DOI: 10.5061/dryad.37pvmcvg2
title: Evolutionary trade-offs may interact with physiological constraints to maintain color variation

description: Animal coloration is a multifaceted trait with many ecological roles and
 related to a variety of developmental and physiological processes.
 Consequently, coloration is often subject to a variety of selective
 pressures, leading to the evolutionary maintenance of variation. In this
 study, we investigated hypotheses related to the maintenance of dorsal
 color variation in wood frogs (Rana sylvatica). First, we tested for
 multimodality, and whether color correlates with body size or condition or
 varies by sex or age-class. We combined behavioral trials with visual
 modeling to test for sex recognition. We also considered visual models for
 predators and tested for an interaction between discriminability indexes
 (JND) of color channel (chromatic vs. achromatic) and predator type (birds
 vs. snakes), as well as for a within individual trade-off between the JND
 of chromatic and achromatic coloration. Finally, we tested for disruptive
 viability selection on color using predation trials, and for antagonistic
 directional selection between viability selection and reproductive
 investment of females. We found that wood frogs present continuous color
 variation that does not correlate with body size or condition, but that
 changes with age. Wood frogs present subtle sexual dichromatism, but we
 found no evidence for a role of color in sex recognition. Instead, we
 discuss the possibility that sex differences might, at least in part, have
 a demographic explanation. Predator visual models indicated that wood
 frogs cannot solely rely on dorsal coloration for camouflage. Moreover,
 different predators might present selective pressures in different color
 channels, while individuals’ achromatic and chromatic coloration trade-off
 in JND. Therefore, different selective pressures caused by different
 predators might interact with ontogenetic changes and
 developmental/physiological trade-offs to maintain color variation. We
 found no relationship between color and survival or reproductive
 investment, suggesting further work is required to fully understand
 selection on color. Our results highlight the importance of understanding
 evolutionary trade-offs and developmental/physiological constraints in
 combination with one another, and suggest the potential for an interaction
 between these proximate and ultimate mechanisms in the evolutionary
 maintenance of variation. These results likely extend beyond color
 expression in amphibians, and exemplify a more general process for such
 evolutionary outcomes.
description: 1) Digital images color data was obtained using MICA plugin in
 ImageJ (see manuscript).  -- Raw files are named:
 "ResultsMICA" for the frogs, and
 "MICA_Standards_noOverexposed" for color callibration
 cards.  -- Data used in analyses was transformed, and
 is saved as "HSVdata_Pictures" 2)
 Spectrophotometry files were imported into R using the package pavo (see
 manuscript).  -- Data used in analyses was transformed,
 and is saved as "CombinedSpecs",
 "backgroundSpecs_NegValuestoMin", and
 "frogSpecs_NegValuestoMin" 3) Ocular media
 data:  -- frogs: digitized from: Govardovskii VI, Zueva
 LV. 1974. Spectral sensitivity of the frog eye in the ultraviolet and
 visible region. Vision Research 14:1317–1321.  --
 kestrel: downloaded from: Lind O, Mitkus M, Olsson P, Kelber A. 2013.
 Ultraviolet sensitivity and colour vision in raptor foraging. J Exp Biol
 216:3764–3764. (<strong>Be careful, correct
 kestrel data is accessed through the correction note:
 https://jeb.biologists.org/content/suppl/2013/04/25/216.10.1819.DC1</strong>)  -- snake: obtained from B. Simões, data from: Simões BF, Sampaio FL, Douglas RH, Kodandaramaiah U, Casewell NR, Harrison RA, Hart NS, Partridge JC, Hunt DM, Gower DJ. 2016. Visual pigments, ocular filters and the evolution of snake vision. Molecular Biology and Evolution 33:2483–2495.
description: - kestrel and snake ocular media data are not provided within
 data folders since obtained from external sources -
 directory structure: codes and datasets are split into 3 main
 folders: 1) "Codes-Experiments" = codes and
 data in reference to behavioral experiments 2)
 "Codes-Pictures" = all codes and data for analyses in which
 color data obtained from digital images were used 3)
 "Codes-Specs" = all codes and data for analyses in which color
 data obtained from spectrophotometry were used -
 additional analyses / figures for response to reviews are found in folder
 "revisions"