Selection on females can create 'larger males'

In many bird and mammal species, males are significantly larger than female s. The prevailing explanation for larger-sized males is that sexual selecti on drives increased male size. In addition, researchers commonly assume tha t the extent of dimorphism indicates the strength of selection for increase d size in males. Here, through reconstruction of ancestral morphology for m ales and females of one large avian clade we present data that contradict t his assumption and illustrate that selection for decreased female size expl ains 'male-biased' dimorphism ca. 50% of the time. Our findings are also in consistent with ecological niche partitioning between the sexes and increas ed breeding benefits from reduced female size as general explanations for t he evolution of size dimorphism within the clade. We conclude that it is in correct to assume sexual dimorphism results from a single selective factor, such as directional sexual selection on increased male size. Rather, we su ggest that the selective forces leading to sexual dimorphism may vary betwe en species and should he tested on a case-by-case basis using a phylogeneti c approach.