Signal design and courtship presentation coincide for highly biased delivery of an iridescent butterfly mating signal

Sensory drive theory contends that signalling systems should evolve to optimize transmission between senders and intended receivers, while minimising visibility to eavesdroppers where possible. In visual communication systems, the high directionality afforded by iridescent colouration presents underappreciated avenues for mediating this trade-off. This hypothesis predicts functional links between signal design and presentation such that visual conspicuousness is maximised only under ecologically relevant settings and/or to select audiences. We addressed this prediction using Hypolimnas bolina, a butterfly in which males possess ultraviolet markings on their dorsal wing surfaces with a narrow angular reflectance function. Males bearing brighter dorsal markings are increasingly attractive to females, but also likely more conspicuous to predators. Our data indicate that, during courtship (and given the ritualized wingbeat dynamics at these times), males position themselves relative to fema...