Sunday, 22 May 2011

Cold-blooded Mammals

Short-eared elephant shrew, another member of the family
Mammals are warm-blooded animals. That is, they maintain a consistent body temperature regardless of, and usually higher than, that of their surroundings. It is, after all, one of the defining attributes of being a mammal. Yet, while virtually all healthy mammals have no real difficulty in keeping their body temperature high, a great many of them don't always bother.

Probably the best known example of this is hibernation. Many animals living in climates with cold winters enter a long "sleep" when their food becomes scarce, and don't wake up until the following spring. But this isn't sleep of the sort that humans experience daily, but something much more dramatic. Their whole body shuts down to a minimal level of activity, and body temperature drops to only a fraction above that of their surroundings - essentially, they become cold-blooded. Of course, unlike reptiles and amphibians, they aren't doing anything much while they are in this state, so we shouldn't draw too many parallels, but they can't be truly said to be warm-blooded at these times, either.

Hibernating animals don't actually spend the entire winter inactive. Every few weeks or so, they wake up for a few hours before returning to hibernation. Even then, they often don't eat, surviving off fat reserves they have built up in the autumn, although its clear that they've had to wake up to do something. Quite what isn't immediately clear, although there have been a number of suggestions. Maybe its to do with their water balance, or perhaps its their immune system - and, of course, there's no obvious reason why it couldn't be more than one thing.

Nonetheless, the ability to remain completely inactive for weeks at a time is quite impressive. But hibernating mammals are not the only ones that can partially shut down their bodily functions. Many animals do much the same thing on a daily basis, entering an inactive, near cold-blooded, state for just a few hours at a time, especially during long winter nights. Although this does tend to happen when the weather is cold and food is scarce, this form of daily torpor is not quite the same as hibernation, and is usually much less dramatic.

The body temperature of animals that enter daily torpor does drop dramatically when they do so, although not as far as true hibernators (which can reach 0°C), and is generally rather higher than the outside temperature. So, not exactly cold-blooded, but at least lukewarm-blooded. The fact that they still have to spend much of the day awake, and that the torpor - while requiring far less energy than mere sleep - isn't as extreme as true hibernation means that such animals do still have to eat, albeit rather less than usual. It helps save energy at times of hardship, but not that much.

In a recent paper, South African researcher Fritz Geiser examined torpor in the Cape elephant shrew (Elephantulus edwardii). The elephant shrews are an odd family of long-nosed insect-eating mammals found predominantly in southern Africa. Despite the name, they are not closely related to the shrews familiar in places such as Europe and North America - ironically, it turns out that they're closer to elephants. Not, it has to be said, that they're especially close to elephants, either, and, while they are usually placed in an order of mammals separate from all others, their closest relatives appear to be other fairly obscure small African insectivores. To avoid confusion with actual shrews, they are sometimes called "sengis" instead, but I'll stick with what seems to be, for the time being, the more popular name.

Elephant shrews      Tenrecs      Golden moles
      ^                 ^               ^
      |                 |               |
      |                 |               |        Elephants, etc.
      |                 -----------------               ^
      |                         |                       |
      |                         |                       |
      ---------------------------                       |
                  |                                     |
                  |                                     |
                  ---------------------------------------
                                    |
                                    |


The relationships shown above are by no means universally accepted, with many other proposals having been made based on various factors. For instance, golden moles may be closer to elephant shrews than they are to tenrecs, and, while the aardvark should probably be on here somewhere, its not at all clear where.

The cape elephant shrew lives in dry rocky regions across much of South Africa, an area that might well be expected to run short on food at times of the year, although it had been previously thought not to undergo torpor at all. Clearly, they don't hibernate, since they don't put on any particular weight during the autumn, and are relatively easy to find and capture during the winter. To see whether they can go into torpor for shorter periods, the experimenters took some elephant shrews to their laboratory and lowered the temperature over several days to see what happened.

If the elephant shrews were like most other animals that can experience daily torpor, what you would expect is that, for a few hours each day, the animals' body temperatures would plummet as they conserved energy, and then rapidly return to normal. But that wasn't quite what happened. It seems that not only are cape elephant shrews capable of daily torpor, but they're remarkably good at it, compared with most other mammals that have been studied.

As the temperature of the air dropped, so the elephant shrews became more and more likely to become torpid, until by the time it had reached 9°C, they spent at least some time every day in this condition. That's pretty much what you'd expect, but what wasn't was how deep the torpor was and how long it lasted. The normal body temperature of these animals is around 38°C, but, when fully torpid, it dropped to less than one degree above the ambient temperature of the room. That wouldn't be so unusual for a true hibernating animal, but it's remarkably cold-blooded for one that only does it for a few hours at a time.

Indeed, while the animals certainly couldn't be said to be properly hibernating, some of them were able to maintain their torpid state for more than just a few hours. Around ten hours a night was fairly common, but, once it got really cold, at times they managed over 24 hours at a time, with the longest being 44 hours. 44 hours is nothing to a real hibernator, but its unusual for any animal that can enter and leave torpor on a daily basis to manage more than twelve. In short, cape elephant shrews appear to be about half way between regular hibernators and those that experience normal daily torpor.

One might well ask why it doesn't bother to go the whole distance, and hibernate properly. The main problem the elephant shrews have here is that they don't seem to be able to put on large amounts of fat at a time, which means that they simply have to wake up to feed at fairly regular intervals. Fortunately, a South African winter is much less cold than one in, say, northern Europe. It rarely snows heavily, and many of the insects that elephant shrews like to eat - such as ants and termites - don't disappear in the winter. Some animals simply wouldn't get the chance to eat in the dead of winter, but the cape elephant shrew, while it might find things difficult, still has the opportunity. By sleeping for a long period of time, it can drastically reduce its need for calories, while still allowing it to go out to find food every now and then.

Another oddity was that, on those occasions when they went into torpor for over 24 hours, they did so much more quickly (that is, their body temperature dropped more rapidly) than when it was just for shorter times. The reason that's surprising is that it's generally assumed that how long an animal spends in torpor depends on such things as what the weather happens to do while its asleep. But, in this case, the animals must have "known" in advance that they were in for a really long kip, suggesting they have more control over their bodily functions than is often supposed. Indeed, they were more likely to have a long bout of torpor if they hadn't been fed recently, suggesting that it was a response, at least in part, to the need to conserve energy.


[Picture from Wikimedia Commons. Cladogram adapted from Seiffert 2007]

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