Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [59]
One of the noctuid winter moths at rest on a beech twig.
Nocturnal hypothermia is common in hummingbirds because of their small size, although if energy supplies are available and temperatures are not too low, the birds don’t have to resort to this option. In the black-chinned Archilochus alexandri and the Rivoli’s Eugenes fulgens, torpor is used only in an energy emergency (Hainsworth, Collins, and Wolf 1977). Similarly, in the broad-tailed hummingbird (Selasphorus platycercus), which successfully rears its young in the energetically near-marginal conditions of the Rocky Mountains, can go torpid even when incubating on the nest if energy crises result from rainstorms and low nighttime temperatures. In other hummingbirds, torpor occurs even in very fat birds. For them it serves as a mechanism to conserve their energy resources needed for migration (Carpenter and Hixon 1988). The Anna hummingbird (Calypte anna) found from northern California to Baja California, regulates its daily energy budget less by nocturnal torpor than by daily gain of energy stores, increasing its body mass by over 16 percent during the course of the day.
There is obvious benefit of torpor, provided the risk of losing physiological control in an environment where temperatures dip too low, is not too great. Some hummingbirds are unable to respond to lowering temperature by shivering if they cool down to 20°C (Withers 1977). These are species (Calypte anna and Selasphorus sasin) living where they don’t encounter temperatures lower than 20°C (southern California). Others, from colder mountain environments, regulate not only a high body temperature when active but also a low one when in torpor (Wolf and Hainsworth 1972). Two other hummingbird species (Panterpe insignis and Eugenes fulgens), from the high cool mountains of Costa Rica and western Panama, not only regulate but are capable of spontaneous arousal from body temperatures of as low as 10° to 12°C. As already mentioned, the arctic ground squirrel, a hibernator, was later shown to do the same at even much more impressively low temperatures. Some hamsters (Lyman 1948) and pocket mice (Tucker 1965) have also been observed to first allow themselves to become torpid but then retain the ability to resist cooling below a specific, much lower body temperature threshold.
One cannot predict what golden-crowned kinglets do in any specific area and under specific conditions. All we can be reasonably sure of is that they likely engage in some torpor, but very deep torpor is probably not an option. A kinglet in a windy winter night at -30°C would have to remain ever-alert. If it should stop shivering for several minutes, it would quickly freeze as solid as a teaspoon full of water.
There is, of course, a way out of the kinglet’s quandary, and that is to find a microenvironment, such as a verdin’s nest stuffed full of body warmers. However, such options are limited in the north woods. This habitat contains neither verdins nor birds who build nests like them. Kinglets are too large to burrow unnoticed into the feathers of an owl, as do hippoboscine flies in winter (and summer). And they are too small and frail to burrow into the subnivian zone under the snow like grouse do. They obviously can’t avoid freezing to death by diving under ice-crusted waters. And yet I’ve seen dippers (relatives of wrens) in midwinter in Yellowstone Park jump into the icy swift water of the Lamar River, disappear from view, and later pop up along the edge of the ice.
I’m not implying that I think even for one second that dippers, or kinglets, can stay down and hide in some crevice under the bank like a frog or a trout. There are lots of obvious reasons why they don’t, but why couldn’t they have evolved that ability? Much that animals have evolved to do would have seemed impossible to us, if experience had not taught us otherwise. And few inhabitants of the winter world have evolved more ingeniously, more bizarrely even, than turtles, frogs, and insects.
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TORPID TURTLES UNDER ICE
Snapping turtles are