Recently I watched some very small mammals scuttling through the meadow grasses, easily running under the fallen stems. They were shrews—smaller than most mice. There are about ten species of shrews in Alaska (including three on different Bering Sea islands); three species are known from Southeast. One of the local species is the water shrew, which can scurry over water surfaces using the hairy fringes on its feet. More common are the somewhat smaller dusky and cinereous shrews, which generally weigh about four or five grams (two grams equals about seven hundredths of an ounce). For purposes of comparison, a penny weighs about three grams, according to ADFG Wildlife Notebook.
Although our shrews are very small, elsewhere in Alaska there lives a rather rare species called the tiny shrew. It really earns its name, because it averages less than two grams (actually 1.8 g) in body weight.
There appears to be a limit to how small a ‘warm-blooded ‘animal can be, and it is roughly two grams. The smallest bat in the world is reportedly the hog-nosed bat (average1.9 g) of south-east Asia. The smallest bird in the world is the bee hummingbird of Cuban forests, weighing between 1.6 and 2.3 g (depending on what reference one consults). For comparison, our rufous hummer weighs about three or four grams.
Why should there be a lower size limit for warm-blooded animals –birds and mammals (and some dinosaurs, but I will ignore them for present purposes)? Being ‘warm-blooded’ means that these animals use their metabolism to generate heat and regulate body temperatures (this is termed ‘endothermy’—internally determined body temperature), and their body temperatures are normally kept rather constant. By contrast, the ‘cold-blooded ‘ animals (almost all the other animals) typically have a body temperature close to that of the environment, although they may bask in the sun to raise their temperature or retreat to a burrow to cool off; these animals are called ‘ectotherms’ (externally determined body temperature).
Metabolic regulation of body temperatures costs a lot of energy. The smaller the body size of an animal, the greater its surface area compared to its volume. And it is through the body surface that much heat is lost. To keep the body warm, a warm-blooded animal generates heat metabolically, which means that the animal has to eat a lot to fuel its metabolism. So the very small warm-bloods, with relatively great surface areas compared to volume, have high metabolic rates and may have to eat several times their own body weight in food every day. (Imagine a one-hundred pound person eating four or five hundred pounds of food every day….). Shrews are voracious, consuming insects and spiders and even mice that are larger than themselves, and they stay active all day and all year. Hummingbirds, on the other hand, can conserve some energy by becoming torpid at times: reducing their metabolic rate and regulating the temperature at a somewhat lower level than when they are active.
These basic facts of warm-blooded life for small animals raise several questions. For example, why can hummingbirds use torpor to conserve energy but shrews (apparently) do not? Also, small mammals and many birds are born at a very tiny size, much smaller than their parents, and only gradually develop the ability to thermoregulate, depending for some time on their parent(s) for body warmth. Must they get close to the two-gram size limit before thermoregulation becomes possible? That would mean, for instance, that young bee hummingbirds, tiny shrews, and hog-nosed bats don’t become really warm-blooded until they reach adult size. What are the consequences of that for parental behavior?
And very basically, why is approximately two grams the lower size limit? Why not one gram?
Is the metabolic cost of keeping warm the only factor contributing to this apparent limit? Could there be a limit to the rate at which food-derived fuel can be delivered to the cells where energy is generated? (this could preclude the even higher metabolic rates that would be needed at a still smaller body size.) Perhaps there are some other factors too, such as the problem of packing vertebrate-style functional digestive organs, circulatory system, and brain into a very small package.
As usual, lots of questions—thinking about them is part of the fun.