Human Vision Fails to Distinguish Widespread Sexual Dichromatism among Sexually "Monochromatic" Birds

Historical scenarios of evolution of avian plumage coloration have been called into question with the discoveries that most birds can see UV light (which normal humans cannot), and that UV-reflecting plumages are widespread in birds. Several examples of sexual dichromatism not detectable with human...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2005-08, Vol.102 (31), p.10942-10946
Main Authors: Eaton, Muir D., Orians, Gordon H.
Format: Article
Language:English
Subjects:
Animals
Aves
Biological Evolution
Biological Sciences
Birds
Birds - genetics
Birds - physiology
Color Perception
Colors
Evolution
Feathers
Female
Gender discrimination
Humans
Male
Male animals
Pigmentation - genetics
Plumage
Receptors
Sex Characteristics
Spectral reflectance
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Summary:Historical scenarios of evolution of avian plumage coloration have been called into question with the discoveries that most birds can see UV light (which normal humans cannot), and that UV-reflecting plumages are widespread in birds. Several examples of sexual dichromatism not detectable with human visual capabilities suggest that our categorizations of plumages as sexually mono- or dichromatic might often be incorrect. Nonetheless, given the limited taxonomic scope of those examples, the vast majority of sexually monochromatic birds are still treated as such without question in avian research. Herein, I show that >90% of 139 species, in a broad sampling of presumed sexually monochromatic passerine birds, were actually sexually dichromatic from an avian visual perspective, based on comparisons of plumage reflectance data using a visual model of color discrimination thresholds. The taxonomic ubiquity of this result suggests that many existing interpretations of evolutionary patterns of sexual dichromatism in birds are erroneous. The visual model used herein provides a method for quantifying sexual dichromatism, revealing that most (58.7%) feather patches sampled lie along a continuum of dichromatism between avian and human discriminatory abilities and could represent unrecognized sexually selected signals. Sexual dichromatism in this study rarely resulted from intersexual differences in UV coloration alone, emphasizing the need for analysis of bird coloration in relation to the full extent of avian visual discriminatory abilities, including, but not limited to, UV-visual capabilities.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.0501891102