These various independent studies document important consistencies among common weasels, which in their turn illustrate important differences among populations and recurring problems with research methods. When and where prey are abundant, common weasels reach good numbers ("good" here means only "sufficient to support a study," because weasels can never be described as "abundant"), they have small home ranges, and they may even allow partial home range overlap with neighbors. Where prey are scarce, weasels are few, they do not establish home ranges, and transients do not linger. Therefore, differences in prey abundance are predictably correlated with differences in numbers, home range sizes, and defensive behavior of common weasels.
Every study done so far has reported that males' ranges were larger than females', often double or more in size. Males use more space than females, in part because they have larger bodies. At first inspection, males' home ranges appear disproportionately larger than their larger food requirements would require, compared with females. Many other predatory mammals show this same pattern, for example, martens and black bears (Powell 1994; Powell et al. 1997). Recently, however, Yamaguchi and Macdonald (2003) showed that decreased availability of prey caused by home range overlap requires male minks to have extra large home ranges; that is, the home ranges of males are not really disproportionately large because they have less food than previously thought. Ultimately, the reason for the sexual differences in home range sizes is probably related to a complex interaction of food requirements, the importance of different prey for members of the two sexes, and the males' need to know where females are located before the breeding season starts (Chapter 14).
Estimates of areas differ also because different methods have been used to estimate home ranges. The advance of technology has brought benefits, but at a price—it has permitted live trapping, which by definition interferes with what needs to be observed (the movements of animals around their ranges), to be replaced with radio-location data, which is inevitably short term. Moors (1974) and Pounds (1981) studied weasels in the same habitat on the same study area but with trapping and radiotelemetry, respectively, and calculated very different estimates of home range sizes. Both studies demonstrated that weasels avoided agricultural fields and concentrated their activities in the thick vegetation along walls and hedgerows. But the boundaries and areas of ground attributed to each individual changed with the two authors' different assumptions on how far weasels will venture from the cover of stonewalls and hedgerows.
Likewise, King (1975c) and Macdonald et al. (2004) also studied weasels in the same area, one with trapping and the other with radiotelemetry, but they concentrated on different habitats and produced very different estimates of home range areas. Inevitably, King's trapping records in the woodland, ranging from three to 109 recaptures per resident animal (totaling 348 recaptures over 27 months) tell us less about the details of weasel movements than Macdonald et al.'s radiotelemetry data from the farmland covering from 10 to 177 hours per animal (totaling more than 5,000 fixes over 103 days), but the same basic method of quantifying weasel home ranges, using minimum convex polygons, was used in both studies. The different limitations of the available field methods still determine what we can learn about these elusive little critters.
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