidence that males follow one of two contrasting tactics: they either patrol a small area, controlling access to the females there, or roam about over a wider range, presumably hoping to sneak past their more sedentary counterparts. Males that use either of the tactics can be distinguished by the different underwater calls that they make, although its unclear whether these calls are to intimidate rivals or attract females, or if that varies between tactics. Either way, a given seal will usually stick with one particular approach that works for them, even though they could change if they wanted to. It is also, perhaps, worth noting that the detailed features of the calls that they make vary across their range, and it's generally agreed that there are distinct subspecies at the Pacific and Atlantic ends of their distribution.

Like those of hooded seals, the pups of bearded seals are born already having shed most of their fluffy fur, although they also have a colour not that different from the adults. While this does make them more obvious against the ice, it means that they can dive from an early age, and, indeed, they will do so only a few hours after birth, if it's the best way to escape a polar bear. Suckling is nothing like the mad dash seen in hooded seals; weaning takes two to three weeks, and the corresponding weight gain is much slower. Having said which, it's still quite fast by the standards of similarly sized land mammals, and it has been calculated that baby bearded seals have a dietary intake of 154 MJ  (37,000 Calories) a day - about fifteen times what an adult human needs. Oddly, considering that they still only have one pup at a time, female bearded seals are one of only three living species of the animals to have four teats, rather than the usual two.

That the pups can dive almost from birth is surprising in view of that they have a foramen ovale, or "hole in the heart" until about two weeks of age, which really ought to prevent that sort of activity. It's likely that they have anatomical features in the lungs and elsewhere that prevent this from being the case, and also that they are only diving as a short-terms means of escape, not for actual feeding. In general, their diving activity remains inferior to that of adults for some months, but considering that females don't become sexually mature for five years, and males for six, there's obviously quite a long "adolescence".

This brings me to the end of the seals of the Arctic. But, while most seals prefer cold waters, even if, like the harbour seal, those waters don't necessarily have to be frozen, there are a few species that actually prefer warm, and even tropical, climes. It is to those that I will turn next...

[Photos by NOAA Fisheries, in the public domain, and Michael Haferkamp, from Wikimedia Commons. Cladogram adapted from Higdon et al. 2007 and Fulton and Strobeck, 2009.]

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