Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [69]
The beaver’s feet and tail stay cold and are not furred. Like the kinglet’s legs, the beaver’s and muskrat’s feet and tails have a specialized blood circulatory anatomy that helps to prevent body heat from escaping from these body parts. The principle is that if it is costly and difficult and not necessary to keep the extremities warm, then it’s better to try to economize and keep them cold. Nevertheless, despite these adaptations of keeping the body hot, rapid heat loss is still inevitable during prolonged dives into ice water.
When a muskrat leaves its lodge and submerges itself under the ice, it not only diminishes its oxygen supply, it also immediately starts to cool. As it forages, it may periodically come up to breathe at a push-up or a feeding platform, where it can replenish its oxygen supply, but it can still be losing heat at a high rate. Most voluntary dives by muskrats last less than forty seconds, although the rats can store enough oxygen to stay down for several minutes. An active rat does not, and presumably cannot, allow its body temperature of 37°C (similar to ours) to drop more than 2°C (MacArthur 1979), and the rat’s solution, like that for lack of oxygen, is to store up a surplus of heat. Just prior to diving into ice water in winter (in contrast to summer), a muskrat increases its body temperature on average by 1.2°C (MacArthur 1979). Then, after returning to the lodge, the rat shivers and expends energy at a high rate to heat itself back up to 37°C (MacArthur 1984b).
Although the rats’ physiological response suggests that their dip into cold water is anticipated, do they really plan ahead? Perhaps, but if so, then the white-faced hornets I have mentioned previously in reference to their insulated papier-mâché nests may do so as well. Some years ago, I took on the brave, or foolish, task of measuring hornets’ body temperatures, grabbing and stabbing them with an electronic thermometer as they left their nests. I learned by my experiments that prior to leaving their nests at low air temperatures (2°C), the hornets shiver to heat themselves up to 39°C. On the other hand, when they leave on a warm day, at 22°C, they warm themselves up to a body temperature of only 35°C. (When attacking, they heat all the way up to 41°C.) That is, they appear to be as clever as a muskrat, or vice versa. I suspect, however, that body temperature is much too important and constant a concern for any animal to be able to rely on mere cleverness to consistently produce the correct responses.
13
FROZEN FROGS ON ICE
In the fall after the birds have left, I often hear birdlike chirps coming from the woods, especially after it warms up for a few days. I’ve tried numerous times to sneak up carefully and identify a caller, but so far I have discovered little more than that it was hiding on or near the ground. Whenever I’ve come close enough to almost step on the sound source, it went silent and I saw nothing.
By chance I found a passage in the writings of the famous nineteenth-century American naturalist-writer John Burroughs, who succeeded in tracing apparently similar sounds. Walking in the woods in his native New York on the last day of December 1884 when it was so unusually warm that bees were flying outside their hives, he paused in the shade of a hemlock tree where he heard a froglike sound from beneath the wet leaves on the ground in front of him. Determining the exact spot from where the sound came, he lifted up the leaves and found a wood frog. “This, then” Burroughs concluded “was its hibernaculum—[where] it was prepared to pass the winter, with only a coverlid of wet matted leaves between it and zero weather.” (We now know a great deal about how amphibians survive the winter, but we still don’t know why some of the ones that have loud mating choruses only in the spring after the ice goes out, may