Maxy Web

The muskox native to Greenland and the Canadian Arctic grows its own sweater. The fur closest to its skin is known as qiviut. Like that of the Arctic fox, it grows for winter and is shed when temperatures warm up in the spring.

An adult muskox can shed up to 3.5kg (8 lb) of this underfur, which can be spun to make what many consider the warmest wool in the world. The long guard hairs can reach the ground on older animals, meaning their shaggy coat can be over 1m (3ft) long in places.

"You wouldn't describe a sheep as furry"

Still, the idea that they are the furriest animals on Earth won't satisfy most people. First, we have to agree what fur is

Even if you've embraced the hipster beard phenomenon, you probably don't consider yourself furry. Likewise, you wouldn't describe a sheep as furry. Yet fur, hair and wool are chemically the same thing: a fibrous protein known as keratin that animals also use to make quills, nails, hooves and horns..

"The winter-time coat ranges between 45-60mm [long], but offers a very high thermal insulation compared to its length," says Stephen Midgley of the Arctic Fox Center in Súðavík, Iceland.

"Altogether the arctic fox can tolerate very low temperatures of -70 °C. It always surprises you just how thin they really are when you pick them up and all the fur compresses against their little bodies."

The reason this coat can protect the fox from such extreme cold, despite being only a few inches thick, is down to its structure.

Beneath the longer outer hairs is a softer, denser layer that accounts for 70% of the fur. This traps a layer of air next to the skin. Air is a good insulator because it's not very dense, so heat only moves through it slowly.