Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [84]
In the field, Zapus builds aboveground summer nests of closely woven fine grass and dry leaves, with a small difficult-to-find entrance to the side. In the fall the mice leave these summer nests and burrow into the ground, where they hibernate. In captivity, mice began digging and making underground nests for hibernation during the last part of September, when temperatures dropped to 5°C. At first the mice’s periods of torpor lasted only a few hours: They awoke frequently, for a few hours and then a few days, before settling in to prolonged torpor of more than two or three days at a time. In Ithaca, New York, hibernating mice have been dug up as late as the end of April (Hamilton 1935), and Sheldon (1938a) even found them in torpor in her live traps in the last part of May. Napaeozapus also “became sleepy” in September and October, and like Zapus they then also left their aboveground nests and built new underground nests for hibernation. Like deer mice, jumping mice find new quarters in the winter. Theirs, thankfully, are not with me.
16
SUPERCOOL(ED) HOUSEGUESTS (WITH AND WITHOUT ANTIFREEZE)
The miceapparently get inside through tiny holes and by chewing and pulling out the oakum fiber plugs between logs. In this they may be aided by the stronger red and flying squirrels, which also pull out the oakum for use in their nests. Whether independently or working together the mammals ultimately provide the main means of entrance for a crowd of insects.
The crowd is always snug inside our Maine cabin, come winter. It consists mostly of cluster flies. According to Harold Oldroyd, who wrote the fly bible, the 1964 Natural History of Flies, there are several species of these robust members of the genus Pollenia. Most of them are several times the size of the more familiar housefly. Pollenia are calliphorid, or “flesh” flies. The larvae of North American native species eat the flesh of dead animals, but the most common flies in the cabin, Pollenia rudis, were introduced from Europe and their larvae parasitize earthworms; they eat them alive from the inside out. Big and bristly, these flies are not handsome, like the shiny metallic green-and-blue native Pollenia, which never enter the cabin. Already in the fall the Pollenia rudis perch in crowds on the logs outside the cabin and sun themselves. When it starts to cool, they slip through the cracks. By November most have made their way inside, but at that time they remain unobtrusive unless I build a roaring fire in the woodstove. Then within minutes they come poking out of the cracks and crevices, and if it is still daylight hundreds or thousands gather in buzzing masses at each of the eight windows making a collective hiss. They apparently perceive the warmth as the return of spring and take it as their signal to try to leave by flying directly to the light at the windows. If given a chance to exit, at least they do not try to stay on, unlike some other previously mentioned guests. Even on the coldest days they rush out instantly when I open the windows wide but they can fly only a short distance before the cold grips them and they plummet immobile to the snow. I’ve captured hundreds of the flies at my windows, painted them with dabs of red paint, and then released them outside to see if they return. When I used my Dustbuster a few days later to vacuum them up by the cupfuls at the windows, I got mostly new ones but there were some returnees.
I’ve become curious about their overwintering adaptations because I haven’t found any of them in the wild. Temperatures can dip to -30°C outside (and the cabin is unheated for most of the winter).