Sexually asymmetric colour-based species discrimination in orangethroat darters

Female mate preferences, male mate preferences and male-male competition can all potentially play roles in promoting speciation by sexual selection. The orangethroat darter, Etheostoma spectabile, is a sexually dichromatic fish in which males compete for access to females. We quantified male and female responses to conspecific versus heterospecific fish under normal light, dim light and blue light, the last of which impaired colour perception. We found that behavioural isolation was likely mediated primarily by male behavioural discrimination against heterospecific fish, with females showing no evident mate preferences. Furthermore, male aggression towards conspecific rivals was reduced in blue light, suggesting that the evolution of male coloration may contribute to speciation through male-male interactions.