What's in a band? The function of the color and banding pattern of the Banded Swallowtail.

Eunice J Tan,Antónia Monteiro,Bodo D Wilts,Brent T K Tan

doi:10.1002/ece3.6034

Abstract

Butterflies have evolved a diversity of color patterns, but the ecological functions for most of these patterns are still poorly understood. The Banded Swallowtail butterfly, Papilio demolion demolion, is a mostly black butterfly with a greenish‐blue band that traverses the wings. The function of this wing pattern remains unknown. Here, we examined the morphology of black and green‐blue colored scales, and how the color and banding pattern affects predation risk in the wild. The protective benefits of the transversal band and of its green‐blue color were tested via the use of paper model replicas of the Banded Swallowtail with variations in band shape and band color in a full factorial design. A variant model where the continuous transversal green‐blue band was shifted and made discontinuous tested the protective benefit of the transversal band, while grayscale variants of the wildtype and distorted band models assessed the protective benefit of the green‐blue color. Paper models of the variants and the wildtype were placed simultaneously in the field with live baits. Wildtype models were the least preyed upon compared with all other variants, while gray models with distorted bands suffered the greatest predation. The color and the continuous band of the Banded Swallowtail hence confer antipredator qualities. We propose that the shape of the band hinders detection of the butterfly's true shape through coincident disruptive coloration; while the green color of the band prevents detection of the butterfly from its background via differential blending. Differential blending is aided by the green‐blue color being due to pigments rather than via structural coloration. Both green and black scales have identical structures, and the scales follow the Bauplan of pigmented scales documented in other Papilio butterflies.

Highlights

Animals have evolved a bewildering diversity of color patterns
Our results clearly show that the transversal greenish-blue band of the wildtype is more effective in deterring predation than a nontransversal band or a gray colored band of either type
The distortion of the transverse band is clearly affecting predation risk. We suggest that this indicates that the transverse band confers a protective advantage in the form of coincident disruptive coloration, as it disrupts the true shape of the butterfly

Summary

| INTRODUCTION

Animals have evolved a bewildering diversity of color patterns. Some of these color patterns are used to signal to the opposite sex (e.g., Baldwin & Johnsen, 2009; Engelking, Roemer, & Beisenherz, 2010; Lim, Land, & Li, 2007), but perhaps the majority help in providing protection from potential predators (e.g., see reviews by Stevens and Merilaita (2011), Merilaita, Scott-Samuel, and Cuthill. We hypothesized that the transverse band of the Banded Swallowtail may be a form of coincident disruptive coloration that disguises the shape of the butterfly, preventing recognition by predators, while its greenish-blue color functions to disrupt the butterfly shape through differential blending. To test the protective benefits of both the transversal band and its green-blue color simultaneously, we created a grayscale variant with a distorted transversal band These four different types of paper models were. We hypothesized that if the butterfly coloration has evolved to background match its environment to reduce detection and predation, the blue-green color of the wing scales should originate from pigmentary absorption rather than to structural colors This is because structural colors are usually iridescent, that is, the color changes depending on the observation and illumination angle. We show that pigments are involved in producing this blue-green color resulting in the butterfly having a matt appearance with no iridescence

| MATERIALS AND METHODS

Findings

| DISCUSSION