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Effects of directionality, signal intensity, and short-wavelength components on iridescent warning signal efficacy

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Abstract

Many bright colors function as visual signals and are produced by structural mechanisms that result in their being iridescent. Iridescent colors differ from other types of animal coloration in that they vary in both hue and intensity, as the angle of viewing and illumination change relative to the color surface. Iridescent colors can serve as warning signals by deterring predation on distasteful animals. Due to their directional and mirror-like reflection, iridescent signals have the potential to (1) vary in appearance with each approach; (2) create a flashing signal; (3) under the right conditions, be more intense than diffusely reflecting signals; (4) display different hues with different positioning of the sun, predator, and prey; and (5) display angle-dependent camouflage, which could all influence warning signal effectiveness. Here, we examine how signal intensity, short-wavelength hue, and variation in appearance affect the response of domestic chickens (Gallus gallus domesticus) to warning signals. Variation did not affect their response. Higher intensity signals were more effective in terms of predator avoidance than were lower intensity, and blue-violet signals were more effective than were blue and blue-green. This could influence the effectiveness of an iridescent warning signal in nature if the prey displays more intense, blue-violet coloration. With the properties we tested, we found no cost to having warning colors that are iridescent and that there may be benefits in the ability to change hue and display a more intense signal. We suggest future research to examine the effects of iridescent flashing and angle-dependent camouflage on warning signal effectiveness.

Significance statement

Iridescent colors have only been recently shown to function as warning signals, but we know very little about how they function to deter predation. Here, we show that birds are more likely to change their behavior in response to higher intensity and blue-violet signals compared to low intensity and blue and blue-green signals. The intensity results are consistent with previous experiments, but the role of short-wavelength hue on warning signal effectiveness has never been tested before. Variation due to the shifting appearance of an iridescent signal did not influence the response to the warning signal, consistent with studies in long-wavelength signals. We found no cost to displaying an iridescent signal and potentially an adaptive benefit as iridescent warning signals have the potential to be brighter and display a different hue based on the arrangement of light source, signaler, and receiver.

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Acknowledgments

We thank Kaci Fankhauser, Aly Galt, Sean Hannam, Drew Hensley, Ashley Lewis, Julea Shaw, and Mike Shillingburg for the assistance on this project; Brett Seymoure, Nikos Lessios, Martin Bergman, Kevin McGraw, and Brian Smith for the helpful discussions on the methodology and for comments on the manuscript. We also thank the anonymous reviewers for their helpful contributions. This work was supported by the National Science Foundation (IOS 1145654 to RLR).

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Authors and Affiliations

  1. School of Life Sciences, Arizona State University, Tempe, AZ, USA

    Kimberly V. Pegram & Ronald L. Rutowski

  2. Desert Botanical Garden, Phoenix, AZ, USA

    Kimberly V. Pegram

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  1. Kimberly V. Pegram
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  2. Ronald L. Rutowski
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Correspondence to Kimberly V. Pegram.

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All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted. All procedures used in this study were approved by the Arizona State University Institutional Animal Care and Use Committee (Protocol no. 14-1349R). At the end of the experiment, most of the chickens were adopted for personal use but some males were humanely euthanized. This article does not contain any studies with human participants performed by any of the authors.

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The authors declare that they have no competing interest.

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Pegram, K.V., Rutowski, R.L. Effects of directionality, signal intensity, and short-wavelength components on iridescent warning signal efficacy. Behav Ecol Sociobiol 70, 1331–1343 (2016). https://doi.org/10.1007/s00265-016-2141-z

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