Pinnipeds are fin-footed marine mammals. They differ from cetaceans, dolphins and whales, in that they are able to live out of the ocean and that they do not have blowholes. This group contains eared seals, phocids, and walruses. They are widely distributed throughout the world's oceans.
Eared seals and phocids are commonly confused. Walruses are easier to identify with their characteristic tusks so they will not be the focus of this article. Like many things in the natural world, eared seals and phocids have many shared characteristics, but they have many defining features that make them unique.
Phocids: True Seals
Close your eyes, take a moment and picture a seal. Chances are pretty good it was of a cute, barking, big-flippered marine mammal balancing a ball on its nose. It may come as a surprise, but when most people think of a seal they really aren't thinking of a seal at all. That cute animal with the ball on its nose is in fact a sea lion. Seals lack the muscles necessary to balance anything on their noses. So what is a seal exactly?
Phocids, 'true seals', lack an ear flap. In other words they do not have external ears. They are still able to hear, their ears are just not clearly defined on the outside of their body. Seals also cannot 'walk' on land. They lack the capability to rotate their hind flippers as would be necessary to walk, like sea lions can. Instead they move with a characteristic inch-worm like movement, moving on their bellies. This makes them rather slow and awkward when moving on land. One other key difference is that when in the water seals propel themselves by a sculling motion by moving their hind flippers and lower body side to side.
An example of a true seal would be the harbor seal, also known as the common seal. They are found in both the Atlantic and the Pacific and can be found in large colonies during breading season. Monk seals, including the critically endangered Hawaiian monk seal, are also categorized as true seals. Oddly enough fur seals are not seals at all. They are really a type of sea lion. This does not help the already confused general public distinguish between seals and sea lions.
Another seal that many people think of is the leopard seal. These creatures are found in the cold waters around the antarctic and are famous for preying upon penguins. They are typically aggressive and are well adapt hunters. In many animated movies about penguins leopard seals, as well as killer whales, often play the villain. Its not that the leopard sea is evil, they are just doing what leopard seals do: eating cute, helpless, little penguins.
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Otariids: The Sea Lion
Otariids are easily distinguished from their phocid cousins by their visible external ear flaps, their large flippers, and their appearance in large groups. Another key distinguishing feature is that their fore flippers, the front flippers, are much more developed than those of the phocids. These large flippers are what sea lions use to move themselves through the water. Otariids can be broken up further into sea lions and fur seals.
The main difference between the two is that fur seals have a thicker coat that includes an undercoat. Sea lions also have a more rounded snout whereas the fur seal has a more pointed nose. As horrible as it is, but one really easy way to tell them apart comes to us courtesy of the Discovery Channel. If you have ever seen an episode of Shark Week chances are you have seen the specials that showcase the great white sharks jumping off the coast of South Africa. Those animals they are hunting are fur seals. The dark, short haired 'seals' that are commonly found in zoos are sea lions. More often than not they are specifically California sea lions, which are known for their characteristic dog-like bark.
What do They Have in Common?
Despite all of their differences seals and sea lions do have several things in common. The first of which is that they give birth to live young, as is characteristic of a mammal, on land or in some cases ice floats and not in the water like whales and dolphins do. Also most seals and sea lions come together in large groups to mate. Typically one male has the 'rights' to several females and will mate with each of them to insure that his genes are passed on. This is most dramatically seen in the elephant seal species. Aside from being the largest species of seal they are the most aggressive and attack and occasionally kill other males during breeding season.
Seals and sea lions also have several physiological traits that they share. One of the most impressive is their dive reflex. During a dive pinnipeds are able to slow their heart beat, known as bradycardia, and shunt blood from their extremities. This allows for them to conserve the oxygen in their blood and use it to move their bodies through the water on their dives.They also have a large volume of blood, most of which is stored in their spleens when not in use during dives. This large amount of blood helps to hold extra oxygen for them to use on their long deep dives. Some dives can last longer than twenty minutes!
When they have young both seals and sea lions feed their pups a fat rich milk. This concoction helps the pups gain weight quickly. This is important because most seals and sea lions are found in cold waters and the young pups need to build up an important layer of fat called blubber. Blubber is a thick layer of fat found just under the skin and it is responsible for keeping marine mammals warm in cold waters. Without a sufficient amount of blubber the pups will die in the water.