In the coldest parts of the world, insulation is essential for survival.
One way is to have a good thick layer of blubber under your skin. But anyone that's seen a walrus flopping across the beach knows that fat can have its downsides. To stay both warm and agile, fat won't do.
The Arctic fox can withstand polar winters, surviving long periods below -20 °C (-4 °F), thanks to some incredible adaptations.
One of the most distinctive is its fluffy coat, which changes colour from brown to white and grows longer for the bleakest season. In certain areas it can double in length; on its underside the longer fur helps insulate the fox from the cold ground when it sits or lies down and even the soles of its feet are furry.
When it curls into a ball, nose tucked under its tail, only the thickest fur is exposed to the elements.
"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.
Air is a good insulator because it's not very dense, so heat only moves through it slowly
"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.
Many polar specialists use this trick.
Human explorers wear down jackets, filled either with the insulating feathers of geese or an artificial fibre that mimics them.
You wouldn't describe a sheep as furry
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.
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 furriest creatures either have a lot of follicles or grow multiple hairs from each one
It is human language that makes distinctions between these structures.
The commonly-held notion is that fur is soft, thick and relatively short. Wool is similarly dense but each strand is fine and grows longer.
Meanwhile, hair is more likely to be sparse and coarse, and can grow staggeringly long. The current world record holder for the longest hair is Xie Qiuping of China, whose 5.62m (18.43ft) of hair is almost as long as a giraffe is tall.
Whatever you call it, the stuff all grows from specialised cavities in the skin called follicles. The furriest creatures either have a lot of follicles or grow multiple hairs from each one.
Chinchillas are rodents that live at elevations of 3,000-5,000m (9,800-16,400ft) in the Andes Mountains of South America.
Sea otters in the north Pacific were hunted to near-extinction
To survive in the bitter cold, they have very dense fur. They achieve this by growing more than 50 fine hairs from each follicle. Their fur is considered to be the softest in the world.
This has not escaped the attention of hunters. Both the short-tailed and long-tailed chinchilla are classified as critically endangered by the International Union for Conservation of Nature, due to severe hunting since the 19th century.
Otters have also come perilously close to extinction due to their pelts. Their thick coats are especially adapted to their semi-aquatic lifestyle.
Again, they have two layers of fur: an inner insulating layer protected by waterproof guard hairs. This prevents water from making contact with their skin and leaching away heat.
Clearly, the sea otter is the furriest creature on Earth
The combination of warmth and waterproofing is exactly what made otter pelts so sought after. Sea otters in the north Pacific were hunted to near-extinction: there were just 2000 left in the wild when the commercial trade in their fur was banned in 1911.
In a 2010 paper, researchers compared the hair densities of Eurasian otters and sea otters. They found that Eurasian otters have up to 80,000 hairs per square centimetre of skin, while sea otters have an astonishing 140,000 hairs per sq cm.
By comparison, Arctic foxes and chinchillas both have 20,000 hairs per sq cm, and a muskox has just 420 per sq cm. Clearly, the sea otter is the furriest creature on Earth.
Why the sea otter? It all comes down to family history and environment.
All marine mammals keep warm with a layer of blubber under their skin, except sea otters.
That is because sea otters belong to a family called the Mustelidae, which also includes badgers, weasels and wolverines. Compared to other marine mammals, they are relative newcomers to life at sea, so rather than evolving a layer of blubber independently they have adapted their existing fur.
The animals and plants that push the limits
This forces them to keep their coats in top condition. Dirt can compromise the waterproofing of their fur, affecting both their body temperature and buoyancy.
To ensure that this doesn't happen, adult sea otters groom and fluff air into the fur of pups to keep them afloat.
When conservationists try to rehabilitate rescued pups, they have to take over this job. They spend hours a day brushing the coats of fluffy babies until they are sea-worthy. Fortunately the pups are already squee-worthy.