In almost any study of trapped weasels, males will be recorded more often than females. The average proportion of males collected in kill traps is usually around 75% in common weasels, and around 60% to 65% in stoats (King 1975a). These figures are even higher in spring and summer. Live trapping or snow tracking of undisturbed live animals may, over a period (especially in autumn), suggest that males and females are in roughly equal numbers (Chapters 8 and 10). Nonetheless, the individuals most often recaptured are always males (King et al. 2003a). The sex ratio of the young at birth averages 1:1 in all weasels, as best as we can tell, and there is no sign of any difference in the mortality rates of males and females as large as the difference in their capture rates. So what causes the excess of males captured?
One possible reason is that males have more opportunities to be caught. If traps are evenly spread out through a suitable habitat, the larger home ranges of the males will include more traps than will the smaller home ranges of females (King 1975a). If the distance between the traps is large, some weasels will not find a trap on their own ground at all, and these are more likely to be females than males (see Table 13.2).
Also, of course, the females of each species are much more likely than males to spend time hunting in burrows or under thick cover where there are no traps. The disparity is increased in spring and summer because many males travel widely then, energetically searching for mates, whereas females tend to be less active, and perhaps also more shy of traps, at that season.
Buskirk and Lindstedt (1989) developed mathematical models to explore these and other possible causes of sex bias in trapping mustelids. They concluded that three factors probably affect the gender balance of captures in all trapping studies: (1) the number of traps on each individual's home range and behavioral differences between the sexes in (2) how they patrol their home ranges and (3) how they approach the traps they find. The "exclusion effect" (some individuals having no traps at all on their home ranges) is a special case applying only if traps are set in a widely spaced grid pattern. Whatever the reasons for the bias, it has important consequences for gamekeepers and conservation managers (Chapters 12 and 13).
Since the enormous fluctuations in population density typical of most weasel populations in good seasons are due to temporarily improved survival of the young, the ratio of young to adults differs distinctly between high and low populations. The effect is similar in both sexes and all ages, however, so density variations have little effect on sex ratio at any age (King & McMillan 1982).
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