Seals form the family Phocidae within the clade Pinnipedia. They are differentiated from sea lions and other pinnipeds by the absence of external ear flaps, among other characteristics. The phocidae family is known as true seals, as opposed to sea lions and eared seals, which are usually incorrectly generalized as 'seals'. In this article, the word 'seals' refers to 'true seals', or 'earless seals', unless otherwise mentioned.
Seals are centered around polar ecosystems, with just one genus found around the tropics. They are highly tuned to their cold, aquatic habitat. Seals are much more at home in water than eared seals, and are consequently much clumsier on land. Unlike penguins or polar bears, which are found only in the southern and northern hemispheres (respectively), seals inhabit both poles, and temperate waters either side of the Equator.
Most seals grow to about 1.1 meters to 2.5 meters in length, and their weights range from 45 kg to 150-200 kg. Larger seals, such as Weddell seals and leopard seals, grow to more than 3 meters long, while weighing more than 500 kg. Elephant seals, one of the largest predators in the world, hugely exceed these dimensions; male southern elephant seals can grow up to 6.5 meters and can weigh more than 4,000 kg.
Excepting cetaceans, seals are the most aquatic mammals. Their bodies have thus evolved to be extremely streamlined. Seals lack external ears (as can be seen in the adjoining image), and they have internal genitals, minimizing water resistance. They can also regulate blood flow to their blubber (layer of fat found in most marine mammals) to better control their temperatures.
When swimming, seals even move in a way more similar to fish than eared seals. They move their hind flippers sidewards, and use the front flippers almost exclusively for steering. Although this makes them slower than eared seals and other marine mammals, their efficient motion allows them to seek and pursue prey much farther from their breeding sites.
Unlike sea lions (which are 'eared seals'), seals can't turn their hind flippers downwards and walk on all fours. Although this allows them to be much more efficient in the water, it necessitates them to drag themselves forward by their front flippers and their abdomens when on land. This behavior has given them the name crawling seals.
Seals are carnivores, and hunt fish and crustaceans. Fish thrive in the -- perhaps surprisingly -- fertile polar waters, and are able to sustain large, permanent populations of all types of seals. Large seals, notably the leopard seal, also regularly feed on penguins. The habitat and location of various species of seals largely determines their diet, since concentrations of various species of fish vary drastically depending upon the location.
Seals are not fussy eaters; they generally feast on whichever fish happens to be present in large numbers in their territory.
Seals spend an overwhelming majority of time in water, and only come to land, or 'haul out', to breed and moult. While breeding, their typically barren habitat causes problems for a nursing female. Nesting sites of a nursing female are often far from the ocean, forcing her, unlike most other mammals, to fast while lactating.
Seal milk is one of the most fat-rich milks, which is needed if the pup is to survive its harsh, cold environment. The fat-rich milk leads to the pup building up a blubber in the first few days, but severely lowers the mother's own fat reserve and energy.
Mothers wean their pups quite abruptly, leaving them at the nesting site and leaving to feed. Until the pup learns to hunt on its own (developing the muscles and lung capacity needed to master diving takes time), it has to live off its own fat stores.
Protected by the relatively low human interference in their pristine habitat, seals are one of the few animals who can still be found in large numbers across their range. However, global warming has already started to eat into their precious habitat, and a few species, notably the monk seals -- a very rare, tropical genus -- are teetering on the verge of extinction.
While collectively seals remain unfettered by growing human expansion, it would be unwise to simply ignore the warning bells from their untouched, unaltered, and irreplaceable habitat.