Sexual Dimorphism in the Body Size of Mustelids (Carnivora): The Roles of Food Habits and Breeding Systems

Abstract

In mustelids males are always the larger sex. In 24 samples from 15 species there is a significant inverse correlation between the extent of sexual dimorphism (measured as the ratio of male to female weight) and male weight. Two explanations for the selective advantages of dimorphism are discussed. The first hypothesis proposes that it reduces intersexual competition for food by enabling each sex to exploit different prey. Available data on food habits are insufficient to test the theory directly. Other objections, however, lead to the conclusion that avoidance of competition is not the primary advantage of dimorphism. The second hypothesis takes into account the polygynous breeding systems of mustelids and the fact that females alone raise their litters. It proposes that small females are favoured because they need less energy for daily maintenance and are probably more efficient in hunting small prey. Because of this they can channel more energy into reproduction than larger females. Large males are favoured by sexual selection and the ability to exploit a wider range of prey. Data are presented for weasels Mustela nivalis showing that towards the end of lactation an average female requires daily about 20% less energy than a hypothetical male-sized female. The optimum sizes of each sex result from different selective pressures, and probably vary independently. Relationships between reproductive strategy, diet and dimorphism are discussed.