Natural History

For a piece on specialization within the bear family, I could have gone with several options.   Sloth bears have evolved to live on termites, and most famously, giant pandas are almost entirely reliant upon bamboo.

But I decided to go a different route.

Sloth bears and giant pandas are just bizarre animals, and compared to the bear family as a whole,  they are pretty puny.

There is another species of bear that is hyper-specialized, though we usually don’t think of it in this manner.

After all, it shares the title of the largest terrestrial carnivoran with the Kodiak bear.

Of course, I’m talking about the polar bear.

Polar bears are often classified as marine mammals.

Now that should tell you something about them.

No other bear is so reliant upon the sea for its sustenance.

Of all the bears, they are the only ones that live almost exclusively on meat.

They can handle absolutely frigid conditions on the arctic sea ice, and they can swim in the most chilling waters of the Arctic Ocean– often swimming for as much as 60 miles at one stretch in order to access better hunting grounds and to return to the land.

You’d think such a large predatory animal would essentially be invulnerable.

But the truth is they are quite fragile.

Part of the reason why they are so fragile is that they are the youngest species of bear, if they are indeed a full species at all.

Indeed, one could make the case that they are nothing more than a modified but highly specialized brown bear.  The initial split between polar and brown bears may have happened 150,000 years ago. Of course, this was a mitochondrial DNA study, but because we don’t yet have the polar or brown bear genome sequenced, all we  have are these mtDNA studies. (The authors of that last study are working on it!)

150,000 years is a very short in terms of an evolutionary time frame.

And to make matters even more complicated, polar and brown bears have continued to exchange genes through the past 150,000 years.  The popular press likes to make a big deal about the killing of a confirmed polar/grizzly bear hybrid in Canada. (Grizzlies are just a subspecies of the Holarctic brown bear).

However, a more recent study of ancient and modern brown bear and polar bear mtDNA revealed that all modern polar bears descend from a liaison between a polar bear and a female brown bear living in Ireland 45,000 years ago.

American black bears never hybridize with brown or grizzly bears in the wild, but they can in captivity. However, polar and brown bears have done so in the wild, so one might be more willing to accept that the polar bear is nothing more (or less) than a modified brown brown bear.

And as a modified brown bear, it has evolved rapidly from a truly generalist ancestor.  Brown bears once range from all of Europe and most Asia. They also colonized a huge chunk of North America, including Mexico and even parts of the Eastern and Midwestern United States. (When Europeans came to the Americas, the brown bear was restricted to the Western half of the continent. It still lived in Mexico, but it was not found in the East.)

If count the polar bear as a brown bear, the brown bear species was also able to colonize the ice cover of the Arctic Sea.

But in order to do that, these modified brown bears had to give up many of their generalist adaptations.

Exactly what polar bears lost from the ancestral brown bear has only recently been revealed. These adaptations have only been extant in polar bears for about 20,000 years, which makes their specialization very recent.  The evolution of the main traits that separate a polar bear from a brown bear are actually more recent than the first signs of domestication in wolves, which appeared over 30,000 years ago.

Its head is proportionally smaller and flatter, which allows the polar bear to shove its head into seal dens and breathing holes.  The main prey of the polar bear on the ice are ringed and bearded seals– especially the pups of both species. Although these animals are relatively large, neither weighs as much as a fully grown male polar bear, and because the bears hunt mostly the pups of these species, they are essentially hunting little blobs of fat.

If your diet is little more than little blobs of fat, you aren’t going have the same selection pressures that maintain the robust skull of the generalist brown bear. Using finite element analysis, it was revealed that the polar bear’s skull has many more structural weaknesses than the brown bear.  A brown bear can kill a moose, but it is very hard for a polar bear to do the same.  Polar bears have roughly the same strength of bite as the brown bear, but it is much more potentially damaging to their skulls if they try to bite as hard. (Polar bears rarely attack walruses, despite what you’ve seen on TV.)

The polar bear is an ice seal hunting specialist.

It is a master at living on the sea ice and stalking and digging up seals.

However, as we all know, the sea ice in the arctic is staying in place for fewer and fewer weeks every year. Although some will argue about the exact cause, we are still experiencing a warming planet.*

And that’s causing polar bears to remain land bound for more weeks out of the year.

It’s also opening up land in the North for brown bears to colonize.

And that allows the two bears to meet once again.

Brown bears will thrive in a warming climate. Not only do they have more robust skulls.  They also have larger molar teeth for really chewing plant matter. Polar bear molars are relatively small.

Bears have to learn from their mothers what to eat, and if polar bear sow doesn’t teach her cubs how eat vegetation, they will likely starve if no meat is available.

Of course, they can learn to eat vegetation as adults, but they simply are physically less adept at this diet than grizzlies are.

And that means that the holarctic brown bear will outcompete the polar bear, and those that it doesn’t outcompete, it will absorb into its gene pool.

The brown bear’s generalist diet has served it well.

But it had to specialize to colonize the arctic ice..

And because it specialized so much, the form that evolved to do so cannot survive in a world without ice– at least in its current form.

However, I don’t think the polar bear will go extinct.

I think that hybridization will become very common between polar and Holarctic brown bears, and through that hybridization, brown bear traits will be introduced back into the polar bear population.

As we’ve seen with Eastern coyotes, hybridization with wolves has introduced several beneficial traits to allow them to specialize in hunting deer in the Eastern United States and Canada.

I don’t see why this hybridization wouldn’t be beneficial to polar bears in a changing climate.

However, I think that we are still operating under the assumption that polar and Holarctic brown bears are separate species, and thus, any time they crossbreed, it’s polluting the gene pool.

I think this is a very wrong way to think about polar and brown bears, for it is exactly that same sort of Victorian typological thinking that we are forced to deal within the dog fancy.

Nature doesn’t consider them separate species. If a polar bear is in estrus and brown bear is around, they will mate, and the offspring they will produce will be fertile.

Polar bears have always existed with occasional infusions of brown bear blood, just a domestic dogs existed for thousands of years with occasional additions of wild wolf blood– and still do in many locations.

A polar bear is a good example of what happens to a generalist species when it evolves a specialized form.

This is a bit of warning to all dog people. The Anglo-Saxon dog fancy has always celebrated the specialist dog and excoriated the notion of a generalist dog. Even some of the bad science about dogs adopts this framework.

However, we have seen time and again what happens to dog breeds that have become too specialized. Like the polar bear, they become masters at one particular thing, and when that thing disappears, the dog either adapts into becoming a fancy breed, develops a new set of talents, or becomes extinct. The baiting bulldog of England became a fancy breed once bull-baiting was banned– much to its detriment. The St. John’s water dog managed to survive on in the retrievers, which are very much multipurpose dogs, but the turnspit, the little short-legged dogs that ran giant “hamster wheels” to turn spits of meat over open fires are now extinct.

Specialization is fraught with perils– whether we’re talking domestic animals or wildlife.

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*I really don’t want this post to lead to a discussion about the causes of climate change. The purpose of this post is to talk about perils of specialization, typological taxonomy, and the evolution of a specialize form of brown bear called the polar bear. I have my views on the issue, but you’re entitled to yours.  However, this is one argument that gets old very fast.