Adaptive Significance of Sexual Dimorphism in Weasels

Abstract

Field data on Mustela erminea L. were used to examine the adaptive basis for the sexual dimorphism in weasels. Two hypotheses were considered: one proposed by Brown and Lasiewski, and which regards the dimorphism as a result of intersexual selection favouring different food exploitation by males and females; both sexes possibly exploiting and defending an area in common. An alternative hypothesis is suggested in this paper and considers the dimorphism primarily as an adaptation for different roles of males and females in reproduction; small females being selected for on bioenergetic grounds and large males in connection with competitive mating. Intersexual selection favouring different food exploitation has given further advantage to small females and large males and might have increased the dimorphism. Field data showed no long-lasting pair bonds between males and females; movements and behaviour of males during the breeding season indicated a polygynous or promiscuous mating system with the females alone rearing the young. The two sexes exploited different average size of prey and used different hunting techniques (females hunted more in small rodent tunnels). Food of males and females greatly overlapped in autumn and winter when both sexes exploited small rodent populations in the same habitat. In spring and early summer males partly switched to alternative prey, associated with different habitat selection and extensive movements at mating time. Scarcity of basic food small rodents and increased availability of alternative prey tended to encourage habitat and food segregation. The data support the author's hypothesis.