CHARACTER DISPLACEMENT, SEXUAL DIMORPHISM, AND MORPHOLOGICAL VARIATION AMONG BRITISH AND IRISH MUSTELIDS

Five native terrestrial mustelids are found in Great Britain. Only thr ee of these occur in Ireland, Farmed American mink have recently estab lished feral populations on both islands. We studied inter- and intras pecific size relationships, sexual size dimorphism, and morphological variation among these mustelids. We viewed each sex as a separate morp hospecies, skull length as a measure of body size, and the upper canin e tooth as the organ used to kill prey. Geographic variation was low i n both islands, so we considered the mustelid population of each islan d a single unit. Community-wide character displacement (evidenced by e qual size ratios) was found among British mustelids for canine diamete r. For skull length it was seen only when the largely vermivorous badg er was excluded. When we added feral mink the regular pattern disappea red, but when we substituted the mink for the polecat, which is now re stricted to parts of Wales and adjacent England, community-wide charac ter displacement was manifest. For Irish mustelids size ratios were no t equal, but the pattern for canines was more regular than for skull l engths. Adding the local feral mink did not result in a regular patter n, but addition of the mink and exclusion of the badger yielded equal ratios for skull length but not for canines. These patterns plus publi shed empirical data support a hypothesis of prey size partitioning. Th e significant differences in size between some of the British and Iris h populations of the same morphospecies suggest the possibility of eco logical release among Irish mustelids, whose populations originally de rive from British ones. In particular, canine sexual size dimorphism i s greater for Irish pine martens, stoats, and mink, as would be expect ed if there were fewer competitors. For the marten and the stoat, Iris h females have evolved to be strikingly smaller than their British cou nterparts, in each case approximating the size of the male of a missin g species (polecat for the marten, weasel for the stoat). For skull le ngth there is no consistent pattern. Finally, morphological variation is greater in Ireland for five of six morphospecies, as predicted by t he niche-variation hypothesis.