Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [62]
A PROBLEM THE TURTLES and other winter water dwellers face that is closely related to food or energy supply, is oxygen supply. All turtles breathe air with lungs, but many species spend the entire winter without the opportunity to take a single breath of air, since they remain locked in under the ice. I have on rare occasions seen a turtle rowing along in slow motion just under the clear ice of an early October or November freeze-up. The turtles’ longest dives of the year are then just beginning, or may have already been in progress for a month. The duration of a turtle’s dive is determined by its ability to get oxygen from the environment and by its ability to accumulate an oxygen debt in its tissues. For dives of a winter duration in an air breather, that precludes engaging in vigorous, or almost any exercise, which snappers manage nicely.
The first sheet of clear transparent ice that covers the pond still allows sunlight to penetrate to the pond plants, and their photosynthesis still produces oxygen at low rates. The oxygen they give off dissolves in the water and is sealed in. As snow later covers the ice, less light penetrates to power the plants and their vegetative portions die. Now they start to rot, and to remove oxygen from the water.
Low water-temperature is an advantage to many organisms, because cold water absorbs and retains more oxygen than warm water. Most of the active water animals have gills to take in that oxygen. The aquatic insect larvae have them—dragonflies, damselflies, caddisflies, stoneflies, mayflies—as do the overwintering tadpoles of green frogs, bullfrogs, and some others. However, none of the few adult insects living underwater have gills, even those which are fully aquatic. That’s probably because the adults have only secondarily invaded the water, and their air-breathing mechanisms have continued with special adjustments added for life in the water. As adults, they were evolutionarily locked in to air-breathing.
Diving beetle adults and aquatic bugs carry air down with them. The predacious diving beetle Dytiscus, which captures tadpoles and small fish (and one of whose larvae killed my snapping turtle), carries a bubble of air hidden under its wing covers that it may expose to the water so that oxygen can diffuse in. Some other beetles, Hydrophilidae, and the back-swimming bug Notonecta, have their ventral body surface covered with a thin film of air (called a plastron) that shines silvery in the water. Like the Dytiscid’s air bubble, this air layer is connected to their air-using tracheal system, and oxygen used up from the air film attached to their bodies is replaced by oxygen from the water passively following the concentration gradient. As a result they can stay active even as ice covers the water.
The water under the ice provides an ideal environment for the animals that can breathe there. It is the one assured refuge from freezing, and many predators are excluded. For centuries it was presumed that birds spent the winter there. Not knowing much about bird biology and evolutionary constraints, it could long ago have seemed logical to observers that the swallows in the fall that skimmed closely over the water surface would spend the winter in the mud under the ice, since frogs, salamanders, and the myriad insects that emerge as adults from the water in the spring fly off and often live far from water. Of course, birds don’t hibernate in the mud, and it is not because an impossible physical hurdle stands in the way for evolution of such capacity. The major problem is probably evolutionary inertia. You can’t convert a jet plane to a prop plane, or vice versa. But what you can do is improve each, up to a point. As with the adult air-breathing insects, birds are historically locked into air-breathing. You can’t just make them water-breathing. And turtles?
Present-day turtles include the already-aquatic species that are evolutionarily predisposed or preadapted for life in the water. Even