Why are walruses so chubby?

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.