Hall (1951) noted that the southern limit of regular whitening in longtails is not a sharp line, with white animals on one side and brown ones on the other, but a broad zone up to about 350 km wide in which white, brown, and pied animals could be found in various combinations. He mapped the position of this zone as running for most of its length along one side or other of the 40th parallel, but extending north on the West Coast (Figure 3.4). From Figure 1.5 it is possible to work out that this zone corresponds very roughly to the southern limit of regular snow cover at least 2.5 cm thick and lying for about 50 days a year.
Virtually all North American stoats east of the Cascades turn white, because they all live in mountain or continental climates; only the stoats of the coastal Northwest Pacific stay brown in winter. In Britain, the transition zone for stoats runs through a reverse S-shaped curve lying between 52°N0 and 56°N: White coats are common in Wales and Scotland; brown is the rule in England. Coastal Holland straddles the transition zone at 51°N. Across Europe, however, the moderating influence of the Gulf Stream is left behind, and the southern limit of whitening follows the snowline southeastward.
The climate conditions required to set off winter whitening are apparently not the same for all species of weasels, or even for the same species in all places. Minnesotan stoats turn white much later than Alaskan ones, and return to brown earlier (Feder 1990:108). British stoats seem to whiten in relatively mild winters: The hilliest parts of Britain where white stoats may be seen are much less snowy than the country occupied by white longtails in the United States, even though the British ones are 1,800 km further north. The same is true of the stoats of British stock introduced into New Zealand. By contrast, Russian stoats behave just like longtails; in Byelorussia (50°N to 55°N) the transition zone for stoats again coincides with the southern limit of stable snow cover lasting at least 40 days a year (Gaiduk 1977).
The southernmost limit of regularly white Mustela nivalis in western Europe lies at a much higher latitude than for Mustela erminea. All-white weasels
are a great rarity in Britain, even in the north where stoats regularly achieve full ermine. Salomonsen (1939) calculated, from the geographical distribution of white and brown winter skins from Greenland, Scandinavia, and Britain, that the critical minimum temperature associated with whitening in common/least weasels is a full 5°C lower than that for stoats. He concluded that, paradoxically, the smaller species is much more resistant to the cold. But from what we know about the metabolic stress endured by small mammals in the Arctic, this seems an unlikely explanation; and besides, there is a better one.
In southern Sweden, nivalis stay brown all winter as far as about 59°N to 60°N (the latitude of the Orkney Islands, in the far north of the British Isles); then there is a narrow boundary zone, about 100 km across, roughly from Norrkoping through Stockholm to Uppsala, beyond which all nivalis turn white. But this boundary does not, as in erminea and frenata, mark a transition zone where some individuals of one species turn white or not according to the local climate. It marks the meeting of the two distinct subspecies of nivalis (see Figure 1.13), which have different histories and genetic makeup, and which are distinctly different in their summer coats as well as in winter (Stolt 1979). The intriguing implication is that M. n. vulgaris evolved at a more southerly latitude than M. n. nivalis, and so in the vulgaris group the allele for winter whitening is (or has become) rare, whereas in the nivalis group it is widespread or even fixed (Zima & Cenevova 2002). British common weasels all belong to the vulgaris group; hence, the fact that they do not turn white in Britain has more to do with genetics and evolutionary history than with climate.
The same is true of the stoats of northwestern North America that were the subjects of Feder's experiments. Those that now live on the Pacific coast probably moved north after the glacial periods from a refuge somewhere on the continental or coastal mainland, whereas those that now live in Alaska could have been living on the wide expanses of ice-free Beringian tundra throughout the Pleistocene (Eger 1990). Their different ancestry explains their different average size and winter dress.
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