Sex reversal induces size and performance differences among females of the African pygmy mouse, Mus minutoides

Abstract

Differences in biological performance, at both intra- and inter-specific levels, have often been linked to morphology but seldom to behavioural or genotypic effects. We tested performance at the intraspecific level by measuring bite force in the African pygmy mouse, Mus minutoides. This species displays an unusual sex determination system, with sex-reversed, XY females carrying a feminizing Xchromosome. XY females cannot be differentiated from XX females based on external or gonadal morphology; however, they are known to be more aggressive. We found that bite force was higher in XY females than in other females and males. We then performed geometric morphometric analyses on their skulls and mandibles and found that the higher performance of XY females was mainly explained by a greater overall skull size. The effects of the X chromosome thus go beyond feminization, and extend to wholeorganism performance and morphology. Our results also suggest limited effects of behaviour on bite force.