VARIATION IN BODY-SIZE, SEXUAL DIMORPHISM AND AGE-SPECIFIC SURVIVAL IN STOATS, MUSTELA-ERMINEA (MAMMALIA, CARNIVORA), WITH FLUCTUATING FOODSUPPLIES

Most hypotheses attempting to explain the evolution of pronounced sexu al dimorphism in body size in the three species of weasels (Mustela er minea, M. frenata, M. nivalis) assume that sexual dimorphism is a long -term adaptation, associated with the different reproductive strategie s of the two sexes. We here examine an auxiliary hypothesis which pred icts that the degree of sexual dimorphism may also vary over the short -term because when food is temporarily abundant, sexual selection shou ld favour a greater growth rate of males than of females. This hypothe sis concerns a phenotypic response which could introduce temporarily i ncreased variation into an existing genotypic trait. We document the p resent size and sexual dimorphism of stoats introduced last century to New Zealand from Britain in relation to between-year variation in foo d supply in a single habitat (forests of southern beech, Nothofagus. s p.). Southern beech trees produce heal ll crops of flowers and seed at 3-5 year intervals, which are associated with very variable supplies of important prey of stoats, including several species of seed-eating birds, litter-feeding insects, and feral house mice (Mus musculus). Al ternative prey are scarce. Regressions of condylobasal length and head -body length on mouse population indices were significant in both sexe s. Mean condylobasal length was larger in both male and female stoats born after a heavy seedfall compared with those born in non-seedfall y ears. However, the largest males born in years of heavy seedfall were removed by selective mortality before dhe age of 3 years, so the condy lobasal lengths for old (greater than or equal to 3.0 yr) males conver ged on a common mean regardless of food supply in their birth year. Se xual dimorphism did not vary with food supplies (as reflected in seedf all records ol mouse population indices) al any age. First-year surviv orship, at least from the age of independence, was significantly negat ively correlated with density of stoats in the summer of their birth y ear. (C) 1997 The Linnean Society of London.