Size and Temperature

Geographical variation in the body sizes of animals is common, and many explanations have been offered since the nineteenth century. At that time, many zoologists still believed that it should be possible to explain much of the riotous variety of life in terms of simple, formal "Rules," comparable with those of physics and chemistry and based on the same properties of energy and matter that govern inanimate things. For example, warm-blooded animals living in cold climates have to expend a huge proportion of their total energy budget on keeping warm, and the smaller they are, the greater the relative expenditure and the

Figure 4.3 Geographical variation in condylobasal length of male common and least weasels in (A) Europe, (B) Asia, and (C) North America. Our arbitrary contour break in Scandinavia falls north of the boundary between least and common weasels, shown in Figure 1.13. See comments in Figure 4.2. (Data from Hall 1951, Holmes 1987 and unpublished data, Kratochvil 1977, Meia 1990, Meia & Mermod 1992, Morozova-Turova 1965, Ralls & Harvey 1985, Reichstein 1957, and van Zyll de Jong 1992.)

Figure 4.3 Geographical variation in condylobasal length of male common and least weasels in (A) Europe, (B) Asia, and (C) North America. Our arbitrary contour break in Scandinavia falls north of the boundary between least and common weasels, shown in Figure 1.13. See comments in Figure 4.2. (Data from Hall 1951, Holmes 1987 and unpublished data, Kratochvil 1977, Meia 1990, Meia & Mermod 1992, Morozova-Turova 1965, Ralls & Harvey 1985, Reichstein 1957, and van Zyll de Jong 1992.)

Figure 4.3 (continued)

greater the danger of irreversible chilling. In some species of mammals, individuals living in colder climates have larger bodies than their relatives in the south, and shorter ears and tails.

In 1847, Bergmann suggested that the larger size of northern mammals is a matter of energy conservation, since large mammals have relatively less surface area exposed to the cold air in relation to the mass of the body in which heat is generated. Bergmann's Rule therefore predicts a steady increase in body size northward in related mammals and birds living in habitats equally exposed to increasingly severe environmental conditions. The same logic underlies Allen's Rule, which explains the relatively shorter ears and tails of northern mammals as a means of reducing the area of vulnerable appendages from which heat may escape.

Weasels in general are particularly sensitive to thermal stress at low temperatures (Chapter 2). Metabolic inefficiency costs them very dearly in the vast, cold northern parts of their ranges, so one might expect them to be prime

Condylobasal Length
Figure 4.4 Geographical variation in condylobasal length of male long-tailed weasels. See comments in Figure 4.2. (Data from Hall 1951, Holmes 1987 and unpublished data, and Ralls & Harvey 1985.)

examples of Bergmann's Rule. A simple comparison between skull size and latitude, however, does not confirm this expectation. Only in North America are stoats substantially larger in the north of their range than in the south (Eger 1990); common weasels in Eurasia are just as substantially larger in the south, and all the rest are more or less indifferent (King 1989a). Moreover, the American stoats that are larger in the north can be said to be so only by comparison with their exceptionally small relatives further south in North America; they are not larger than their relatives at the same latitudes in eastern Eurasia. Within single regions spanning a range of climates, Bergmann's Rule does not explain the local variation of stoats in Russia, Europe, or New Zealand (Petrov 1962; King & Moody 1982; Erlinge 1987).

Figure 4.5 Skins from the collection of the Natural History Museum, London, arranged to represent the range of variation in body size of adult male Eurasian weasels with latitude (except that the Swiss specimens are out of order). Above, stoats; below, common and least weasels. Origin of specimens, left to right; stoats: Switzerland, Norway, France, Hungary; common and least weasels: Switzerland, Norway, France, Hungary, Italy, Egypt. Scale of centimeters at left.

Figure 4.5 Skins from the collection of the Natural History Museum, London, arranged to represent the range of variation in body size of adult male Eurasian weasels with latitude (except that the Swiss specimens are out of order). Above, stoats; below, common and least weasels. Origin of specimens, left to right; stoats: Switzerland, Norway, France, Hungary; common and least weasels: Switzerland, Norway, France, Hungary, Italy, Egypt. Scale of centimeters at left.

Figure 4.6 Skins from the collection of the Natural History Museum, London, arranged to represent the range of variation in body size of adult male Eurasian weasels with longitude. Above, stoats; below, common and least weasels. Origin of specimens, left to right, both species: England, Germany, European USSR, Asian USSR.

Figure 4.6 Skins from the collection of the Natural History Museum, London, arranged to represent the range of variation in body size of adult male Eurasian weasels with longitude. Above, stoats; below, common and least weasels. Origin of specimens, left to right, both species: England, Germany, European USSR, Asian USSR.

Figure 4.7 Skins from the collection of the Natural History Museum, London, arranged to represent the range of variation in body size of adult male and female long-tailed weasels with latitude. Origin of specimens, left to right: Colombia (female right, male left), northeastern United States, Canadian Northwest Territories.

Figure 4.7 Skins from the collection of the Natural History Museum, London, arranged to represent the range of variation in body size of adult male and female long-tailed weasels with latitude. Origin of specimens, left to right: Colombia (female right, male left), northeastern United States, Canadian Northwest Territories.

Bergmann assumed that temperature is directly correlated with latitude. At low elevation this is generally correct, and the skull lengths of weasels in North America are about as well correlated with temperature as with latitude. But temperature is also correlated with elevation. The high mountains that cross Eurasia from east to west at low latitudes (e.g., the Alps, Caucasus, and Tien Shan) therefore confuse the simple size-latitude correlation for that part of the world. For example, stoats and common weasels living high in the mountains of Switzerland are all distinctly smaller than those in the surrounding countries, and least weasels survive there as a separate, southern population isolated from their relatives in the cooler north (Güttinger & Müller 1988). This problem can be removed by comparing size directly with mean annual temperature; the result shows clearly that both sexes of both species of weasels in Eurasia are smaller in colder environments—the very opposite of what Bergmann's Rule predicts.

This result is puzzling only until we remember that weasels evolved their long, thin shape in response to the opportunity to hunt small rodents in their own burrows and under snow. This means that, for much of the time that the northern weasels are hunting, they are not exposed to the full rigors of the winter air temperatures as much as their southern relatives are. So, comparing the sizes of northern and southern weasels in terms of Bergmann's Rule is logically invalid. Besides, the relationship between size and temperature cannot be crucial for weasels, since the difference between the males and females living in one place is often at least as great as that between the northern and southern members of either sex. We need, then, to find another explanation for the general north-south variation in size of the weasels as a group.

Because the size of a body affects many of its functions, and the best size for one function is not necessarily the best for any of the rest, the actual size of the whole animal is usually a compromise. Sandell (1989) proposed a model predicting the best sizes that male and female stoats could be. Sandell designed his model to take into account the differences in the reproductive roles and in the seasonal energy requirements of males and females. He assumed that the total amount of energy available to any animal in any one day, expressed as a multiple of its basal metabolic rate, is limited—which certainly seems reasonable for weasels (Chapter 2). Then Sandell allowed for the important facts that a weasel's priorities for energy expenditure are not the same all year round, and that they are not the same in the two sexes.

In winter, the huge costs of keeping a small thin body warm in a cold climate load the loss columns of a weasel's budget with high "overheads." Consequently, during winter the most important thing for weasels of both sexes is to economize on their daily expenditure of energy as much as possible. The best way to do this is to be extremely good at hunting, so as to be able to spend the least possible time each day out of the den searching for food. For any combination of values measuring foraging efficiency and air temperature, there will be a theoretically "optimum" body size, which will be the same for both sexes. In the breeding season, however, the equations must be different. In the intense competition for mates, larger males tend to be more successful, so during the breeding season the optimum size for males is larger than in winter. Conversely, even in the breeding season, foraging efficiency is still the most important consideration for females, since it determines the number of young they can rear. The optimum size for a female in summer is, therefore, controlled by the same variables as in winter.

Real animals, of course, vary in size individually, and the biogeographical history of their species is important too (Chapter 1). Sandell's work suggests that the most successful animals in any given place should be the ones that have chanced on the best compromise between possible sizes at different seasons. Sandell's model could in theory predict the optimal body size for members of each weasel species in each place, and, thereby, predict geographical variation in body size of weasels, if only we could accurately measure foraging rates. That has not yet been done.

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