As a general rule, terrestrial mammals are furry, while aquatic ones are fat. It doesn’t work across the board: sea otters rarely leave the water but have the densest fur of any mammal.
And of the handful of mostly hair-free landlubbers out there, including elephants, rhinos and humans, at least one is prone to plumpness.
But it’s still a useful distinction. Whales, dolphins, hippos, dugongs, manatees and walruses are all at the bald and well-padded end of the spectrum.
Fur and fat are both effective insulators. Fur traps a layer of warm air against the skin, and otters are proof that it can work in water, too.
But a fur barrier has a major disadvantage in aquatic environments, because the air gets squeezed out under pressure, which means that it’s little use for diving animals. Otters only get away with it because they stick to shallow waters.
Most aquatic animals, though, do dive, so they rely on a different sort of barrier against the cold – a thick layer of fat under the skin called blubber. In the Arctic, there’s no shortage of cold to keep out, so a walrus has more than its fair share of blubber. Just how much depends on how well fed the animal is, and varies with the seasons. In early winter it may be 15cm thick.
A walrus’s skin and blubber combined can account for nearly half its entire body mass. That’s more than half a tonne for a typical adult male weighing 1,200kg (the biggest males reach about 1,700kg). Females average about 800kg (up to a maximum of 1,250kg).
Walruses aren’t the biggest mammals in the sea (the great whales have that one covered). They aren’t even the biggest pinnipeds (male southern elephant seals can exceed three tonnes). They might not be the fattest, either (a well-fed Weddell seal can appear almost spherical). But there is something particularly flabby and amorphous about them.
Their bodies seem to spread out in all directions on land. The totally naked, pinkish, blotchy skin doesn’t help either. When they first emerge from cold water, they are an even pastier grey, because the blood vessels in their skin constrict to conserve heat.
Then there are the fleshy nodules, or ‘bosses’, around the head and neck of mature males, which may signal dominance to rivals and provide protection from their pointed tusks. Meanwhile, walruses’ pinniped relatives (seals, sealions and elephant seals) have at least retained a short, sleek coat of hair, despite their reliance on blubber for insulation, which preserves their modesty to some extent.
You only have to watch a walrus ‘walking’ on land, using its four flippers as legs, to see that this is a species intended for the weightless conditions of the sea. The bigger the animals grow, the harder it becomes to lift their bellies off the ground. The result is a gait that is more maggot than mammal
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