Between Ice and Hard Times
By Peter Marchand
In an otherwise unbroken snow cover, the fresh tracks of a river otter captured my attention. I followed them, skirting obstacles too difficult for my clumsy snowshoes, ducking under dense, snow-laden branches, and suffering uncountable avalanches on the back of my neck, all the while straining for a glimpse of my quarry. That I never caught up with it did not surprise me, but what the otter led me to did. Its tracks took me to a sizable beaver pond, and there, as I stood on the bank and followed the trail with my eyes to the far side of the smooth, flat clearing, my chase ended in a black hole.
Curious, I started out across the ice, nervously alert to every sound my steps created. As I approached the hole, I saw that the ice at its edge was not very thick. I could also see mud in the otter's tracks where it had come back out of the water to sit in the snow, and there appeared to be spots of blood here and there. If the otter had brought mud up from the bottom, I reasoned, the water couldn't be too deep, and it apparently had found something to eat. I had to look in that hole.
Peering in through the reflection of sky on the water's surface, I saw that a spring at the bottom of the shallow pond was keeping the ice open, its slightly warmer water constantly rising to the surface. There, under a window of light and in the comfort of water only a few degrees warmer than the rest of the pond, a mass of salamanders-adult red-spotted newts-had gathered amid the pond weeds, weaving themselves into a loose tapestry of legs, tails, and sparsely flecked bodies. As I watched incredulously, individuals constantly left and returned to the mass, swimming quickly to the surface and diving back again, darting off to the sides, out of, and back into, my view. Hastily clicking off photos, I searched my memory for anything like this that I had seen or heard of before. I drew a blank (although I have since uncovered a 1932 report of similar observations)-and as I reluctantly left to race the fading light home, questions overwhelmed me.
Was the activity that I had observed normal for overwintering newts? Would it have made any difference if that "window" had not been there to let in light and oxygen? And what of the otter, dependent as it is on aquatic resources? Was it just lucky that day to find a hole in the ice? How does it cope with a normally impassable barrier? Captivated by my "discovery," I delved into the literature and began a systematic study of the pond in winter to parallel my ongoing investigations of life under the snowpack.
For half the year or more in northern climes, ice complicates life in the pond. Acting as a lid on the aquatic ecosystem, the ice limits the amount of nutrients, energy, and oxygen that enter the system. The pond becomes a world unto itself. It is a world of extremely stable temperatures, however, with bottom waters never exceeding 39 F and upper waters in contact with surface ice always at 32 F. (Water reaches its maximum density at 39 F, so water that is warmer or cooler than this rises. As winter progresses, water temperature at the bottom of a shallow, ice-covered pond may gradually fall below 39 F, but it is never higher.) This physical quirk of water-the closeness of its freezing point and temperature of maximum density-sets narrow temperature limits within which all aquatic plants and animals must operate during winter.
For an ectothermic animal-one whose body temperature remains basically in equilibrium with its surroundings-the water temperature is of little consequence. Since these animals have no stable body temperature to maintain, the effects of cold water show up mainly as sluggishness: A drop in temperature causes chemical reaction rates to go down, which leads to a decline in metabolic rates. A number of fishes, such as the bluegill and the sunfish, are quite lethargic during winter, remaining responsive to physical stimuli but hardly able to get out of their own way. Some, like the American eel, appear to play an active role in depressing their metabolism even more than dictated by the cold alone. Many organisms go through winter this way, conserving energy and oxygen in a state sometimes referred to as semihibernation.
The salamanders that I observed that winter afternoon, however, were not lethargic by any means. As ice fishermen know well, some fish species-lake trout, for example-remain active throughout winter, pulling hard when caught under the ice. In fact, a number of aquatic organisms respond to the winter cold by adjusting their metabolic rates upward. They usually accomplish this through increased enzyme production, which catalyzes more chemical reaction. These organisms benefit by remaining active at a time when competition from other species is reduced; the drawback is that they continue to consume oxygen just when it is in shorter supply.
For semiaquatic mammals, which must maintain a relatively high, constant body temperature independent of their environment, immersion in cold water has wholly different implications. Water siphons heat from a warm body much faster than does air, even considerably colder air. And every time a mammal dives under the ice, the water presses the animal's coat against its body, displacing much of the air trapped by its fur and significantly reducing the coat's insulative value. As potentially dangerous as cold water may be, these mammals have no choice. Muskrats, for instance, are active all winter long but typically do not store food and, therefore, must forage under the ice many times every day.
To counteract the repeated wetting of their fur, muskrats constantly groom themselves with oily secretions from the Hardarian gland, which is located in the eye socket and discharges into the nasal passages and corner of the eye, and from sebaceous glands situated at the opening of hair follicles. These water-repelling oils reduce water-to-skin contact, thus maintaining some of the insulative integrity of the muskrat's dense underfur while the animal is underwater.
Grooming does not do much for a naked tail, however. The tail of a muskrat or beaver, with its bare and disproportionately large surface area, probably helps dissipate excess heat in the summertime, but this "radiator" function would be disabling in winter were it not for a heat-shunting mechanism that keeps the tail from becoming a serious liability. When the animal enters cold water, its surface vessels constrict, forcing blood into deeper capillary beds and cooling the skin, thereby reducing heat loss through conduction. Within the tails of muskrats and beavers (as well as in the beavers' feet), the deeper capillaries form a dense network in which veins and arteries are in sufficient contact that arterial blood flowing outward, on its way to the extremities, gives up much of its heat to the colder blood returning from the tail in adjacent veins. This countercurrent heat exchange prevents heat from draining away from the body without compromising the delivery of oxygen to the tail.
Both muskrats and beavers employ yet another wintertime survival tactic. Robert MacArthur of the University of Manitoba discovered that before diving into cold water, muskrats sometimes pause at the edge of a plunge hole and, while sitting quietly, elevate their body temperature by about 2 F. MacArthur calculated that this seemingly modest increase would add nine minutes to the time a muskrat can stay under the ice before its body temperature drops back to normal. This might seem a moot advantage, since a muskrat's maximum aerobic endurance-how long it can hold its breath-is only one minute, but MacArthur has also demonstrated that muskrats scavenge oxygen from air bubbles trapped under the ice, thus prolonging their stay underwater considerably.
Beavers also raise their temperature before a dive, only about half as much as muskrats, but because of their larger mass, they derive about the same benefit. When muskrats and beavers do become chilled, they can return to their dens, huddling with nest mates or -generating heat by burning up their energy-rich brown fat.
These tactics work well for muskrats and beavers, whose food, although under ice, is never far away, and whose communal social habits keep them dry and well groomed much of the time in lodges often many degrees warmer than their surroundings.
The river otter is not so fortunate. Facing uncertain access to water and a scarcity of many of its usual food items, such as insects and amphibians, the otter ranges over a large foraging territory in winter, exposed to cold air temperatures as well as icy water. Generally solitary hunters in winter, otters tend to utilize many different shelters rather than a single den. Alone and carrying only modest amounts of body fat (sacrificing reserve energy and the insulative benefits of fat for increased agility), the river otter relies heavily upon the superior quality of its winter pelage to survive the cold. Its fur is four times as dense as the muskrat's, and the underfur is crimped to entrap more air. In addition, the outer guard hairs, themselves partly hollow and good insulators, are equipped with scales that interlock with those of adjacent hairs to protect the underfur while diving. The otter's tail, apparently lacking an effective countercurrent heat-exchange mechanism, is particularly well insulated.
But it is not by fur alone that river otters survive winter. Unequipped to cut their own holes in the ice, indeed unequipped to excavate their own burrows for shelter, north-country otters depend heavily on beavers for both. In a study in the boreal forests of Alberta, a group of researchers at the University of Calgary found that in winter, otters almost invariably sought lakes with beaver lodges, or bog ponds with steep banks containing old beaver dens through which they could enter the water. Sometimes otters deliberately rifted dams on beaver-impounded streams, lowering water levels and thereby gaining access to food and shelter beneath the ice. So dependent on beavers were the otters that the northern limit to their range coincided with that of beavers, even though otter prey could be found farther north.
Although many questions remain, the otter in search of food that stormy day and the salamanders it found among the pondweed told much of the story, as we know it, of the aquatic ecosystem in winter. For cold-blooded species adapted to the low temperatures and oftentimes low oxygen levels in the winter pond, life may not be so bad. For the semiaquatic mammals, however, a long winter can bring grave hardship. The one difficulty these mammals cannot endure is getting frozen out of their shallow ponds or marshes. In times of low snow cover and extended cold, when water threatens to freeze to the bottom, these animals sometimes find themselves caught in a squeeze between the ice and hard times.
Peter Marchand is director of research at the Catamount Biological Station in Woodland Park, Colo. A version of this story appeared in Natural History, March 1997, pp. 54-57; copyright© Natural History Magazine, Inc., 1997.