Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)
ICED-IN WATER RODENTS
The snapping and painted turtles that come in June to lay their eggs in the sun-warmed sand in our driveway would not be here were it not for the beavers, whose dam meanders across a valley between wooded hills. The beavers predate all of us humans in this landscape, having been here for thousands of years, except when they were once temporarily driven out by trappers due to a fashion in hats in Europe. Their dam remained. Parts of it are ancient. It has likely been broken and torn out thousands of times, but it will always be repaired or rebuilt. This dam holds a shallow pond of several acres, which is where the turtles come from in June, and also where they return in the winter to be safely under the ice. Without beavers this would be unbroken forest. There would be no painted or snapping turtles, no bullfrogs, green frogs, mallards, Canada geese, dragonflies, giant predacious water beetles, snipe, Virginia rails, willow flycatchers, yellow warblers, red-winged blackbirds, sunfish, minnows, catfish, kingfishers, great blue herons, mink, or muskrats. It is a rare day that I do not pause at this beaver bog to soak up the marvels. I record what I see to keep it for later. The following is a journal entry on the day before the final freeze-up.
9 Dec. 2001
It was 20 F yesterday morning under clear blue skies, but by afternoon high clouds were drifting in. Very promising—snow is surely on the way.
I wake up in the night, look out, and see white ground. I can not sleep any more as I anticipate a dawn with the first real, powdery snow, after the slush we’ve had so far. I get up, grab a cup of coffee, and head down the driveway to the beaver pond at 6 am. Since nights now last almost 15 hours, it is still night when I get there.
Fine feathery snow crystals drift down. There is not a breath of moving air. The sharp clean smell of this new snow prickles my senses and excites. Within a minute I stand at the edge of the pond feeling peace, and just barely hearing the tinkling of snow crystals falling on my jacket. They amplify the stillness.
The freeze-up is late this year. The pond was only covered yesterday with its first thin sheet of ice. Before that its surface reflected the dark shadows of the surrounding pines, now light from the moon that is barely visible through thinning clouds illuminates a white expanse except for two dark bare patches of water. There the snow is wetted into a layer of gray slush. One patch is around the beaver lodge at the opposite side of the pond near the dam, and the other patch is by the mound of decaying cattails where the geese nested in the spring. I can just barely make out two black lumps along the edge of this slush-patch. They look like stumps, but I don’t remember ever seeing any stumps near there. They would not attract my attention, except that they have no white topping of fresh snow on them. There is snow on every blade of cattail leaf, and on every twig of alder and arrowwood bushes around me.
Nothing has changed twenty minutes later. It is still dark, and I’ve not heard a peep of a bird, nor seen a wiggle of the two black shapes although I imagined that one moved slightly. But in the dark, under gently falling snow, one can all-too-easily imagine all sorts of things, wonder about them, and come to absurd conclusions based on unrestrained imagining. After another 20 minutes, with the first lightening on the eastern horizon, come the first calls of goldfinches. In the daytime I recently had seen a flock of about 80 feeding on white birch seeds. The birds stayed closely bunched, and like one organism they synchronously and erratically flew off the tree, circled, and alighted again at the same tree. They hung like Christmas ornaments from the twigs and twirled around them to extract the seeds out of the fruiting cones. Showers of cone bracts drifted down and peppered the snow on the ground. The goldfinches are now, in the dawn, flying in search of another fruited birch, to begin their daily work of fattening up to fuel them through the next night.
There is a sudden whirr of wings from a heavy bird behind me. It is probably a grouse coming out of cover in the thick pines to feed on buds up in the bare branches of a poplar or birch. The snow is not yet deep enough to tunnel into to hide and stay warm.
A movement catches my eye: A long black shape comes loping onto the pond from the edge of the cattail patch. A mink. After coming out onto the pond, the water weasel lopes over to the larger of the two muskrat lodges, briefly examines it, and continues on to the next one nearby, the smaller one.
I hear a musical twitter of a flock of snow buntings who are flying by, high overhead. They’ve come from north of Hudson Bay. They are the snow birds, and winter is not far behind. Two crows fly by, cawing loudly. The mink pauses a few seconds along-side the muskrat lodge, then runs on across the pond, past the two still immobile dark shapes along the slush hole, and then on into the cattails on the other side.
I stand as still as before. I’m mesmerized. It is getting light now. The snowflakes continue their soothing rustle on my jacket. A raven croaks in the distance, from where it comes every morning at dawn. Tree sparrows, migrants from the Arctic tundra, stopped in the bog a few days ago to fatten up on seeds. Their sweet-sounding sing-song notes resound back and forth, as they stay hidden close to the ground under the alders at the pond edge. They’ll be gone in a few days. Suddenly I hear a tinkling-rustling sound right next to me. I look down and see a blade of sedge wiggle. A tiny load of snow slides off. A flash of movement. A moving black dot. It’s the eye of an immaculately white weasel. The weasel disappears, reappears somewhere else as if by magic from under snow-covered grass. I see now a fresh mouse track. Probably deer mouse, given the tail-drag and the long stride. Flushed prey? The weasel dashes from one clump of grass to another, crossing the mouse’s trail. The weasel stands up, extending its slender six inch body toward a tiny rustle, looks in that direction, then dashes off. In seconds it is back, standing tall and looking at me. Fearless, focused, improbably alert, and powered with unbounded restless energy, it soon again disappears from sight. (Following its track, I later found where it had dragged something, leaving drops of blood on the snow.)
I’m becoming ever-more curious about those two immobile black lumps on the ice. Are they the heads of otters peering up out of the water? Two miniature hunched-up beavers? Maybe muskrats.
It is light, finally, and one of the lumps slides into the slushy water. Muskrat it is. Its companion stays put. Within about one minute the diver is back, sculls briefly around the slush pool, and hauls out next to its companion. It sits up and preens its fur. The other looks around, and then also slips down for a dip. Their lodge looking like a miniature beaver lodge, is within a few seconds’ swim under the ice, but the rats stay outside in the open on the ice. (I found them here off and on for the rest of the day whenever I checked.)
By the next day the pond was totally frozen over. The entrance to the rat’s mud and cattail castle, of soon-to-be-rock-solid cattail-reinforced ice, was under water. It had been the rats’ last chance to see daylight. Now they would be sealed in for almost half the year. Only the warm spring sun melting the ice will eventually release them. I suspect they have no notion of what lies ahead in the months to come—and for that matter, neither do I.
Usually when I gaze over the pond in the weeks and months after the ice has sealed it in, I see only the reminders of the pulsating life it supports and harbors. I see the sedges along the edge that in late April or May shoot up like sharp green lances. In winter they are bent into mounds and weighted down with snow. The cattails that hide the deep nests of red-wings cradling light blue eggs with purple squiggles are limp and lifeless. The nests of cedar waxwings, catbirds, goldfinches, and kingbirds are long abandoned and exposed on the leafless arrowwood bushes. They will soon fall to the ground and be reclaimed by the soil. However, three conspicuous structures rising from the water are ready for life. They are the shelters of water rodents, ready for occupancy. They were made for overwintering (see Chapter 5, “Nests and Dens”).
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 more out of synch with every passing day (Bovet and Oertli 1974). That is, the beavers probably experience constant night. Of course this makes no practical difference to them in the perpetually dark, safe world under the ice. A schedule is then irrelevant.
Muskrats presumably experience a constantly dark winter world similar to that of beavers, and they have evolved to solve the same problems of energy shortage and keeping warm. But muskrats don’t build dams. The two I had watched at dawn depend on the water provided by beavers. They build a house specifically for winter. I had seen two in front of me near a patch of cattails, and the one closest to me, like most others, is a conical, two-foot-high pile of dried cattail leaves scraped into a heap and patched together with mud dredged up from the bottom of the shallow water where it is built.
The muskrat’s house is a partial solution to extreme cold as studied in detail by Robert A. MacArthur from the Zoology Department of the University of Manitoba. Muskrats also huddle, but weighing only about 2 pounds to the beaver’s 40 to 60 pounds, they lose heat more easily and their need to huddle is greater. That need for heat is met by becoming more tolerant for fellow muskrats. Even nonkin may gather together in a lodge and thereby gain several advantages. They warm the lodge, huddle (Bazin and MacArthur 1992), groom each other, and apparently stimulate each other to go out and forage (MacArthur, Humphries, and Jeske 1997). Beyond that, the muskrats rely on physiology to a greater extent than beavers do, making up for what they don’t solve by behavior.
Like beavers, muskrats are nonhibernators that maintain a high body temperature and thus need continuous fuel for their high metabolism. Unlike northern beavers, however, they don’t collect a cache of food prior to winter (nor do southern beavers). They are therefore forced to continue foraging. They feed on plants on the pond bottom, but being locked in under the ice presents problems for a warm-blooded air-breather. Unlike the torpid turtles, they need to inhale oxygen, and a lot of it, because swimming is hard exercise. Their only source of oxygen may be in the lodge, where its concentration can be low and that of carbon dioxide high, due to the respiratory needs of fellow lodge occupants and slow rate of gas diffusion through the solidly frozen lodge walls.
Muskrats have solved their problem of access to oxygen like other mammals that dive for a living. But they do it better specifically in the winter (MacArthur 1984b). In the winter muskrats carry more oxygen in the blood by increasing the number of red blood cells with their oxygen-binding molecules, hemoglobin. They also have stores of oxygen in the muscles, where it is held by a special protein, myoglobin. Myoglobin is the oxygen-binding protein that colors meat red. With 42 percent more oxygen stores in the body in winter than in summer (MacArthur 1984b; 1992a), muskrats gain more underwater foraging time and/or foraging range. Additional foraging range is achieved by making feeding shelters in the fall that look like miniature lodges, reaching about a foot above the water surface. I suspected that the second, smaller, muskrat lodge that I saw was one of these. Muskrats may also make “push-ups” of vegetation later on, on the ice where they can find cracks (MacArthur 1979). Both kinds of shelters are built within swimming range of the main lodge and serve like the breathing holes that arctic seals maintain. Here they can come up for a breath of air or to feed on roots brought up from the bottom. Additionally, they may exhale air bubbles that get trapped under the ice, and air from these bubbles can later be used to extend dive durations (MacArthur 1992b).
Limits of underwater foraging in winter also depend on temperature. Beavers and muskrats are of a select group of animals that can swim in ice water because of their extraordinary fur, which keeps their skin dry by trapping a layer of air next to their skin. This insulative air layer, which solves some of the problem, is the result of a luxuriantly thin and fuzzy underfur and highly specialized thicker “guard hair” that is long and glossy and protects the underfur (felted into hats, it almost spelled the beaver’s total demise).
The beaver’s feet and tail stay cold and are not furred. Like the kinglet’s legs, the beaver’s and muskrat’s feet and tails have a specialized blood circulatory anatomy that helps to prevent body heat from escaping from these body parts. The principle is that if it is costly and difficult and not necessary to keep the extremities warm, then it’s better to try to economize and keep them cold. Nevertheless, despite these adaptations of keeping the body hot, rapid heat loss is still inevitable during prolonged dives into ice water.
When a muskrat leaves its lodge and submerges itself under the ice, it not only diminishes its oxygen supply, it also immediately starts to cool. As it forages, it may periodically come up to breathe at a push-up or a feeding platform, where it can replenish its oxygen supply, but it can still be losing heat at a high rate. Most voluntary dives by muskrats last less than forty seconds, although the rats can store enough oxygen to stay down for several minutes. An active rat does not, and presumably cannot, allow its body temperature of 37°C (similar to ours) to drop more than 2°C (MacArthur 1979), and the rat’s solution, like that for lack of oxygen, is to store up a surplus of heat. Just prior to diving into ice water in winter (in contrast to summer), a muskrat increases its body temperature on average by 1.2°C (MacArthur 1979). Then, after returning to the lodge, the rat shivers and expends energy at a high rate to heat itself back up to 37°C (MacArthur 1984b).
Although the rats’ physiological response suggests that their dip into cold water is anticipated, do they really plan ahead? Perhaps, but if so, then the white-faced hornets I have mentioned previously in reference to their insulated papier-mâché nests may do so as well. Some years ago, I took on the brave, or foolish, task of measuring hornets’ body temperatures, grabbing and stabbing them with an electronic thermometer as they left their nests. I learned by my experiments that prior to leaving their nests at low air temperatures (2°C), the hornets shiver to heat themselves up to 39°C. On the other hand, when they leave on a warm day, at 22°C, they warm themselves up to a body temperature of only 35°C. (When attacking, they heat all the way up to 41°C.) That is, they appear to be as clever as a muskrat, or vice versa. I suspect, however, that body temperature is much too important and constant a concern for any animal to be able to rely on mere cleverness to consistently produce the correct responses.