Assessing Sexual Dicromatism: The Importance of Proper Parameterization in Tetrachromatic Visual Models

PLoS One. 2017 Jan 11;12(1):e0169810. doi: 10.1371/journal.pone.0169810. eCollection 2017.

Abstract

Perceptual models of animal vision have greatly contributed to our understanding of animal-animal and plant-animal communication. The receptor-noise model of color contrasts has been central to this research as it quantifies the difference between two colors for any visual system of interest. However, if the properties of the visual system are unknown, assumptions regarding parameter values must be made, generally with unknown consequences. In this study, we conduct a sensitivity analysis of the receptor-noise model using avian visual system parameters to systematically investigate the influence of variation in light environment, photoreceptor sensitivities, photoreceptor densities, and light transmission properties of the ocular media and the oil droplets. We calculated the chromatic contrast of 15 plumage patches to quantify a dichromatism score for 70 species of Galliformes, a group of birds that display a wide range of sexual dimorphism. We found that the photoreceptor densities and the wavelength of maximum sensitivity of the short-wavelength-sensitive photoreceptor 1 (SWS1) can change dichromatism scores by 50% to 100%. In contrast, the light environment, transmission properties of the oil droplets, transmission properties of the ocular media, and the peak sensitivities of the cone photoreceptors had a smaller impact on the scores. By investigating the effect of varying two or more parameters simultaneously, we further demonstrate that improper parameterization could lead to differences between calculated and actual contrasts of more than 650%. Our findings demonstrate that improper parameterization of tetrachromatic visual models can have very large effects on measures of dichromatism scores, potentially leading to erroneous inferences. We urge more complete characterization of avian retinal properties and recommend that researchers either determine whether their species of interest possess an ultraviolet or near-ultraviolet sensitive SWS1 photoreceptor, or present models for both.

MeSH terms

  • Animals
  • Color Vision*
  • Female
  • Galliformes / physiology*
  • Male
  • Models, Neurological*
  • Photoreceptor Cells / physiology
  • Sex Characteristics*
  • Signal-To-Noise Ratio
  • Visual Perception

Grants and funding

Funding was provided by the Natural Sciences and Engineering Research Council of Canada in the form of a Canada Graduate Scholarship to P-P.B. as well as Discovery and Equipment grants to S.M.D, an Ontario Graduate Scholarship to P-P.B., as well as a Field Museum Visiting Scholar Grant and a Collections Study Grant from the Richard Gilder Graduate School in association with the American Museum of Natural History to K.J.