Species in which the sexes equally exhibit colourful ornaments are an issue for evolutionary theory. Among several hypotheses, sexual selection for mutual mate choice and social selection for signals of behavioural dominance are most commonly supported. We examined the previously documented sex-similar size of yellow-orange ear patches in the king penguin, Aptenodytes patagonicus. This species is monogamous and pairs just before reproduction. Raising a chick requires considerable effort by both parents, as they alternate care of their single offspring with foraging at sea. The size of the ear patches appears to signal aggressive territoriality in the breeding colony for both sexes. However, experiments suggest that females prefer large patch size during mate choice, and males do not prefer this trait. We tested whether the size of the coloured ear patch was influenced by sexual selection for couples that had recently paired. We used analyses of covariance to compare the size of the ear patch to a measure of body size and then tested for the difference between males and females. Males were 6.2% larger in ear patch width and 7.7% larger in ear patch area than females, and the distance between the ear patches over the head was 7.5% smaller in males, with all differences highly significant. Consequently, sexual selection appears to favour larger ear patches in males, possibly because of an excess of males that promotes female choice. Social selection also appears to favour the evolutionary maintenance of ear patches of males, and thus both types of selection may contribute to enlarged ear patches.