Pinnipeds

The thirty-three (33) species of Pinniped can be divided into three (3) distinct families. The first group is the family Phocidae, consisting of eighteen (18) species referred to as true seals or 'earless seals' due to their lack of external ear flaps or pinnae. The second group is the family Odobenidae that is represented by one species, the walrus. The walrus is easily distinguished by its tusks and great size. Like the 'true seals' the walrus has no external ear flaps. The third group is the family Otariidae, including the fourteen (14) species of fur seals and sea lions. Fur seals and sea lions are recognisable by their external ear flaps (pinnae). The fourteen species consists of nine species of fur seals and five species of sea lions.

As with whales and dolphins, seals were once land-dwelling mammals that took to the sea approximately twenty million years ago. The seals did not evolve as fully aquatic mammals however, and must return to land (or ice) to bear and rear their pups; some species spend much of their time on land between feeding trips. Despite this fact, seals have undergone many changes or adaptations to become highly efficient and elegant swimmers.

Water is a very different medium to air, and is harder to move about in than air. Water is also an excellent conductor of heat which is why a land-based mammal, such as a human, can not spend extended periods in the ocean without losing a lot of body heat. Lastly, water is a great conductor of sound, which allows communication over long distances; particularly useful as seals have good hearing. So from this information it is clear that changes or adaptations were required for the seals' land-dwelling ancestors to become efficient in their new aquatic medium. Changes occurred to the body shape, method of locomotion, method of maintaining body temperature and the senses such as eyesight, hearing, vocalising underwater and their whiskers.

All seals have a sleek, elongated body shape rather like a torpedo. Their streamlined body shape evolved to create as little resistance as possible in water - a very dense element. The external appendages of seals have been reduced from those of their land ancestors. The head merges with the rest of the body without an obvious neck, in keeping with their sleek body shape. The fur, when wet, is very smooth to allow water to glide over it easily. The thick layer of blubber (fatty tissue) just beneath the skin aids streamlining by smoothing out the contours of the body and any structures that may protrude such as the skeleton.

In water seals become very buoyant. This effectively makes them weightless; suspended in their water environment. Therefore they do not require the supporting limbs that keep a land mammal's body clear of the ground. Seals modified their limbs by shortening the arm and leg bones now hidden within the body, however these are still very strong.

The bones of the hands and feet have elongated and are bound together in a web-like structure of skin to form flippers. These paddle-like blades are streamlined like the wings of an aircraft and are perfect for propulsion.

The Otariid's (Fur Seals/Sea Lions) fore flippers are their main source of power when swimming. Their fore flippers are longer than their relations, the Phocids (true seals), with cartilage extending past the digits (finger bones) for extra length and strength. Fur Seals and Sea Lions sweep through the water with powerful down strokes of the fore flippers that rotate at the wrist joints allowing the streamlined flipper to cut through the water on the return stroke. The Fur Seals and Sea Lion's hind flippers are also elongated, with cartilage extending past the digits. Their toes are joined with a web of skin and strong nails are found on the inner three toes of the hind flippers, which are ideal for grooming. The hind flippers are used for steering (and sometimes for paddling in confined quarters), and when not in use are left to trail behind the body completing the streamlined shape.

On land, Fur Seals and Sea Lions are the most agile of all seals. They are able to turn their hind flippers forward for walking. For slow walking the fore flippers are moved alternately with the hind flippers being brought up together (almost to the fore flippers) and the belly being held clear of the ground. For faster movement on land, Fur Seals and Sea Lions can break into a gallop by moving the fore flippers together, arching the back and bringing the hind flippers up more quickly in a series of bounds. The roughened skin on their flippers provides grip on rocks or ice. If approaching a seal on the beach, it is important to note that if threatened the seal may become aggressive, and some species can out-run a human.

Locomotion in True Seals

The Phocid's (True Seal's) main source of power when swimming comes from its rear flippers and hind quarters. When swimming a true seal presses its fore flippers against its body, where there is a slight indentation in the blubber for the flippers, to complete the streamlined form. In Phocids, the fore flippers are generally smaller than Otariids (Fur Seals & Sea Lions), being short and blunt in comparison, with strong claws on each digit. The Antarctic phocids' fore flippers are longer and more pointed than the northern phocids' with less developed claws. True Seals propel themselves by alternate strokes of the hind flippers, which are spread to take advantage of the web on the power stroke and relaxed on the return stroke. In addition to their hind flippers, True Seals use side to side movements of their hind quarters, rather like that of a fish, to propel themselves. When moving slowly True Seals may swim differently, holding the fore flippers out from the body for stability.

True Seals have far less mobility on land than Fur Seals and Sea Lions. They crawl on their bellies in a caterpillar-like fashion, with the seal taking its weight on its chest while bringing the pelvic region forward. It then transfers its weight to the pelvic region while it thrusts its chest forward. As mentioned, True Seals have limited mobility on land, however on ice they are able to move much faster by sliding across the ice. They are said to be able to move across ice as fast as a person can run. Both sides of a True Seal's fore flippers and hind flippers are covered with fur and there is no definite sole on the flipper. The strong claws found on a True Seals fore flippers are used for grooming. Some True Seals use their claws for grip when hauling out on rock or ice and others may use their claws to scrape snow and ice to keep a breathing hole open or to create a breeding lair in the ice.

The normal body temperature of a mammal is 37 degrees Celsius, however water conducts heat at a greater rate than air, therefore a land mammal loses body heat very quickly when immersed in water. To overcome this problem seals have two important adaptations that help them to maintain a normal body temperature. Seals possess a thick layer of blubber below their skin, surrounding their entire body with the exception of the head and flippers. Blubber (fatty tissue) does not conduct heat well therefore it is an effective means of preventing heat loss from the seal's body. Most seals have a layer of blubber that is 7 - 10 cm thick. Blubber can also act as a food store, helping female seals to produce milk to feed their pups and allowing seals to go without food for periods when they come ashore to breed, give birth or moult. As the blubber does not cover the head and flippers of a seal, there is a need to reduce heat loss from these areas. This is done by reducing the blood flow to these areas, which in turn causes a reduction in heat loss. Blood flow may also be reduced to other areas of the body surface when required, particularly useful when diving at depths where the water is much colder.

Another adaptation that helps a seal retain heat is the layer of fur covering the body. Sea Lions and True Seals have a coat of fur (sometimes quite sparse in some species), but Fur Seals have developed this method of insulation further than the other seals.

The fine hair fibres trap a layer of warm air around the roots, which provides insulation. The fine but dense hair or under fur is supported by stiff guard hairs that make up the outer coat. Glands in the skin produce oil that helps the fine tips of the under fur repel water, therefore the water only penetrates the guard hairs. This form of insulation is effective on land and when nearer to the surface of the water, however at greater depths the pressure of the water pushes the trapped layer of air out, reducing insulation. Blubber is far more effective in these situations.

Seals have excellent senses. Their eyesight is particularly good and this is especially useful underwater where light levels are low. It is said that seals only see in black and white, which gives them better vision in low light environments. Seals have large eyes that are almost spherical in shape. The large size of their eyeballs enables the eye to take in more light, therefore providing them with excellent low light vision. A seal's vision is equally as important on land as under the water. On land a seal's sight is important to assess the intentions of an intruder by the way it moves and its posture, and to look out for signs of danger. When out of water, seals often have to cope with very bright light conditions. To protect their sensitive eyes, a seal's pupil contracts to a tiny pinhole. This protects the eye from the increase in light but does not effect the eyesight. A seal's eyes are also protected by secretions, which look like oily streaks as it runs from the eyes.

As with their eyesight, a seal's hearing is highly developed both in and out of the water. The bones of a seal's middle ear are much larger than those of a land mammal, and seals can pin point the location of a sound underwater quite precisely. The Weddell Seal.

Seals can produce calls underwater that can be heard 30 kilometres away by other seals. On land seals are very vocal when communicating with each other. Hearing is an important sense on land, for instance a female seal and her pup use calls to enable them to distinguish each other from the rest of the mothers and pups. Male seals (bulls) make roaring sounds to establish dominance and territorial boundaries. The various species produce a wide range of sounds from grunting, barking and clicking to howling, rumbling, shrill chirping sounds and even eerie bell-like calls.

Seals use their extremely sensitive whiskers to detect vibrations underwater such as the movements of a fish swimming. This enables them to catch prey in the dark or in environments where there is little visibility. Seals push their whiskers forward when hunting and capturing their prey to allow successful detection of its vibrations. The whiskers are highly efficient for capturing prey, as it has been known that a seal, which was blind in both eyes, was still able to catch more than enough food for itself and remain healthy. The whiskers are also used on land for interaction between individuals. If the whiskers are held forward it is usually a sign of aggression. A female seal may also drive off the unwanted attention of a bull seal by attempting to bite its whiskers which irritates them, often causing the bull to retreat. During the Dolphin Marine Magic show, when a seal balances a ball on its nose, it uses its whiskers to sense the movement of the ball allowing the seal to keep it perfectly balanced.

While a seal's sense of smell is not used underwater as the nostrils are sealed, smell is said to be an important sense for seals on land. Seals often have a strong scent, particularly breeding males, which indicates that smell plays some part in social interactions. Further evidence that smell is important is that, as well as vocalisation, a mother seal will smell her pup if it has been separated from her to recognise it as her own.

Like all mammals, a seal's hair needs renewing at certain intervals; therefore the process of moulting takes place. Moulting is more obvious in true seals than fur seals and sea lions. Fur Seals and Sea Lions lose their old coats and renew them gradually; therefore moulting is not so noticeable. True Seals, on the other hand, replace their coat every season over a short period. Fur and skin is shed in bits and pieces, often leaving the seal looking very untidy. Blood supply to the skin increases where the new coat is growing, so many species of true seal spend time huddled together on land during moulting to avoid great heat losses.

Fur Seals often choose rocky beaches safe from storm tides for breeding grounds, whereas Sea Lions tend to prefer sandy beaches, and True Seals choose to breed either on sandy beaches, or in cracks or holes in ice depending on the species. For Phocids and Otariids there is competition within the breeding males to establish a hierarchy and determine territorial boundaries, or with some species, to maintain exclusive access to a female. This is done by displays and loud vocalisations, or by engaging in fights. The females (cows), in many species, instinctively return to the colony where they were born to give birth to their own pups. It is usually the females that decide which territory, within the colony, they will give birth and mate in rather than the bulls choosing a heirem.

The reproductive cycles of Phocids and Otariids differ in that, although gestation periods (mating to birth) are generally the same being an annual cycle, the beginning of this cycle may occur in different months for different species or individuals. There are two exceptions to this and they are the Australian Sea Lion and the Walrus who both have unique breeding cycles. The Australian Sea Lion has a gestation period of eighteen months and the Walrus has a gestation period of fifteen months. The basic process however, is the same for all other species of seals. This begins with mating approximately 6 - 20 days (depending on the species) after giving birth, when the cows come into oestrus (heat). The cows continue to rear their newborn pups. After mating and fertilisation of the egg, there is a period of approximately three months where implantation of the egg is delayed. The delay allows births and mating to occur at the same time, when males and females congregate on land together which can be a dangerous time for many seals. The delay gives the cow time to recover from the initial strain of lactation and rearing a young pup before the new embryo begins to develop.