Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [68]
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,