Mapping the Patterns

Only two of the three North American species are represented in most places. The most widespread combinations are least weasels and stoats in the north and longtails and stoats in New England and the west. All three are found together in a broad stretch of country from Minnesota across to Pennsylvania. If longtails are present, they are always the largest, and if least weasels are present, they are always the smallest. Stoats may be either the larger or the smaller of a set of two, depending on which the other one is. In the southern United States and south into Mexico and beyond, longtails hold the stage alone.

In North America, stoats become larger in the Northwest, where they are about the same size or a little smaller than the northeastern Eurasian stoats just across the Bering Strait (Eger 1990). The smallest American stoats, in the South and especially in the Southwest, are considerably smaller than European common weasels. Stoats elsewhere range in size from much smaller to much larger than longtails (Figure 4.2, Table 4.1). By contrast, North American least weasels and longtails are no larger or smaller in any particular direction (Figures 4.3 and 4.4); one can find longtails of roughly the same size from Canada right down into South America. Most North American least weasels are very small, and the next smallest are their nearest relatives in the Old World, those inhabiting Siberia. In parts of the southwestern United States, longtails are smaller than West European stoats and stoats are smaller than West European common weasels.

Stoats and least or common weasels live together over most of Eurasia, and of the two, stoats are always the larger. Both are smallest in the Far East and north of Eurasia, and become larger toward the west and south (Figures 4.2 and 4.3). Both stoats and common/least weasels living at high elevation in the Alps, Caucasus, Altai, and Tien Shan Mountains are particularly small. Otherwise, the largest Eurasian stoats are found at the southwestern edge of their range (from the Netherlands across the North European Plain), and the largest common weasels are found south of the southern limit for stoats, particularly in Egypt, where they are much larger than stoats are in eastern Siberia. The general patterns are very obvious from skins (Figures 4.5 to 4.7), and the finer details plotted on continental-scale maps show a mosaic of local variations (Ralls & Harvey 1985; Meia 1990; van Zyll de Jong 1992; Reig 1997; Abramov & Baryshnikov 2000). The details of these local variations, however, do not affect the general trends, which are unmistakable at continental scale.

Why are the patterns of variation in the individual weasel species so confusing and contradictory? There is no obvious reason why the smallest stoats should be found in the south and west of North America but in the north and east of Eurasia. Both are harsh environments, certainly, but they are harsh in different ways—the high Rocky Mountains present different challenges to animals compared with the vast cold expanses of Siberia. Even more difficult to explain are the smaller-scale trends that contradict the general regional pattern. For example, stoats vary over a much greater range of sizes in North America than Eurasia, while common weasels vary much more in Europe than in North America. Although common weasels in Europe generally tend to become larger southward, in Britain the largest common weasels are in the north (Table 4.1). Even within Britain, the general north-south cline is interrupted by local variations.

If we view all the weasels as a single group, however, the continental-scale pattern is the same in both the Old and the New Worlds. Weasels in general are relatively small in the far north, both in North America and in Eurasia. The mean skull length in stoats, always the largest species in the north, seldom exceeds 46 mm right around the Pole, and least weasels measure 30 to 32 mm in Alaska

Figure 4.2 Geographical variation in condylobasal length (skull size) of male stoats in (A) Europe, (B) Asia, and (C) North America. The females generally vary in the same way, although the degree of difference between them is locally variable. The distribution of size classes is schematic only. (Data from Eger 1990, Güttunger & Müller 1988, Hall 1951, Heptner et al. 1967, Holmes 1987 and unpublished data, Kratochvil 1977, Meia 1990, Meia & Mermod 1992, Ralls & Harvey 1985, Reichstein 1957, Reig 1997, and Schmidt 1992.)

Figure 4.2 Geographical variation in condylobasal length (skull size) of male stoats in (A) Europe, (B) Asia, and (C) North America. The females generally vary in the same way, although the degree of difference between them is locally variable. The distribution of size classes is schematic only. (Data from Eger 1990, Güttunger & Müller 1988, Hall 1951, Heptner et al. 1967, Holmes 1987 and unpublished data, Kratochvil 1977, Meia 1990, Meia & Mermod 1992, Ralls & Harvey 1985, Reichstein 1957, Reig 1997, and Schmidt 1992.)

Figure 4.2 (continued)

and Canada and 33 to 35 mm in Russia. The largest weasels are always found in the south: Longtails in the United States and common weasels in Egypt reach 50 to 53 mm in skull length or more.

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