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How many color metrics do we need? Evaluating how different color-scoring procedures explain carotenoid pigment content in avian bare-part and plumage ornaments

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Abstract

For a variety of technical and conceptual reasons, biologists have come to use several different methods to quantify the colors of animals. However, the relative abilities of these different color-scoring procedures to capture variation in the actual color-generating mechanisms—pigment or structural composition of the integument—have never been tested systematically. Here, we examined which commonly employed color metrics predict carotenoid content of ornaments in three avian species (house finch Carpodacus mexicanus, mallard duck Anas platyrhynchos, and zebra finch Taeniopygia guttata). We used spectrophotometry to measure reflectance spectra from beak and feather tissue, calculated numerous color metrics (e.g., hue, chroma, brightness, principal components, and tetrahedral color space position) from these spectra, and determined carotenoid content at the site of color measurement with high-performance liquid chromatography. We found that several principal component, tristimulus, and avian visual model metrics significantly correlated with carotenoid content of house finch feathers and duck beaks. Carotenoid content of mallard beaks was most closely correlated with brightness and saturation metrics, whereas in house finch feathers, carotenoid concentration was best captured by hue and saturation metrics. According to tristimulus scores and visual models, we found that the ultraviolet portion of the spectrum was not an essential predictor of variation in carotenoid content. Also, visual model chromatic contrasts generally were not significant predictors of carotenoid content, although some achromatic contrasts and tetrahedral color space vector parameters were. Our results indicate that numerous methods, especially tristimulus scores, are suitable for capturing pigment-based color variation in two carotenoid-containing ornaments, and we discuss the merits and shortcomings of these different approaches. In contrast, there were no significant relationships between any color metrics and the carotenoid content of zebra finch beaks, suggesting that other color-generating mechanisms besides carotenoids may contribute to color variability in this species.

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Abbreviations

B:

Brightness

H:

Hue

HPLC:

High-performance liquid chromatography

LWS:

Long wavelength sensitive

MWS:

Medium wavelength sensitive

PC:

Principal component

PCA:

Principal components analysis

S:

Saturation

SWS:

Short wavelength sensitive

UV:

Ultraviolet

UV-Vis:

Ultraviolet and human-visible

UVS:

Ultraviolet sensitive

VS:

Violet sensitive

θ:

Theta in visual model; relative stimulation of SWS, MWS, and LWS photoreceptors

φ:

Psi in visual model: stimulation of the UV/V sensitive photoreceptors

r :

R in visual model: chromaticity or spectral purity

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Acknowledgments

We would like to thank E. Tourville, A. Bascuñán, and V. Peng with laboratory and animal assistance, and several anonymous reviewers for comments that greatly improved the manuscript. Funding was provided to KJM by the National Science Foundation (IOS-0746364, IOS-0910357, IOS-0923694, and IOS-0925633).

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Authors and Affiliations

  1. Arizona State University, School of Life Sciences, PO Box 874601, Tempe, AZ, 85287-4601, USA

    Michael W. Butler, Matthew B. Toomey & Kevin J. McGraw

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  1. Michael W. Butler
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  2. Matthew B. Toomey
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Correspondence to Michael W. Butler.

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Communicated by E. Fernandez-Juricic

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Butler, M.W., Toomey, M.B. & McGraw, K.J. How many color metrics do we need? Evaluating how different color-scoring procedures explain carotenoid pigment content in avian bare-part and plumage ornaments. Behav Ecol Sociobiol 65, 401–413 (2011). https://doi.org/10.1007/s00265-010-1074-1

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