Why Polar Bears Still Make Dens

Hibernation is something that's generally associated with smaller animals, such as bats and rodents. It's likely to be a particularly useful trait for them, since they otherwise need to eat regularly to survive and this would be difficult in winter. Bears are a clear exception to this, snoozing away the winter despite being particularly large animals, and it seems that hibernation has been a key feature of their evolution for some time, distinguishing them from other carnivorans, such as wolves, that live in similar climes.

One of the first things we have to point out in this respect is that it isn't universally agreed by scientists that bears do, in fact, hibernate. This is obviously a debate about terminology, rather than the bear's actual behaviour, and it hinges on whether a bear's body temperature drops enough while it's denning for the activity to be truly considered as "hibernation". It seems fair to say that the usual tendency these days is to consider that what they're doing is close enough to count, but not everyone agrees. For today's purposes, however, I'm going to use what some might argue is a layman's understanding, rather than a strict biological term.

The bears most associated with hibernating are probably the brown (Ursus artctos) and American black (U. americanos) species, the former spread across most of the temperate zones of the northern hemisphere, the latter across most of North America. The longest hibernation time for a bear reported in the scientific literature is apparently one referring to American black bears in southern Alaska, clocking in at 212 days, with the local brown bears said to be sleeping for almost as long.

But these are extremes, due to the equally extreme climate. Brown bears in some parts of Spain may not hibernate at all, and even as far north as Kodiak Island, some may skip particularly mild winters. Similarly, American black bears in are well-known not to hibernate in the southern parts of their range. This suggests a fair degree of adaptability, with the bears able to tailor their annual activity to the local conditions. This may have helped them recolonise Europe more rapidly than any other carnivore after the ice sheets retreated at the end of the Last Ice Age, as well as their relatively rapid colonisation of North America.

How closely related brown and American black bears are to one another is not entirely resolved. This is partly because many of the modern species of bear seem to have arisen over a comparatively short span of time during the Ice Ages, making it difficult to tell in which order the various splits occur. Early studies based primarily on mitochondrial DNA tended to support the idea that the American black bear's closest relative is the Asiatic black bear (U. thibetanus) but more recent ones adding in a wider number of nuclear genes favour the brown and American black species being much more closely related, as do those using the Y chromosome.

In fact, Asiatic black bears, which live from Iran to Japan and the Russian Far East, have denning habits similar to those of their cousins, so, whichever of the two schemes is correct, it's perhaps unsurprising that the brown and American black bears should also behave in much the same way. Moreover, we have evidence from the isotopic composition of their bones that the extinct cave bears (U. spelaeus) of Europe also hibernated - which, in fact, is probably what they were doing in the caves where their fossils are commonly found. And these, too, are known to be very close relatives of at least the brown bears and probably also the American black.

But one thing that essentially all genetic studies agree on is that the closest relative of the brown bear isn't one of the black bears, but the polar bear (U. maritimus). Indeed, the two species may have parted company as recently as 620,000 years ago, implying a very rapid series of physical changes as well as adaptations to extreme cold and to an entirely carnivorous, seal-based, diet.

But, despite being such close relatives to three species that do all hibernate in much the same way, polar bears don't do so. Right?

Well, yes, but perhaps it's not quite that simple. It is certainly true that polar bears don't hibernate even in the sense that brown and black bears do; they don't go to sleep for a long time whenever it gets cold and snowy because... well, the obvious reason. But they can reduce their metabolic rate when they are low on food, being able to boost it back up again when they find a large and tasty fat-rich seal to eat (which can keep them fed for about a week). This is far less effective than the hibernation of brown and black bears, although sometimes the polar bear does go as far as constructing a den to shelter in, since they are still at least somewhat active, but it evidently helps during times of shortage.

Which, in their case, is more likely to be in the summer when the ice is melting and they can't hunt in the usual way than it is during the winter.

But hibernation isn't only important to brown and black bears as a means to survive the winter. It's also when they give birth. Thus, even in places where males and non-pregnant females don't bother to hibernate, pregnant females have to do so, albeit for much shorter periods than they would otherwise. 

Pregnant polar bears also retreat into dens before they have their cubs, and this does occur in winter, just as it does for their close relatives - presumably so that the cubs emerge in the spring when the weather is less inclement. Furthermore, while they are doing this, the mother polar bears do reduce their metabolism, as they necessarily must do, given that they aren't eating for up to eight months. This, seemingly, is something built into their genetics, and it's natural to wonder how far back it goes in the bear family tree.

Sun bears and sloth bears are tropical species, the former living in Southeast Asia, and the latter on the Indian subcontinent. Neither species hibernates, and neither does the more distantly related spectacled bear of the tropical Andes. But pregnant sloth bears do construct underground maternity dens, just like more northerly bear species. They don't seem to go to sleep after they do this, but they do apparently go for weeks without food, ensuring that they don't leave the newborn cubs alone and unprotected. 

While formal studies are few and far between, spectacled bears have been reported to behave similarly in the wild, and sun bears at least make the attempt in captivity. Going even further down the evolutionary tree, giant pandas also fast for around three weeks after giving birth although, again, they don't hibernate in any meaningful sense.

This suggests that denning, as a habit among bears, probably predates the split between the spectacled bears and all of the other non-panda species, and that the general method of reproduction, with newborn bear cubs being extremely small vulnerable, and the mother compelled to put aside her own feeding to look after them, may be earlier still. 

Indeed, there is some fossil evidence that the giant short-faced bear (Arctotherium angustidens) of Ice Age South America at least reduced their metabolism prior to giving birth as brown, black, and polar bears do today. It, however, belongs to the spectacled bear lineage, which potentially places that as originating not long after the split with the pandas. 

Dating the origin of winter hibernation is trickier, although if Asiatic black bears (which hibernate) are really more closely related to sun and sloth bears (which don't) than to the American black, it's likely that the two tropical Asian species lost the ability when they moved south to warmer climes, rather than never having had it in the first place. Alternatively, it could have arisen twice, as the arrival of the Ice Ages compelled bears to co-opt a metabolic process they already had available for a different purpose.

[Photo by Alexander Lessier, from Wikimedia Commons.]