Data from: 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 females in such a way as to simultaneously maximise three components of known or putative signal conspicuousness: brightness, area, and iridescent flash. This suggests that male signal design and display have coevolved for the delivery of an optimally conspicuous signal to courted females. More broadly, these findings imply a potential signalling role for iridescence itself, and pose a novel example for how signal design may coevolve with the behavioural context of display