There are 33 species of pinnipeds alive today, most of which are known as seals. Pinnipedia is made up of three main groups: The walrus, which is the only living member of the family Odobenidae; the eared seals of Otariidae, including numerous kinds of fur seal and sea lion; and the earless seals, known as true seals or Phocidae. Despite the name, earless seals have ears—they’re just hidden beneath the surface of their skin.

Pinnipeds can be found on every continent on Earth, though most species occur in cold-water environments. Thick layers of fat, also known as blubber, keep the animals warm, in addition to dense fur. Walruses are the exception to the rule, as these large, tusked pinnipeds have nearly hairless bodies.

Seals range greatly in size, from the gargantuan southern elephant seal, which can weigh more than a pickup truck, to the relatively slender, 100-pound Baikal seal.

While there are many differences among the species, all seals have feet shaped like fins. In fact, the word pinniped means "fin-footed" in Latin. Those fin-shaped feet make them supreme swimmers, and all pinnipeds are considered semi-aquatic marine mammals. This means they must spend some part of their lives on land or sea ice, usually during the mating and birthing seasons.

With so much time spent in water, some species like the elephant seals have evolved the ability to hold their breath for up to two hours and dive to depths of more than 6,500 feet looking for food. (Read more about how diving animals can stay underwater for so long.)

Nearly all seal species are reliant on marine habitats, though some will enter estuaries and rivers in search of food. An outlier is the Baikal seal, which spends its whole life in Lake Baikal, a freshwater lake in Siberia.

Evolutionarily speaking, seals are thought to be most closely related to bears and the group of animals that includes weasels and otters, as well as skunks, raccoons, and red pandas.

Varied diets

All seals eat other animals, and most rely on fish caught out at sea. But a few species break the mold.

For instance, leopard seals make a living hunting down penguins and even other seals. And walruses survive on a diet of clams and other shellfish, which they detect with their highly sensitive whiskers and then suction up from the seafloor with their powerful mouths.

There is also a species of pinniped known as the crabeater seal, which lives in Antarctica. However, these animals don’t eat crabs at all—at least not the kind you would think. Instead, these seals use highly specialized teeth to filter water for tiny, abundant crustaceans known as Antarctic krill.

Pinnipeds in peril

Historically, hunters have heavily targeted pinniped species for their fur, a practice that drove some species to extinction: The Caribbean monk seal, for instance, disappeared from the planet in the 1970s. Though hunting is more regulated today, the animals still face many threats, including lack of food, entanglement in fishing gear, and conflict with fishermen. Some species, like the Hawaiian monk seal and Mediterranean monk seal, remain very close to extinction.

However, climate change represents the single largest threat to many species of pinnipeds, especially those that rely on sea ice. Various species of Arctic seals rely on ice for breeding, for instance, while walruses use seasonal ice formations to forage for food farther from shore. (Read how climate change is changing what leopard seals eat.)

Ringed seals build caves in the snow and create holes in the ice that give them access to the ocean. If that snow melts earlier than usual, it will put the animals at greater risk of predation from polar bears.

For harp seal pups, survival depends on its first ten days From the minute the pups are born, they enter a race against time.