Species Diagnoses

The general weasel appearance is common to all three species (Figures 1.9. and 1.10). The differences among them hinge mainly on size, color, and reproduction (Table 1.1). Size and color are obvious to the eye, so one might think that they would be easily defined and reliable guides. Most of the early taxonomists based their species descriptions on size and color, often using the standard practice of describing whole species—or, at any rate, whole populations—from one or a few "type" specimens. But the three weasel species between them occupy an enormous geographical range (Figure 1.11), across which climate, habitat, and prey vary equally enormously. Under these different conditions, each weasel species has evolved considerable variation in external appearance, especially in size (Chapter 4), in the details of summer pelage, in the length and color of the tail (Table 1.1), and in whether or not they turn white in winter (Chapter 3).

Until the early 1800s, the names of European weasels were applied to their North American counterparts, except that the species' distinctions were not made consistently. From then into the twentieth century, many local variants of North American weasels, and sometimes even brown and white individuals of the same species in the same place, were described as separate species. The "type-specimen" method of nineteenth-century taxonomists led to glorious confusion and to dozens of new species and subspecies descriptions. Hosts of new names confound the literature of that period. As more and more specimens were collected, however, taxonomists realized that M. erminea is a circumboreal species and should have the same scientific name in both North America and in Eurasia. During the second half of the twentieth century, least weasels in North America have also become generally accepted (but not by Reig 1997) as belonging to M. nivalis, instead of to the separate species M. rixosa.

The color patterns of weasels are not much help in distinguishing the species, because they vary so widely. In summer coat, the head, back, all or most of the tail, and the outsides of the legs of almost any weasel are rich chestnut in color, but range from sandy tan to dark chocolate. The throat, chest, and belly range from pure white through creams and yellows almost to apricot in some longtails. The feet often have white markings, and the outer edges of the ears

Least Weasel Line Drawing

Figure 1.9 Least weasel (Mustela nivalis nivalis), common weasel (M. nivalis vulgaris), North American stoat and west European stoat (M. erminea), and long-tailed weasel (M. frenata) in summer coat. The inset shows an example of the facial patterns of long-tailed weasels, often called bridled weasels, in the North American Southwest. European stoats are about the same size as longtails, distinctly larger than North American stoats. The five weasels are drawn to scale.

Figure 1.9 Least weasel (Mustela nivalis nivalis), common weasel (M. nivalis vulgaris), North American stoat and west European stoat (M. erminea), and long-tailed weasel (M. frenata) in summer coat. The inset shows an example of the facial patterns of long-tailed weasels, often called bridled weasels, in the North American Southwest. European stoats are about the same size as longtails, distinctly larger than North American stoats. The five weasels are drawn to scale.

Long Tailed Weasel Winter

Figure 1.10 Least weasel (Mustela nivalis nivalis), common weasel (M. nivalis vulgaris), North American stoat and west European stoat (M. erminea), and long-tailed weasel (M. frenata) in winter coat. Common weasels never turn completely white in winter, while stoats, least weasels, and longtails do not turn white in the southern parts of their ranges where winters are mild. The five weasels are drawn to scale.

Figure 1.10 Least weasel (Mustela nivalis nivalis), common weasel (M. nivalis vulgaris), North American stoat and west European stoat (M. erminea), and long-tailed weasel (M. frenata) in winter coat. Common weasels never turn completely white in winter, while stoats, least weasels, and longtails do not turn white in the southern parts of their ranges where winters are mild. The five weasels are drawn to scale.

Table 1.1 Distinctions between the Species of Weasels1

M. nivalis

M. erminea

Table 1.1 Distinctions between the Species of Weasels1

M. nivalis

M. erminea

n. nivalis

n. vulgaris

e. hibernica

others

M. frenata

Authority

Linnaeus

Erxleben

Thomas &

Linnaeus

Lichtenstein

and date

1766

1777

Barrett Hamilton 1895

1758

1832

English name

least

common

Irish stoat2

stoat

long-tailed

used here

weasel

weasel

weasel

Other names

grass weasel, miniver

Irish weasel

ermine, short-tailed weasel

Summer coat

straight

wavy

straight or wavy

straight (Eurasia),

straight

demarcation line3

wavy (N. America)

or wavy

Tail tip

brown

brown

black

black

black

White in winter

yes

no

no

maybe

maybe

Tail length4

<25%

<25%

30-45%

30-45%

40-70%

Diapause

no

no

yes

yes

yes

1. For species distributions, see Figure 1.11; for sizes, see Chapter 4.

2. This form is also present on the Isle of Man, Islay, and Jura.

3. Demarcation line: the line separating the brown from the white summer fur.

4. Tail length expressed as a percentage of head-body length.

1. For species distributions, see Figure 1.11; for sizes, see Chapter 4.

2. This form is also present on the Isle of Man, Islay, and Jura.

3. Demarcation line: the line separating the brown from the white summer fur.

4. Tail length expressed as a percentage of head-body length.

Short Tailed Weasel Distribution
Figure 1.11 Global distributions of the three northern weasel species Mustela nivalis (common and least weasels), M. erminea (stoat or short-tailed weasel, ermine), and M. frenata (longtailed weasel). (A) Europe. (B) Asia. (C) North America.
Common Weasel

are often fringed with white in stoats. The boundary between the brown and white fur is irregular in common weasels, often with odd spots or large patches of brown among the white (Figure 1.12). This pattern is also found in many longtails, in many stoats and least weasels in North America, and in some 80% of Irish stoats.

Common weasels usually have a brown spot, called the gular spot, behind the angle of the mouth (sometimes joined by an isthmus to the rest of the brown area), though least weasels do not. On common weasels, and on other weasels with crooked belly lines, the details of the belly pattern are individually unique and constant through successive molts (Linn & Day 1966); on one common weasel live-captured 54 times (on the right in Figure 1.12), the pattern remained unchanged through two molts (King 1979). Stoats, longtails, and least weasels living in cold climates turn white in winter; stoats, longtails, and common weasels living in mild climates stay brown.

Every stoat and longtail has a distinct, bushy pencil of long, black hairs on the tip of its tail. This mark distinguishes the members of these species reliably

Figure 1.12 The irregular, individually distinct patterns of brown and white blotches on the bellies of three English common weasels. The unique patterns can be used to identify individuals. (Redrawn from sketches made by C. M. King of anesthetized, live animals in the field.)

Sketches White Tailed Weasel

Figure 1.12 The irregular, individually distinct patterns of brown and white blotches on the bellies of three English common weasels. The unique patterns can be used to identify individuals. (Redrawn from sketches made by C. M. King of anesthetized, live animals in the field.)

from common and least weasels, though individual common and least weasels may have a few black hairs at the tips of their tails. Longtails in southwest North America have attractive patterns of white and brown markings on their faces, which explains why their alternative names in these parts of the country is "bridled weasels" (Fagerstone 1987).

Common and least weasels in Europe are very different in appearance, even in the few places such as Sweden and Switzerland where they live close together (Stolt 1979; Güttinger & Müller 1988). Only the least weasel is the true winter-white "snow-weasel" described and aptly named as M. nivalis by Linnaeus in 1766; evidently he did not distinguish the southern one, so that was named M. vulgaris by Erxleben in 1777. Later, vulgaris was reduced from full species to subspecies status, although Erxleben's name remains attached to the subspe-cific epithet. The two are now regarded as sibling subspecies of one polytypic species, M. nivalis Linnaeus. Yet in color patterns and winter whitening, in reproductive cycles, and consequently in population dynamics, they are more different from each other than stoats are from longtails, suggesting that the idea of recognizing them as two species has some merit (Reig 1997).

Two strong counterarguments to the two-species idea, however, hold greater weight. First, if size and color pattern are indeed critical characteristics separating the two "potential" species, they should vary together, but they do not. Mediterranean common weasels are all very large, but in some populations they have straight belly lines (Type I), in others they have crooked lines (Type II), and some populations have both (Figure 1.13). The two patterns are controlled by the two alleles of a single gene, of which the one producing the Type I straight line pattern typical of nivalis nivalis is dominant over the one controlling the Type II crooked line pattern of nivalis vulgaris (Frank 1985). Second, both forms have the same number of chromosomes (n = 42), are fully interfertile, and inherit size and belly pattern independently (Frank 1974; Mandahl & Fredga 1980; Frank 1985; Zima & Cenevova 2002). M. nivalis in Sicily have the same straight belly line as those in northern Scandinavia, but they weigh several times more.

If common and least weasels are clearly not separate species, yet still obviously are different, the only alternative policy is to recognize them as separate subspecies. In fact, the most recent review of the intraspecific taxonomy of Mustela nivalis recognizes 17 subspecies in three groups: a small nivalis type, a large vulgaris type, and large nivalis (boccamela) type (Abramov & Baryshnikov 2000). Ideally, it is best not to recognize subspecies as valid unless they represent incipient speciation (Whitaker 1970), so here we ignore most of the named subspecies of all three species.. But the differences between the nivalis nivalis group and the nivalis vulgaris group, which presumably developed while their ancestral populations were isolated in separate glacial refuges, may represent an advanced stage of the speciation process, interrupted just before it was completed. The modern common and least weasels are different in many ways but definitely still interfertile, as Frank (1985) proved, so under the conventional

Least Weasel Distribution

Figure 1.13 The approximate distributions of the two patterns of coat color of Mustela nivalis in Europe, designated type I and type II by Frank (1985). Figure 1.9 shows the type I pattern on the least weasel and the type II pattern on the common weasel. (Distributions deduced by King [unpublished] from the collection of skins in the Natural History Museum, London. Güttinger and Müller [1988] provided local detail for Switzerland. For a more detailed version of this map extending across the Palaearctic, see Abramov 2000b.)

Figure 1.13 The approximate distributions of the two patterns of coat color of Mustela nivalis in Europe, designated type I and type II by Frank (1985). Figure 1.9 shows the type I pattern on the least weasel and the type II pattern on the common weasel. (Distributions deduced by King [unpublished] from the collection of skins in the Natural History Museum, London. Güttinger and Müller [1988] provided local detail for Switzerland. For a more detailed version of this map extending across the Palaearctic, see Abramov 2000b.)

definition of a biological species they both still belong to M. nivalis—unless they are found to maintain reproductive isolation in the wild. On the other hand, the differences between them are too strong to ignore.

We, therefore, continue to follow van Zyll de Jong (1992) in distinguishing two subspecies (or subspecies groups) of M. nivalis, labeled as M. n. nivalis and M. n. vulgaris. We do not reject the idea of a third (M. n. boccamela) group, but there is not enough information about the ecology of these large southern weasels to treat them separately here. This is not a very satisfactory solution, but the jury is still out on what might constitute the most useful definition of such a variable species, so at the moment we cannot suggest a better one. Recent analyses of morphological variation across the entire geographic range of M. erminea and M. nivalis suggests that both vary extensively across the entire northern hemisphere (Chapter 4), and that describing this variation is difficult even for taxonomists (Meia & Mermod 1992; van Zyll de Jong 1992; Abramov & Baryshnikov 2000). At present, the best we can do in this book is to discuss three species but four distinct forms.

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