Vulnerability to Cold

Every animal must balance, every day, the energy gained from food against the energy spent in getting it. Small mammals have to spend relatively more energy

Table 2.1 Food Consumption by Captive Weasels

Males Nonbreeding females

Table 2.1 Food Consumption by Captive Weasels

Males Nonbreeding females

Species

Wt food/day

Body wt weasel

% bodywt eaten/day

Wt food/day

Body wt weasel

% bodywt eaten/day

Reference

Stoat

57

23

33

14

(Day 1963)

Stoat

19-32

23-37

(Müller 1970)

Common weasel

41

133

33

28

81

36

(Moors 1977)

Least weasel

32

81

40

(Gillingham 1984)

Long-tailed weasel

c. 33

1933)

to maintain a constant body temperature than large ones, because they have a large surface area, relative to their mass, from which heat radiates. For weasels, this problem is even greater than for other small mammals. Their long, thin shape is ideal for hunting through small spaces and burrows, but it has the huge disadvantage of giving them a larger surface area than "normally" shaped animals of the same weight, even when they are asleep (Brown & Lasiewski 1972).

Weasels feel the cold tremendously. Their body temperature is 39°C to 40°C (102°F to 104°F), and their body heat escapes easily, not only because of their shape, but also because they cannot afford to insulate themselves too much. Layers of fur or fat would get in the way and perhaps prevent them from entering vole runways and burrows. The length of a mammal's fur is related to the diameter of its body, not its weight; hence, weasels have shorter fur than regularly shaped mammals of their mass. Their winter fur is hardly warmer than their summer fur.

Most mammals also use fat as an insulation from the cold. Fat is a good insulator, and it produces more energy when burned to run a body. Yet, weasels appear to benefit from storing excess nutrition as muscle instead of as fat (Harlow 1994). Fat is dead weight for a weasel, requiring energy to carry around and slowing a weasel's escape from predators. Muscle, on the other hand, can be used until it must be burned as fuel. So, few weasels carry much fat, and any fat that they do have is confined to dips in the body outline to retain their sleek figures.

The cold northern winters that weasels endure over much of their range must be, therefore, periods of great energetic stress. The simple maintenance of body heat at rest requires twice as much energy as in summer, at three times the cost incurred by a lemming of similar size resting at the same temperature (Casey & Casey 1979). Arctic and alpine weasels at rest absolutely depend upon having a thickly insulated den, which they take from recent prey and improve by lining with fur, as illustrated by Sittler (1995).

However safe and attractive a warm den may be, a weasel cannot stay in it indefinitely. Hunger will eventually drive it out into the cold again, and that means using up yet more energy for running. Curiously, researchers who have studied the energy metabolism of weasels disagree as to whether weasels are restricted in their movements by subzero temperatures. In Alaska, Casey and Casey (1979) estimated that arctic least weasels outside their dens may have to generate up to six times the basic metabolic rate just to keep warm, and they may have little capacity left in reserve to provide energy for active hunting. If so, weasels should stay under the snow when the air temperature drops too far, which is exactly what they do. Stoats tracked by Kraft (1966) in Western Siberia did not emerge onto the snow surface when the air temperature got below -13°C.

On the other hand, Sandell (1989) concluded that stoats in Sweden do not need to generate more than three times the basic metabolic rate under any conditions, and that, since 75% of the energy expended during activity is released as heat, a running weasel does not need to spend any extra to keep warm whatever the temperature. If this second interpretation is correct, an active weasel is virtually independent of air temperature and is safe even in severely freezing conditions so long as it keeps moving.

Whether these results actually contradict each other is not clear. The authors used different species, methods, and materials and their results may apply to different situations. For example, weasels may avoid hunting above the snow because they cannot afford to stop moving, even when stops are necessary for foraging. Or, weasels may remain below snow in extreme cold not only because no prey venture above snow at those temperatures, but also to avoid exposure to hawks and owls. Or, the differences in body size and geographic origins between the weasels studied in Alaska and in Sweden were enough to introduce real differences in the results. In any case, it seems safe to conclude that weasels balance their winter energy budgets with little to spare, and in the far north they probably survive at the limits of their metabolic capacities.

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