Inter‐ and Intraspecific Character Displacement in Mustelids

Abstract

In a search for possible community—wide character displacement (manifested as equal size ratios between adjacent morphospecies in a size—ranking), we examined the weasels (Mustela) of North America and also the mustelid—viverrid guild of Israel. We measured condylobasal skull length (CBL) and maximal diameter of the upper canine (C sup L) on museum specimens of the two or three weasel species sympatric at each of eight North American sites. We found substantial geographic variation and, at each site, pronounced sexual dimorphism of each species, as well as geographic variation in the degree of sexual dimorphism. With conspecific males and females viewed as separate "morphospecies," we found little variance about the mean measurement for a morpho—species at each site, and almost no overlap of distributions between sympatric morpholo—species. In site of the geographic variation, at seven of the eight sites the size ratios for C sup L between adjacent morphospecies in a size—ranking were remarkably equal, while the analogous analysis for CBL showed ratios tending towards equality at only four sites. Indications of equal ratios were largely absent for both variables when sexes were analyzed separately. For a more taxonomically diverse "mustelid—viverrid" guild of five species in Israel, the 10 ranked morphospecies had extraordinarily equal size ratios for C sup L but not for CBL, a result maintained if sexes were analyzed separately. These results, plus information on feeding behavior of mustelids, seem consistent with a hypothesis of competitive character displacement, but many critical data remain uncollected. There is no direct evidence that prey are limiting to either guild, data are scant on what the morphospecies actually eat, and there is no functional analysis for these species of the role of canine or skull size in ability to kill prey of different sizes and types. Sexual selection based on canine display or fighting might well contribute to sexual dimorphism. Though direct observation is lacking, it is also possible that canines may be used in threats or fighting in interspecific encounters. However,it seems unlikely that such canine use could account for the entire pattern of size ratio equality, as it would be quite coincidental that size ratios sexually selected between sexes within species should happen to equal those, however selected, between the male of each species and the female of the next larger species.