RUI: Evaluating Mechanisms of Selection for the Maintenance of Sexual Dimorphism

Amherst College, Amherst MA

Investigators

Abstract

Sexual differences are widespread in animals, including humans, and biologists recognize three mechanisms for their evolution: 1. Sexual selection resulting from mate competition and mate choice; 2: Natural selection resulting from sexual differences in reproductive roles of males; and 3. Natural selection resulting from competition between the sexes for food. Field experiments under natural conditions will be used to examine the roles of natural and sexual selection in the maintenance of sexual dimorphism. Although studies demonstrate that sexual selection and reproductive role mechanisms operate in natural populations, almost nothing is known about the role of food competition in the maintenance of sexual dimorphisms. The problem is that in many species, sexual differences in diet are often associated with sexual differences in size, making it unclear whether dietary differences are the cause or the consequence of dimorphism. The proposed studies avoid this problem by focusing on sexual dimorphisms in feeding anatomy: the beak size and shape of a hummingbird, which can be matched to the size and shape of the flowers the birds visit. Consequently, the proposed studies will be the first to unambiguously link sexual differences in feeding morphology to sexual differences in feeding performances at natural food sources, and to separate the effects of natural from sexual selection on body size and anatomy.