the weasels, the smallest of the true carnivores, are among nature's more recent inventions. Both their relatively recent origin and their small size are due to their evolutionary profession, or "niche," as supreme specialists in the difficult art of hunting rodents in confined spaces. All the details of their ecology and behavior can be explained by the idea that they have evolved to exploit a particular kind of high-risk, high-reward resource—the boom-and-bust, unstable populations of voles and lemmings. They developed this skill well before the ice ages began, so that now they are among the very few predators that can overwinter in the simple and savagely inhospitable (to us) environments of the contemporary snowy north (Figure 2.1). They are outstandingly successful in this profession, and their natural distributions are among the largest of any mammals in the world.
The weasels have also spread south to milder climates, where they can find more stable populations of larger prey. There, the greatest problems are likely to arise not from fierce cold and unpredictable food supplies, but from the more constant dangers of competition and predation from other species. Nevertheless, their primary home is the circumpolar boreal region, and that is the key to understanding everything about them because it is where they display their marvellous adaptations to best advantage.
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