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The term “seal” is often used to refer generally to pinnipeds, members of a superfamily of mammals by the same name. It is also used specifically to refer to the so-called “true seals” in the family Phocidae. The defining characteristic of a seal is that it is a mammal with flippers, adapted to an aquatic life, and capable of survival both in and outside of the water, unlike dolphins, another group of flippered mammal which you may be familiar with.
All pinnipeds have sleek, muscular bodies which are designed for swimming, and they are also carnivorous. Many have powerful teeth and jaws adapted to make them formidable hunters, and they are also extremely fast, powerful swimmers, able to pursue prey across extended distances. A seal is also capable of remaining underwater for up to two hours without needing fresh air, thanks to the amazing oxygen conservation abilities of pinnipeds.
True seals spend most of their life in the water, and they tend to be very awkward on land. They communicate by grunting and slapping the water, and their rear flippers are more developed than their fore flippers. In the water, true seals move very sinuously, revealing themselves as animals well used to the marine environment. Eared seals, on the other hand, have more developed front flippers, and they are comfortable navigating on land as well as in the water. They are also very vocal; sea lions are one example of eared seals.
The walrus is also considered a pinniped. Walruses are distinguished by their massive bodies and formidable tusks, and they are particularly fond of shellfish. The walrus was once a widespread pinniped, but it is not restricted primarily to the arctic regions of the world. Walruses also have a very unique feeding technique which allows them to crush shells in their mouths, suck out the usable flesh inside, and spit out the remainder, almost like they are vacuuming out the shell.
When you see a performing seal, you are actually looking at a sea lion, a member of the eared seal group. True seals cannot perform the tricks associated with seals in zoos and circuses, as they lack the large front flippers needed to navigate with confidence on land. The seals along the seashore are also not true seals, as you can tell by their vocalizations and well developed front flippers.