Reputation: Polar bears are the friends of penguins

Reality: As far as we can tell, polar bears and penguins have never met outside a zoo

My children have outgrown books in which polar bears and penguins inhabit the same page. But there is an alarming number of them out there and this is one biogeographical crime I cannot forgive.

There is little doubt that polar bears are a very recent offshoot from the brown bear lineage

Polar bears live in the Arctic, near the North Pole. Penguins live on Antarctica and the neighbouring continents, near the South Pole. They are literally poles apart, so don't unite them in fiction just because they look cute together.

If children's authors are looking for a "friend" for their polar bear protagonist, then how about a brown bear?

There is little doubt that polar bears are a very recent offshoot from the brown bear lineage, though there is considerable debate over when this divergence took place.

One analysis suggests that the split began around 5 million years ago. Another places the division at less than 500,000 years ago. Either way, the polar bear is a remarkably recent evolutionary innovation.

This much is clear from the fact that the two species are still reproductively compatible.

When zoos have brokered a cross between a polar bear and a grizzly, they have been rewarded with cubs. These hybrid offspring, variously named "pizzlies" or "grolars", are often fertile themselves.

This is compelling evidence that polar bears and brown bears can be friends

Over the past decade, these beasts have occasionally cropped up in the wild too.

For instance, in 2006 an American big game hunter shot a polar bear in the Canadian Arctic that turned out to be a pizzly. In 2010, an Inuit killed a second-generation hybrid, a cross between a female pizzly and a male grizzly.

This is compelling evidence that polar bears and brown bears can be friends, even if the level of intimacy might not be suitable for children.

There are of course differences, the most notable being coat colour.

The hairs of brown bears are pretty solid. But polar bear hairs are hollow.

There is no direct evidence to support this theory

It's been suggested that this unusual structure may prevent pigment-producing cells migrating into the hair shaft.

The hollow structure of polar bear fur also led to the alluring idea that each strand might act like a fibre-optic cable, carrying warming ultraviolet light directly to the bear's pigmented skin. But when physicist Daniel Koon tested this idea in the 1990s, he found otherwise. "There is no direct evidence to support this theory," he concluded in a 1998 paper.

If polar bears do not use their hollow hair shafts to harness the sun's energy, how do they survive the harsh Arctic environment? The answer is largely down to fat.

Polar bears are so dependent on blubber in their diet it's of little surprise they have substantial stores of fat themselves, sometimes accounting for more than half their body weight.

It remains an enigma how polar bears are able to deal with such lifelong elevated levels of cholesterol

In 2014, Eline Lorenzen of the University of California, Berkeley and her colleagues looked for genetic differences between brown bears and polar bears. Some of the most striking were to do with fat metabolism. The levels of cholesterol in the polar bear's blood are "extreme".

"For polar bears, profound obesity is a benign state," Lorenzen said in a statement.

"It remains an enigma how polar bears are able to deal with such lifelong elevated levels of cholesterol," the authors wrote. The answer may have something to do with some genetic fine-tuning of the polar bear's cardiovascular function.

How will polar bears rendezvous with their brown bear chums? They'll probably use their noses.

With a rapidly warming Arctic, polar bears will have to move further afield to survive

In 2014, scientists revealed that sweat glands on polar bears' feet leave smelly trails all over the sea ice, a lasting odour that other bears can detect.

The paper did not say whether brown bears leave similarly smelly trails that could be used to track them, but it would not come as too much of a surprise.

With a rapidly warming Arctic, polar bears will have to move further afield to survive. Whether they will encounter brown bears more often remains to be seen. But they still aren't going to run into any penguins.