Sunday, 29 January 2017

Pinnipeds: The Difference Between Seals and Sea Lions

Otariid (above) and Phocid (below)
Mammals, being air-breathing animals, mostly live on land. Even when they hunt or graze for food in the water, they generally return to dry ground to do things like sleeping. Nonetheless, an aquatic lifestyle has evolved several times amongst mammals, and four of those lineages survive today. Perhaps the best adapted, because their young are born able to swim, are the cetaceans (whales, dolphins, etc.) and the sirenians (such as manatees). The other two are sea otters and pinnipeds.

There are three recognised families of pinniped alive today: seals, sea lions, and another that includes only the walrus. All are distinguished by having flippers instead of feet, and a lifestyle that requires them to climb out of the water in order to breed and raise their young.

Leaving the walrus aside, the names I've just given to the other two families (and thus the title of the post) are really a bit misleading. This is because a group of animals called "fur seals" actually belong to what I'm calling the "sea lion family" (technically the Otariidae), and in casual usage, the term "seal" is often extended to pinnipeds in general. As a result, if we really want to be accurate, we need some term to distinguish members of the other family from seals more generally. The technical term for these animals is "phocids", but other commonly used terms include "true seals" and "earless seals".

But, for the purposes of this post, I'm just going to call them seals, and refer to otariids in general as "sea lions", even though they actually include fur seals as well as the genuine sea lions.

So, using that definition, what is a seal, and how is it different from a sea lion?

Clearly, there are a lot of similarities between the two. The word "pinniped" literally means "fin-foot", and the animals do indeed have flippers instead of regular walking feet. They are also generally streamlined, spend most of their lives at sea, and have a layer of insulating blubber beneath their skin. They have large eyes, very short tails, and sensitive whiskers. They are all carnivorous, and males are usually (though not always) larger than females. There are also, as one might expect, a number of adaptations to the lungs to allow them to make long, deep dives. In addition to their blood being able to hold more oxygen, due to larger red blood cells, the lungs can almost entirely collapse when they hold their breath, thus protecting from getting the bends when they surface after swimming at depth.

But equally, there are some obvious physical differences that you don't need to be an expert to spot. Relatively speaking, seals tend to have shorter necks and longer torsos than sea lions, and their fur is short over their entire bodies, with no sign of the manes found in male sea lions. The fact that one of the common names for phocids is "earless seals" is indicative of another clear difference: true seals do not have external ears. They do, of course, have ears, and ones particularly adapted to hearing underwater, but where sea lions have mobile ear flaps, just like dogs, deer, and most other mammals, in true seals the ear canal just opens directly onto the surface of the skin, leaving that part of the head entirely smooth and streamlined.

Perhaps the clearest physical difference, however, is in their limbs. All living pinnipeds have absurdly short limbs. The main limb bones are concealed inside the body, so that they have no visible elbows or knees; all that you can see are the ankles/wrists and the feet. Sea lions are capable of moving about on all fours, and can be quite agile on land, clambering across uneven rocky surfaces with alacrity. In particular, their hind flippers stretch forwards and out to the side, helping to support their body weight.

Their fore-flippers are also much longer than those in seals; in the water, it is these flippers that provide almost all the propulsion, while the hind-flippers are used for steering. The flexibility of those rear flippers allows them to be agile underwater, too; sea lions have a tighter turning circle than seals, or for that matter, dolphins.

In seals, however, the hind-flippers project out behind the animal, with their ankles, limb bones, and associated tendons arranged in such a way that it is quite impossible for them to rotate the flippers forward. On land, this is a real problem, forcing the animal to haul itself forward with its fore-flippers and muscular undulations of its body. The hind-flippers might as well not be there, for all the good they do.

Get them underwater, of course, and it's a different matter. Swimming now works in almost the opposite way to how it does in sea lions; propulsion is provided by the hind limbs, while the fore-flippers are used for steering. This has the advantage of increasing stability and speed at the expense of manoeuvrability, and has been described as resembling (somewhat loosely) the way that tuna swim.

The flippers themselves, of course, are formed by elongated toes connected by webbing that is often quite fleshy. Even here though, there is a difference between the two types of animal, although you may have to get fairly close to notice it. Seals have claws on all of those toes, with the claws of the fore-flippers being relatively large, and those on the hind flippers smaller and more slender. Sea lions however, usually have no claws at all on the front feet, and only have claws on the middle three toes of the hind feet. Similarly, the fore-flippers of seals are furry, while those of sea lions are hairless.

Both seals and sea lions need to be streamlined in order for their swimming to be efficient. In addition to their general body form and (in the case of seals) relatively short and inflexible necks, this also requires that the males not have, well... dangly bits that would drag in the water. In both types of animal, the penis retracts into the body when it isn't needed, but the solution isn't quite so simple for the testicles.

This is because male mammals have a scrotum for a reason: for reasons that aren't entirely clear, in order to mature, sperm needs to be somewhat cooler than a mammal's regular body temperature. Which means that the testes have to be somewhat outside the main bulk of the body, something that isn't really an option for pinnipeds. Sea lions achieve this by only developing a scrotum during the breeding season, when they spend a lot of time on land anyway; for the rest of the year, the testes are safely tucked away in the abdomen.

Seals, however, perhaps reflecting their ever-so-slightly-better-adapted-to-water features than sea lions, do not have a scrotum at all. Instead, their testes are stored just beneath the skin, outside of the main body cavity, and not insulated by much in the way of blubber. Blood vessels carrying slightly cooler blood from the hind-flippers also pass close to the testes, and may provide an additional cooling effect.

The ancestral pinniped was a carnivorous animal, related to land-dwelling animals such as bears, raccoons, and weasels, and, more distantly, to dogs, cats, and hyenas. It is for this reason that pinnipeds are no longer considered to be a taxonomic order of their own, as they oncer were. They are instead members of the wider order Carnivora, which includes all those other animals I have just mentioned (among others). One of the key distinguishing features of carnivorans is the presence of flesh-shearing carnassial teeth, but pinnipeds, with their diet of fish, shellfish, and the like, are unusual in having completely lost this particular feature.

Compared with, say, dogs, pinnipeds in general have a reduced number of teeth. Sea lions have 26 to 30 teeth, and seals rather less - allowing them to have a shorter, rounder, snout. Both seals and sea lions have large, sharp, canine teeth, which are often larger in males than in females. Moreover, in sea lions, the pair of upper incisor teeth immediately in front of the canines are often enlarged and sharpened, to the point that they look almost like canines themselves.

Seals are much more variable in both the number and shape of their teeth, making generalisations difficult. In both groups, however, there is a tendency for it to be difficult or impossible to distinguish the premolars from the molars, and some seals don't have molars at all. This is likely due to their diet, which is even more purely carnivorous than that of most land-based carnivorans, given the general lack of vegetation in the open sea. It's notable, for instance, that cats, which are also highly carnivorous, similarly have a reduced number of teeth - although the details are different, since so are the specific sorts of animals they are feeding on.

So much for the differences between seals and sea lions. But there are several species of each, and a fair degree of variety among those species. There are, in short, plenty of differences between seals and other seals, never mind anything else. So, over the next few months, I'm going to try and look at the individual seal species, and how they vary across the world.

[Photos by "Avenue" and "Steenbergs", from Wikimedia Commons.]

1 comment:

  1. I have also seen/heard Polar Bears listed as "Aquatic Mammals" - This may depend on your view of whether Sea Ice counts as "land" or not in determining the portion of their life being spent "on land"

    The one we saw North of Svalbard was certainly a long way from land...