Before hibernating, they construct a den, in which they will spend the next several months, depending on the exact weather conditions. Significantly, mothers of both species give birth in the den, often while they are still asleep. All of which works for a large, omnivorous animal living somewhere that inclement weather makes it hard to forage in winter. But that's not something that will be true of all bears.
There are, in fact, eight living species of bear. Brown bears are found right across the northern parts of the world, in Asia and Europe as well as North America, while American black bears are, as their name suggests, unique to North America. Although they can be found relatively far south, in places like Mexico and Turkey, this is mostly in the mountains, so cold weather is still a problem. The Asian black bear, however, is a different species, and it is found in southern China, Southeast Asia, and the Middle East. Across most of its range, the Asian black bear never hibernates, but they are certainly capable of it and do so in the colder parts of their range - the Himalayas, Korea, Japan, Manchuria, and the Russian Far East.
The remaining species do not hibernate at all. For four of them, this is because they just don't live far enough north for there to be any need to do so. However, in these species, as in the non-hibernating Asian black bears, females still construct dens. These are intended for shorter occupancy and, in some species, may be more obviously temporary affairs than those constructed by their northern relatives but it is still here that mothers give birth to their cubs, protecting them from the outside world until they are old enough to leave in relative safety.
The polar bear (Ursus maritimus) is, of course, in almost exactly the opposite situation. They don't need to hibernate because they feed almost entirely on seals, and they're just as much around in the winter as they are in what passes for the summer that far north. (When I started this blog, incidentally, there was still some concern that genetic evidence showed that polar bears were just a subspecies of brown bear, despite them visibly not being anything of the sort - since then, more complete studies have confirmed our common sense notions. They are different species, and may have been so for over a million years.)
Since they do not hibernate, when it comes to denning, polar bears behave in a similar manner to those species living close to the tropics. That is, only the females construct dens, and then only to raise their cubs. They dig their dens, not in soil or beneath tree branches, but into snow drifts, either on land or on Arctic ice floes. In the latter case, the den will drift with the ice, with one study showing that they travel an average of 385 km (240 miles).
Young bear cubs, like kittens and puppies, are altricial - that is, they are blind, helpless, and absolutely reliant on their mother for survival. Given this, it's unsurprising that hiding them beneath a snow drift is a good way to protect them physically from the dangers out of the outside world. But it's perhaps less obvious that, it also keeps them warm, since that isn't something we normally associate with snow. This, it turns out, is partly because there's a mother polar bear in there with the cubs, but also because warm is relative.
Studies on polar bear dens have shown that they can maintain a steady internal temperature of 0°C (32°F)... which may not seem as it's very warm from a human perspective, but, when you consider that the average January nighttime temperature on, say, Baffin Island is about -30°C (-22°F) that's quite an improvement.
Mother polar bears emerge from their dens between March and April which, not coincidentally, is when the local seals have their pups and also come out of the water to moult. By the time they emerge, the mother bear will not have eaten for up to eight months and lost nearly half her body weight, so you would imagine she's pretty hungry. And yet, rather than immediately heading off in search of edible seals, it seems that the young family group often hangs around near the den for a couple of weeks, and sometimes even a whole month.
This gives the young cubs the opportunity to head back inside to safety if something happens, but it also just lets them get used to the outside. During this period, the mother is doing relatively little - she's probably too exhausted and hungry. But the cubs are active almost all the time that they are outside, running about, playing, digging in the snow and so on. This, it seems likely, allows them to build up their strength and trekking skills for what's potentially an arduous journey to somewhere food is more plentiful.
A recently published survey looked at the behaviour of 70 polar bears after they emerged from their dens along, and on floating sea ice close to the north coast of Alaska. This was done with a combination of previously fitted radio collars and direct observation, making it possible to determine when the bears left their dens, when they left the general area to head elsewhere, and whether or not they had cubs at the time. The later the bears emerged from their dens, the less time they spent before heading off, perhaps partly because the mother would have been more hungry at that time, but maybe also because the cubs were slightly older and could adapt to the outside world more quickly.
Perhaps the most significant finding, however, is how important this gap between emergence and departure is for the young cubs. If she didn't have any cubs - perhaps because they had died before she left the den, or perhaps because she failed to give birth - the mother rarely wasted any time before heading out. Usually leaving within 48 hours, and sometimes almost immediately, it's clear that staying at the den isn't something she does for her own health.
The cubs, however, very much benefited. By the time they leave the den, they should already be a couple of months old, having been born in December or January. But, even so, just staying at the den site for an extra four days increased the chance that the cubs would survive into the spring from 50% to 90%.
This is significant because of the way our world is changing. As the ice retreats from the Arctic, it's likely that many polar bear mothers will enter their dens less fattened up than they might have been, forcing them to head out earlier after they emerge. Perhaps even more importantly, the increasing accessibility of these remote coasts means that humans are more likely to be found there, whether for industrial and transport purposes or for recreation such as snowmobiling and boating. If they are disturbed, mother bears are also more likely to leave their denning sites early, with one study finding that they cut short their time by a full five days on average.
Polar bears are already considered a "vulnerable" species. If we better consider what their requirements are, maybe there is something we can try to do about that.
[Photo by Schliebe Scott, for the US Fish and Wildlife Service. In the public domain.]
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