Sexual dimorphism in trophic structures is a common phenomenon in most animal phyla, and has attracted considerable interest from evolutionary biologists. In most animals, males and females differ in size, sometimes substantially. The usual cause cited for sexual dimorphism in animals is sexual selection acting through female choice and/or male-male competition.
Natural selection acting on the fitness advantages of reduced resource competition between the sexes, however, is also an important alternative evolutionary scenario that can produce sexual dimorphisms. This alternative idea is that sexual differences in body size or morphology may evolve due to ecological causes - that is, the ecological benefit of the sexes occupying different ecological niches, which reduces intersexual food competition and expands the species' overall feeding niche. Adaptation to different diets may also be due to sex-specific nutritional requirements, as for example females may have additional dietary components compared to males - this directly relates to nutrients needed for the production of eggs. However, sexual selection and natural selection for sexual dimorphisms are not incompatible; for example, it may well be that the direction of dimorphism in body size is determined by sexual selection, but the degree to which the sexes diverge is either constrained or amplified by ecological factors.
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