cold-blooded. They do, however, have to wake up from time to time, if only to relieve themselves, and possibly have a light snack.
Bears, on the other hand, cannot really be described as "small mammals". Yet most of them do seem to hibernate. That hibernation is, however, rather different to what, for example, hamsters, are doing. In fact, it's sufficiently different that it's been argued that bears don't really hibernate at all. To be honest, it's a matter of definitions, of what exactly you consider the word 'hibernate' to mean. Today, most zoologists seem to consider 'hibernation' to be a perfectly good word for what bears are up to, although it's certainly possible to come up with a stricter definition.
At any rate, we have to concede that it's a different kind of hibernation to that seen in other mammals. In many respects, for instance, it's not as extreme. The body temperature of bears drops by around 7°C at the most, and even then hovers back to their regular temperature every few days or so. This is hardly 'cold-blooded', since it's still a good deal warmer than their surroundings, although it is at least a step in that direction. At the same time, their metabolism slows down, until they're burning only 25 to 50% of the normal number of calories, and their heartbeat slows dramatically from the usual 55 beats per minute to as little as 9.
Which actually sounds impressive - until you realise that some hibernators can reduce their metabolism to just 1% of its normal rate! On the other hand, though, bears do not eat, drink, urinate, or defecate for the entire six months or so of their hibernation, which is rather more than most smaller hibernators can manage, and they aren't periodically waking to stretch their legs. Surprisingly, despite this lengthy snoozing, they don't suffer any significant muscle or bone atrophy, as one would expect for an animal that doesn't move for several months. It may well be that they are getting exercise without moving, presumably by some sort of muscular twitching.
But it gets weirder, because not only do bears keep themselves exercised while they're apparently asleep, they also give birth. In fact, not only does their entire 'true' pregnancy take place during hibernation, they don't properly wake up for months afterwards. Which means that, for about the first three months, they're also nursing their young as they sleep.
With regard to their giving birth while hibernating, it may help that newborn bears are remarkably small in comparison with their mothers. While most animals have an easier time giving birth than we humans do, there can't be many large mammals that have an easier time of it than bears. By the same token, though, those tiny cubs have to grow rapidly before mum wakes up, and everyone heads outside. That requires a lot of nutritious milk, all of which has to be supplied without the mother eating, or even drinking, for months on end.
The nutrients required to do that, of course, come from the mother's body fat, which she will have built up before hibernation. Pregnant mothers can lose a staggering amount of weight this way, even compared with other hibernating bears (which obviously lose quite a lot anyway). Bear milk is high in fat, and low in sugar, because the mother is likely to have a lot more of the former available, especially over such a long period, and she doesn't want low sugar levels herself.
But there is a problem here, because that won't work during the pregnancy itself. Fat doesn't cross the placenta easily, so the growing baby has no choice but to survive on sugars. At least, that's true in humans, and, if it's also true in bears they should have high blood sugar levels in pregnancy, along with some other changes that somehow allow all this to work.
In a recently published study, Michito Shimozuru and co-workers monitored the body temperature and blood chemistry of pregnant bears, and compared it with their non-pregnant counterparts. Being Japanese, the researchers chose to use Japanese black bears (Ursus thibetanus japonicus) as their test subjects.
These, it should be noted, belong to a different species to the black bears familiar to Americans (Ursus americanus), and are instead related to a number of other kinds of black-coloured bear found throughout eastern Asia. The two species are, however, very closely related, and they are somewhat similar in appearance. Perhaps the easiest way to tell them apart is the presence of large flash of white fur on the chest of the Asian species.
Asian black bears, including the Japanese subspecies, breed between May and August. However, like many members of the weasel family, the fertilised egg stops growing only shortly after it starts dividing, and then waits in the female's womb in a sort of suspended animation. This continues until about the time that the mother enters hibernation in late November, when the tiny embryo implants and starts to grow. From then on, the actual pregnancy lasts only 60 days, a remarkably short time for such a large animal. When the cubs are born around the end of January, they are still tiny, weighing around 370 grams (13 ounces).
These sort of studies have been done on bears before, but what's new about this one is that the researchers were able to compare pregnant with "pseudo-pregnant" bears. Pseudo-pregnancy in bears is probably not something that occurs much in the wild, but it can occur in captivity when the bear mates but fails to actually get pregnant.
The first finding was that body temperature in pregnant and pseudo-pregnant bears drops far less than it does in other hibernating bears. This has been seen before in other bear species, and is probably because that body heat is needed to help the foetus develop, and because of the general metabolic demands of pregnancy. It may, in fact, be part of the reason that the body temperature of bears doesn't drop as much as that of rodents when they hibernate.
That it occurs even when the bear isn't really pregnant probably indicates that it is due to the mother's hormone levels. While we don't know for certain which hormone this might be, progesterone seems a likely candidate, and this is, indeed, known to be produced in false, as well as real, pregnancies.
At the same time, as predicted, blood sugar levels went up during pregnancy. Clearly the mothers were creating sugars in their livers, presumably by breaking down glycerol or other fatty substances, and using these to provide energy to their growing young. What's particularly interesting here is that this didn't happen in the pseudo-pregnant bears, so it can't simply be the result of progesterone or some other hormone that's elevated in pregnancy. Instead, it may well be that the embryo itself is sending chemical signals to its mother to hurry up and give it sugar. When there is no embryo, therefore, nothing happens. It's worth noting that this sort of two-way communication between mother and unborn child is by no means unusual in mammals.
Rather more surprising is that pregnant bears had lower levels of urea in their blood than would otherwise be expected. That urea levels don't soar during hibernation is something of a surprise itself - it's rather what you'd expect if an animal hasn't been able to pee for six months. It seems that, in order to avoid poisoning themselves through lack of urination, bears are able to recycle their urea, using it to help maintain their muscle mass - which explains why they aren't ridiculously weak when they wake up.
But why would this be even more extreme when the bear is pregnant? The most likely explanation is that, because the bear's metabolism is working faster, they also recycle urea more quickly, so there's less of it around at any given time.
All in all, there's clearly a lot going inside a mother bear, even while she's ostensibly asleep...
[Picture by Kabacchi, from Wikimedia Commons]