Multimodal signalling: structural ultraviolet reflectance predicts male mating success better than pheromones in the butterfly Colias eurytheme L. (Pieridae)

In sexual selection, multimodal signals elicit mate choice when more than one sensory modality is activated. However, determining the relative use of each signal is difficult because it requires a comprehensive understanding of the mating system and how this system works under natural conditions. We examined the role of structural ultraviolet (UV) reflectance and pheromones in the butterfly Colias eurytheme. Both traits are important in mediating interspecific interactions and pheromones have been implicated in intra-specific mate choice. UV reflectance, which arises from the presence of a multilayer thin-film interference array, has potential as an honest indicator of male condition, viability and/or age. We investigated the relevance of these signal traits to courtship success by releasing virgin females in the path of free-flying males until each female had rejected and accepted at least one male. This design facilitated a within-subjects (females) analysis of mate choice, thus controlling for potentially confounding variation in intrinsic female receptivity. Principal component analysis indicated that variation across males in UV brightness and pheromones was essentially orthogonal. Females preferred younger males (as subjectively adjudged by wing wear), and while age covaried with UV brightness and almost all pheromone descriptors, UV brightness emerged as the best and most general predictor of male mating success. Our results suggest that this trait serves as an important intraspecific sexual signal in C. eurytheme, and they provide the clearest evidence to date regarding the functional relevance of structural coloration to female mate choice in butterflies. We discuss the preferential use of one secondary sexual characteristic (UV reflectance) over another (pheromones) with regard to evolutionary strategies. (c) 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.