Nutritional aspects of sexual dimorphism in the American mink Mustela vison (Schreber)
Nutritional aspects of size-related sex differences in the diets of free-living mink were investigated in laboratory-based feeding trials with adult farm-bred mink maintained on 'natural' diets. As preliminary studies had shown that carcase utility was virtually complete, the rations presented comprised the minced whole carcasses of wild rabbit Oryctolagus cunniculus, eel Anquilla anguilla, laboratory rats and mice, and domestic fowl. Determinations of gross composition revealed significant differences between these diets; the smaller prey types, including rodents, birds and fish, were found to have a higher ratio of Apparent Digestible Energy to Nitrogen than larger items such as rabbits, although comparisons with data presented by other workers demonstrated that the variations between species within these prey groups are as great as, or greater than, those between the diets themselves. From the results of the feeding trials, it was also apparent that such diets do not differ significantly, either in digestibility or biological value and attempts to classify particular prey items in terms of their nutritional value are, therefore, of limited application in analyses of the feeding ecology of a generalist predator. Nutrient intake was related to diet composition and varied widely between trials, although the mass-specific requirements of females were higher than those of males. Comparisons of gut morphology indicated that, in females, hypertrophy of the alimentary tract may develop in response to increased energy demands. A similar adaptation was evident in both males and females from wild populations, suggesting that the natural diets of free-living mink are generally of a lower quality than the rations fed to commercially raised animals. Feeding trials were also carried out on growing kits from 56 to 105 days post partum. Sex differences in nutrient metabolism were not significant but the growth rates of males were higher than those of females. In both sexes the growth rates of kits feeding on 'natural' diets were lower than those of animals raised on commercial rations. This effect was most pronounced in males, a finding which supports the hypothesis that the degree of sexual dimorphism in this species is dependent on the extent to which the growth potential of males is constrained by dietary regime during the early phases of development.