Some scientists place seals within the order Carnivora, and some in a separate order, Pinnipedia. But there is agreement that seals consist of three families:
- Otariidae: fur seals and sea lions
- Phocidae: ‘earless’ seals
- Odobenidae: walruses.
In the past, there was greater diversity: for example, there were once another 13 species of walrus. Some evidence suggests that fur seals, sea lions and walruses are derived from bear-like ancestors, while the earless seals may have evolved from otter-like ancestors. Molecular studies, however, support the notion that all seals are descended from a common ancestor within the order Carnivora.
Adaptations to sea life
As with all marine mammals, adaptation to the aquatic environment has influenced the body shape of seals. They have become streamlined, their limbs have become flippers (the term Pinnipedia means ‘feather foot’ or ‘wing foot’), and they have a layer of fat to help insulate them in the water and to act as an energy reserve.
Life on land
Seals can be at sea for weeks at a time, sleeping, feeding, and travelling huge distances. But unlike dolphins or whales, they are tied to the land for breeding. They will also spend time on land at haul-out sites when not breeding.
Fur seals and sea lions
Fur seals and sea lions are known as otariid or ‘eared seals’. Of all the seals, they are thought to have evolved first, and yet are the least specialised for life in water. They have external earflaps and still walk on all four flippers when on land. In the water they propel themselves with their front flippers. The New Zealand fur seal and sea lion are examples of otariid seals.
Earless or phocid seals have no earflaps, which makes them more streamlined. They are unable to move their hind flippers under their bodies, so they move with a caterpillar motion on land. In water, they propel themselves with their hind flippers, and steer with their front flippers.
The phocid seals are more specialised for a life in water than eared seals: they dive deeper and for longer and, in males, the testicles are kept internally – a much warmer place when in chilly waters. In New Zealand the southern elephant seal and leopard seal are examples of phocid seals.