Sexually attractive males produce relatively more sons in an externally fertilising fish

According to the mate attractiveness hypothesis of biased progeny sex-ratio, sexually attractive males should produce relatively more sons. We tested this hypothesis with three-spined sticklebacks Gasterosteus aculeatus, a fish species with external fertilisation, in which the intensity of red breeding coloration is a sexually selected trait. We used reflecto-spectrophotometry and visual modelling to assess red breeding coloration from a stickleback’s point of view. More intensely red-coloured males produced relatively more sons as assessed with molecular sexing three weeks after hatching of 34 clutches from different parents. Confounding factors like mortality rate and maternal and paternal effects other than male coloration could largely be ruled out. We discuss potential mechanisms for the role of fathers in sex-ratio allocation.