Sexually dimorphic behaviours, such as parental care, have long been thought to be driven mostly, if not exclusively, by gonadal hormones. In the past two decades, a few studies have challenged this view, highlighting the direct influence of the sex chromosome complement (XX vs XY or ZZ vs ZW). The African pygmy mouse, Mus minutoides, is a wild mouse species with naturally occurring XY sex reversal induced by a third, feminizing X* chromosome, leading to...