Sexual dimorphism is widespread across sexually reproducing organisms. Males and females can differ dramatically in morphology, behaviour and physiology. Some of this dimorphism results from genetic adaptations that reside on sex chromosomes (Mank, 2009).However, many of these phenotypic differences are instead mediated by the differential expression of genes present in both sexes (Ellegren & Parsch, 2007). Sex-biased gene expression has been widely studied and varies among species, tissues and developmental