Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [67]
Water is essential for the beaver’s winter food supply. In the fall, beavers get busy, working, well, like beavers are supposed to. The whole family pitches in and fells trees in the nearby forest. Some of the mature poplars they have felled near my house measure up to 53 inches in circumference, but thankfully the animals preferentially harvest young, fast-growing trees that are dragged away whole.
Trees are limbed and the limbs dragged into the water and then floated out to near the lodge. By freeze-up the beavers have accumulated a brush pile of hundreds of pounds next to their lodge. It presses into the pond bottom and the top of the pile sticks out of the water. Foot-thick, concrete-hard walls keep the occupants safe and warm in the deepest cold. After freeze-up, whenever a beaver needs to feed, it must exit its lodge underwater and swim out to the food cache to bring back sticks to feed from. During such dives, as in other divers, the beavers’ heart rate drops and energy expenditure is reduced to prolong diving duration (to about fifteen minutes).
Cross section of a new dam and lodge with family quarters.
The food cache, though large in size, is generally short in useable calories. Like many other herbivores, including termites and cockroaches, beavers compensate by harnessing cellulose-digesting bacteria in the gut. And then they recycle their rich cellulose diet once more, by eating their fresh feces. Despite all this, they also lay up body fat in the fall. Finally, adults (but not growing kits) save energy in the winter by tolerating greater amplitude of body temperature fluctuations, and reducing mean body temperature by about 1°C (Smith et al. 1991; Smith, Drummer, and Peterson 1994).
After the beaver family is imprisoned under the ice and in the lodge, the only source of air they have access to is through a small vent-hole at the top, which lets in air through a latticework of thick sticks.
The lodge so prominently visible to me near the opposite shore, beyond the hunkering muskrats, would soon be frozen in. What might it be like to spend months huddled in near-absolute darkness, except for a perhaps once-daily dive to feed in the snow-covered, inky, ice-cold water? I surmised that, having adapted to these conditions, beavers could not be too unhappy in them. They are like muskrats, probably placid creatures, and likely not as claustrophobic as I.
Beavers and muskrats are small-eyed primarily nocturnal animals, and their activity is governed by a circadian schedule. We normally start our daily activity when darkness changes to light. But a beaver or muskrat becomes active by leaving its already-dark lodge in the dark of the night or just before dark. How does it know that it is time to get up and going? The proximate answer is that as in us and flying squirrels their circadian rhythms alert them when it is the right time. However, like cheap watches, biological clocks would eventually get out of phase with the day/night cycle. To be useful, they have to be reset periodically by some cue, to synchronize with the external environment. We set our biological clocks using the lights-on transition as a reference signal. Without such a signal, our internal rhythm would gradually bring us out of phase with the external environment, or we would be unable to readjust when we pass into another time zone. Beavers in the winter can apparently lose contact with external light-dark signals, and their activity rhythm, which is slightly longer than twenty-four hours, starts to free-run. As in DeCoursey’s experiments with flying squirrels in constant darkness, each animal gets