Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [58]
The relevant question is how hyperthermic? No body temperature measurements of kinglets in the wild under severe (or any) winter conditions (such as -30°C and wind) are available, so we have no definite answer. The birds would predictably have a great and most urgent need of hypothermia for energetic economy at -30°C, but such regularly encountered temperatures would pose a great risk of freezing to death in birds that become too hypothermic. Birds that get too cold could become unable to respond. Not being able to shiver they might then quickly turn to ice; cooling down risks losing physiological control for being able to generate heat. The trick is to be able, like the arctic ground squirrels, to achieve a physiological state that is technically close to death, while retaining the ability to respond and come back to life on demand. Minihibernation overnight is a good strategy, but only if temperatures in the morning are high enough to allow the animal to passively heat up to the point where shivering is again possible and the bird is able to warm up quickly. Endothermic insects face the same problems as small endothermic vertebrates, but more acutely.
Consider, for example, the tomato sphinx moth (Manduca sexta), whose large familiar green larvae feed on tomato and other solanaceous plants. The nocturnal adult regulates nearly the same body temperature as does a hummingbird while in flight. After flight at, say 15°C, the moth immediately cools and within a minute or two it is torpid. In the evening, if air temperature is 30°C, it needs to shiver for less than a minute to be flight-ready again. But if air temperature is 15°C it must shiver for several minutes. If air temperature is only 5°C lower, however, then the animal is incapable of warming up at all. It would remain in torpor and assuredly freeze to death if temperatures were then lowered to below 0°C. Normally, however, summer-active moths are never subjected to such low temperatures, so they need no defense mechanism to escape death by freezing. Similarly, bats can afford to enter hypothermia to save energy, when they are within the safety net of a cool but not too cold cave. They can slip into torpor, secure that they won’t turn into a block of ice, as long as they overwinter in deep caves where temperatures don’t go below the freezing point of their tissues.
The owlet moths of the subfamily Cuculiinae that are common in New England, face the brunt of the problem of potential lethal freezing. To escape predators (bats) they are active in the winter. Their flight muscles are amazingly cold-tolerant, and they can shiver and warm up even from temperatures as low as 0°C, but they will freeze solid at near -10°C. Nevertheless, they don’t shiver to prevent themselves from cooling to lethal temperatures when subjected to temperatures approaching 0°C. Instead they seek refuge under insulating leaves and snow to avoid getting that cold. Neither the moths nor most ground squirrels, except arctic ground squirrels, are in any real danger because in the microhabitat where they hibernate they are sequestered from very low temperatures and so no alarm response to dangerously low body temperature has evolved in them. In general, few birds find secure shelter from the cold such as that afforded by deep caves or underground burrows. Winter birds may face temperature drops of from well above freezing to -30°C or colder over the course of a single night, and they simply can’t afford to relinquish body temperature control. Ironically, hummingbirds provide a conspicuous model of an adaptive response that applies to many birds in