Abstract
Sex ratios significantly different from 1:1 usually are observed in trap captures of mustelids. Although these ratios could be caused by skewed sex ratios in wild populations, trapped samples consistently are skewed toward males. This apparent sampling bias generally has been attributed to sexual dimorphism of home-range sizes in mustelids, postulated to result in greater exposure of males to traps. The magnitude of differences in rates of capture between sexes appears to be body-size dependent because small mustelids exhibit more strongly skewed sex ratios in trap captures than do large ones. We investigated possible causes of sexual differences in rates of capture of mustelids. Mathematical equations were derived to describe the effect of trap spacing in relation to home-range diameter as a cause of these sex biases. Large trap-spacing values in relation to home-range diameter result in these biases, but this “exclusion effect” (the effect of some smaller home ranges having no traps at all) is only one consequence of sexual dimorphism of home-range size. At low trap-spacing intervals, variations in home-range size result in differential numbers of traps in male and female home ranges (the “trap number effect”), although the density of traps is the same. Computer simulations were used to examine the interaction of known and postulated body-size-dependent factors, including home-range size, rate of travel, sensory acuity, and a body-size-independent factor, dimensionality of trap arrangement, as a possible cause of sexual differences in rates of encounter with traps. Rates of encounter with traps that favor capture of males and that produce greater differences in rates of capture between sexes in small species than in large ones were observed if home ranges were sexually dimorphic, traps were arranged in grids, rate of travel was proportional to the 0.25 power of body mass, and males perceived traps at the same distances as did females. Sex-specific behaviors, either resulting from sexual differences in territory packing, or in the way that animals respond to traps that they perceive, are hypothesized to be contributing causes of differences in rates of capture between sexes.
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