Mammals
Seal
Pinnipedia · also called seals, pinniped
A seal is a carnivore that went half-way back to the sea. It hunts, and can hold its breath for an hour and dive deeper than a nuclear submarine, but it still has to come ashore or onto the ice to rest, moult, and raise its pups. Its ancestors, around 30 million years ago, were bear-like animals on land.
Overview
Seals are the group Pinnipedia, around 33 species, in every ocean and a couple of lakes. There are three families — the true seals (Phocidae), which hump along on land; the eared seals (Otariidae — the fur seals and sea lions), which can walk on their flippers; and the walrus, on its own. All are carnivores, hunting fish, squid, and crustaceans on breath-hold dives.
Built for the dive
A seal exhales before diving, collapses its lungs, drops its heart rate to a few beats a minute, and shuts blood away from everything but the heart and brain, storing oxygen in the blood and muscle instead of the lungs. A Weddell seal stays under for over an hour; an elephant seal dives past 2,000 m. Stiff facial whiskers track the hydrodynamic trail of a fish that passed 30 seconds earlier.
Fast, fatty milk
Seal milk is 40 to 60 per cent fat. A hooded seal pup is weaned in four days — the shortest nursing of any mammal — tripling its weight on near-pure fat while the mother fasts. A blubber layer, up to a third of an elephant seal’s weight, insulates, fuels the fasts, and streamlines the body.
Status
Seals as a group are Not Evaluated. Most have recovered strongly from centuries of hunting, and grey, harbour, and elephant seals now number in the hundreds of thousands to millions. But the Mediterranean and Hawaiian monk seals are Critically Endangered with a few hundred to ~1,500 each, and ice-breeding species are threatened by the loss of the sea ice they raise pups on.
Seal: common questions
What is the difference between a seal and a sea lion?
They are both pinnipeds but in different families, and the quickest tells are the ears and the flippers. A sea lion, and its close relatives the fur seals, has small external ear flaps and long, strong front flippers, and it can rotate its hind flippers forward under the body to walk and even climb on land, moving quite fast. A true seal has no external ear, just a small hole, shorter front flippers, and hind flippers that point permanently backward and cannot be turned forward, so on land it can only hump and wriggle along on its belly. In the water, sea lions swim mainly with the front flippers while true seals power along with side-to-side sweeps of the hind flippers.
How do seals dive so deep for so long?
Through a set of adaptations that let them work on a tiny oxygen budget. A seal breathes out before diving and lets its lungs collapse, so almost none of its oxygen is stored in the lungs, avoiding the bends. Instead it carries a large blood volume rich in haemoglobin and its muscles are packed with myoglobin, both oxygen-binding proteins, so the store is in the blood and muscle. On a dive the heart rate drops to as low as a few beats a minute, blood vessels to the gut, skin, and limbs clamp down, and the brain and heart are supplied almost exclusively while the muscles run partly without oxygen. A Weddell seal can stay under for over an hour and an elephant seal can dive to more than 2,000 metres.
Are seals dangerous to people?
Most seals are not a threat and will flee into the water if approached, but they are large wild carnivores and should be given space. Bites do happen, usually when a seal is cornered, defending a pup, or provoked, and a seal bite carries a real infection risk, including a hand infection called seal finger. Two species warrant particular caution. The leopard seal of Antarctica is a powerful predator that has attacked divers and boats and killed a researcher in 2003. Large bull sea lions and fur seals defending a territory or a harem will also charge and bite. Feeding wild seals is discouraged everywhere because it makes them bold and leads to conflict.
Why do baby seals look like they are crying?
They are not crying with emotion, but their eyes do stream. Seals have large tear glands that constantly produce fluid to protect and lubricate the eye, which matters underwater and against wind and salt, but unlike land mammals they have no tear duct to drain that fluid into the nose, so it simply overflows and runs down the face, especially when the seal is out of water. In pups the effect is very obvious and, combined with the big dark eyes, makes them look tearful. It has nothing to do with distress, and a genuinely alarmed seal pup usually calls loudly rather than looking sad.