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The amazing aquatic abilities of seals
Living in the ocean
Seals spend most of their time at sea and only haul out on land to rest and breed. They swim for hundreds of kilometers, dive to great depths and catching elusive, fast swimming prey. To survive, seals need special abilities to stay warm, stay fed, travel around, and avoid getting eaten.
Firstly, seals need to stay warm in the water. They do this with a combination of fur and blubber.
Fur seals have incredibly dense double-layered fur - hence their name. The outer layer, composed of coarse guard hairs, creates a mirror-smooth surface when wet which, combined with a streamlined shape, allows seals to slip through the water with hardly any drag. The inner layer of fur is made of ultra-fine hairs that trap air for insulation. There are around 50,000 hairs in a square centimeter (humans have only 200 per cm2!).
As a fur seal dives deeper and deeper, more air is squeezed out of the fur by water pressure. This is why deep diving seals like sea lions and phocid seals use mostly blubber for insulation and have only a single layer of fur.
Because seals are so well insulated they can easily overheat out of the water. Sometimes they are seen holding a flipper in the breeze as a way of cooling off. The flippers are the only part of the body not covered by fur and have large blood vessels to help give off heat.
Cross section of fur
Sight: Some seals hunt at depths where little sunlight can reach (80m - 2000m). Others hunt at night because that is when their prey rises from the depths. So seals have large, sensitive eyes for seeing in the dark. They also have flat corneas for focusing underwater, just like the flat glass of a scuba diver's mask.
Touch: A further aid to seeing in the dark comes from their long, sensitive whiskers that can pick up vibrations in the water. Swimming fish leave a trail of turbulence that seals can follow by feeling it with their whiskers!
Smell: Seals don't use smell in the water, in fact they close their nostrils to stop water getting in. On land, though, they use smell for identifying each other (especially between mothers and pups) and for detecting predators.
Hearing: Seals can hear underwater as well as on land. When out of the water they communicate vocally - seals may bark at each other to say "back off!", mothers and pups call to find each other and males make a series of grunts to show how big & tough they are.
The long whiskers of a northern fur seal
Scuba divers and freedivers are always impressed by the abilities of seals. Imagine slipping through the water with hardly any drag, free from a mask and fins!
The phocid seals avoid problems with bouyancy, nitrogen narcosis, barotrauma and decompression sickness ("the bends") by exhaling before they dive. The otariids seals, however, dive with a full breath and exhale on the way back to the surface. The reasons why they do this are still unclear.
Diving seals store oxygen with blood haemoglobin and muscle myoglobin. They can store many times more oxygen than a human in this way. Once diving, the heart rate slows to conserve energy.
Fur seals and sea lions can dive as deep as 150 meters for up to 5 minutes (in Australian fur seals, 80m, 2-minute dives are normal). The phocid seals are even better adapted for diving. Elephant seals are the current record holders - one was recently recorded diving to 2000 meters (yes, 2 km down)!
|Australian fur seal pup swimming|
Foraging is no picnic. If a seal doesn't catch enough food it will burn up its blubber stores and eventually starve. They are also eaten by killer whales and sharks.
An extremely flexible spine makes seals one of the most agile animals in the water. So once a seal has spotted an attacker it has a good chance of escaping it. This is why great white sharks ambush seals at high speed from below, sometimes jumping clear of the water.
Sharks sometimes attack surfers and swimmers who make a similar silhouette to a seal. Humans usually don't spot a shark until its too late and then they aren't agile enough to escape it. In most cases the shark releases the person after biting them and deciding they are too bony to bother eating. Unfortunately that first bite can be fatal.
Like all land animals, humans need lots of bones to support themselves. Marine animals spend most of their time floating in water so they don't need large limbs to hold themselves up. So they usually have a lower percentage of bone in their bodies.
This Australian fur seal has survived two bites from a shark