Male behavior in a swallowtail butterfly (Battus philenor) ensures directional iridescent sexual signal is visible to females during courtship

Iridescent colors in animals may vary with the arrangement of the observer and the light source relative to the animal. When these highly directional colors are used as signals, there may be transmission problems for senders in that the perception of the signal by intended receivers can be greatly affected by the spatial arrangement of the sender, receiver, and sources of contributing light. One potential solution to this problem is for senders to behave in a manner that arranges the positions of sender, light source, and receiver to enhance signal perception by the receiver. We tested this idea by describing the structure of courtship behavior of males of the Pipevine Swallowtail ( Battus philenor ) butterfly and analyzing its consequences for the female detection of the male’s iridescent sexual signal which is used in mate choice by females. During courtship, males perform a swoop maneuver that brings their dorsal hindwing from below to directly in front of the female. Several swoops in rapid succession occur in aerial courtships that lead to copulation. We measured under solar radiation the radiance of the male’s dorsal hindwing as seen by the female during a swoop. Regardless of how the male’s swoop path is oriented relative to the sun, there will be a brief, bright, and saturated flash of blue visible to the female. Our results support the conclusion that male swoops enhance the visibility of the male’s iridescent color signal to the female during courtship. ### Competing Interest Statement The authors have declared no competing interest.