Elsevier

Animal Behaviour

Volume 73, Issue 1, January 2007, Pages 47-54
Animal Behaviour

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

https://doi.org/10.1016/j.anbehav.2006.07.004Get rights and content

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 intraspecific 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.

Section snippets

General

We reared virgin females on alfalfa (Medicago sativa) from eggs laid by female C. eurytheme collected in Chandler, Arizona, U.S.A. in May 1998. Females used in the experiment were kept at 4°C for up to 5 days posteclosion, and were marked with a red felt-tipped pen on both ventral hindwings for identification before release in the field. Female, but not male, C. eurytheme are dimorphic: most are orange-yellow, some are white (alba). We only used orange-yellow phenotype females because alba

Results

Successful courtships lasted significantly longer on average (21.2 ± 4.8 s) than unsuccessful courtships (12.7 ± 3.39 s; paired t test: t47 = 2.93, P < 0.01; Cohen's d = 0.598). Males that were accepted by females were younger (as classified using wing wear) than rejected males (logistic regression: G1 = 12.4, N = 48, P < 0.0005; mean age difference (accepted  rejected): Cohen's d = −1.46), and all potential signal trait variables except for C27 covaried with age (Table 2).

Evaluation of the mean signal

Discussion

Our results suggest three major findings. First, female C. eurytheme prefer to mate with relatively younger males (i.e. males with less wing wear). Second, almost all UV reflectance and pheromone traits covaried with age (Table 2) yet variation among these two trait groups was largely orthogonal and therefore uncorrelated (Table 3). Third, among potential signal traits, the brightness of UV reflectance was the strongest and most informative predictor of male courtship success. Females may also

Acknowledgments

Work described here was submitted in partial fulfilment of a Ph.D. thesis at Arizona State University. We thank Emily Hayes for valuable help in the field and Laura Taylor-Taft and Joe Macedonia for help with the UV analysis. We also gratefully acknowledge Rob Raguso for loaning us his equipment to measure UV reflectance. Extremely helpful comments on this manuscript were provided by David Jennings. This material is based upon work supported by the PEO foundation and the National Science

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