Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [76]
The behavior of the turtles, as such, may not seem unique or surprising, except when seen in terms of the caterpillars’ behavior. The three caterpillars remained on my desk in a jar in moist moss and green grass (one of their food plants) retrieved from under the snow in a field. I had expected them, like the turtles, to immediately start wandering and feeding on the grass when they warmed up. They didn’t. They moved only enough to crawl under the moss and, even while experiencing the warmth of my study, to curl up again in the same hibernation posture that I had found them in. One month later the woolly bears had not moved. They seemed dead. Suddenly, in the last week of March, they all encased themselves in lightly spun cocoons (that incorporate the spiny hairs that they shed). When I tested the frost-hardiness of the pupae by subjecting them to moderately low temperature (-14°C in the freezer compartment of our refrigerator) they froze solid and were dead.
It had previously been reported that cold-hardened woolly bear caterpillars remain unfrozen, even down to about -30°C, through a combination of supercooling and antifreeze. Low temperatures in the fall were reported to stimulate them to convert their glycogen stores into glycerol and sorbitol, and the amounts of these alcohols (up to 5 percent body weight) reduced the freezing point of their blood to about -10°C, and the rest—the prevention of ice formation of the whole animal down to -30°C—was presumably due to supercooling. Could this really be true for the New England population? I recalled having sometimes found woolly bear caterpillars in outdoor winter woodpiles in Vermont, and although I had no reason to study them, I did have the impression that they were occasionally hard frozen. But I didn’t test if they were dead.
I wanted to put my woolly bear caterpillars to a test, and when I found two of them just out of hibernation (the following spring as I was writing this on Easter weekend) I put them (as previously the pupae) into a film vial and to -14°C in the freezer compartment of our refrigerator. Two hours later they were indeed quick-frozen into blocks of ice. They were solid. I could tap the table with them. When thawed out an hour later, they were alive and well!
This had been a severe test, since the caterpillars had already spontaneously aroused from hibernation and since freezing-survival (as I’ll show later) requires slow freezing. Not believing my senses, I immediately repeated the experiment with the same two caterpillars. The result was the same: Woolly bear caterpillars (like the aforementioned geometrical caterpillars that the kinglets eat) do survive freezing—even multiple freezing—whereas the pupae don’t. No wonder my caterpillars had waited so long to pupate after coming out of hibernation.
Cecropia moth cocoon on red maple twig.
Sawfly cocoon (left) on beaked hazel twig with male catkin bud (right).
Another woolly caterpillar, Gynaephora groenlandica (unrelated to the tiger moth), that lives in the High Arctic, has no chance to escape freezing solid. It is routinely subjected to the very much lower temperatures (to near -70°C) on the tundra where snow cover is thin and the ground is permafrosted. This species is one of the very few moths that has evolved to live within 83 degrees of the North Pole. During the short arctic summer the caterpillars briefly thaw out and feed. Because temperatures are low even then, they spend most of the year frozen solid. They only grow slightly in any one year before again freezing solid and their freezing-thawing cycle is repeated thirteen to fourteen years before they are finally ready to spin a cocoon on an exposed rock to catch direct sunlight for heating. They then molt, first into a pupa and then into adult moths that mate, lay eggs, and die a few days later.
The Gynaephora