DOI: 10.5061/dryad.fbg79cp28
title: Losses of sexual dichromatism involve rapid changes in female plumage colors to match males in New World blackbirds

description: Differences in coloration between the sexes (sexual dichromatism) can
 increase or decrease in a species through evolutionary changes in either
 or both sexes diverging or converging in their colors. Few previous
 studies, however, have examined the relative rates of such changes,
 particularly when dichromatism is lost. Using reflectance data from 37
 species of the New World blackbird family (Icteridae), we compared
 evolutionary rates of plumage color change in males and females when
 dichromatism was either increasing (colors diverging) or decreasing
 (colors converging). Increases in dichromatism involved divergent changes
 in both sexes at approximately equal rates. Decreases in dichromatism, in
 contrast, involved changes in females to match male plumage colors that
 were significantly more rapid than any changes in males. Such dramatic
 changes in females show how selection can differ between the sexes.
 Moreover, these evolutionary patterns support the idea that losses of
 dimorphism involve genetic mechanisms that are already largely present in
 both sexes, whereas increases in dimorphism tend to involve the appearance
 of novel sex-specific traits, which evolve more slowly. Our results have
 broad implications for how sexual dimorphisms evolve.
description: We collected reflectance measurements from male and female
 representatives of 37 New World blackbird species
 (<em>Icteridae</em>) in the grackles-and-allies clade. For
 each species, we measured 22 feather patches on study skins of three males
 and three females using an S-2000 spectrometer (Ocean Optics, Dunedin,
 Florida) with an R200-7- UV /VIS reflectance probe (fiber diameter = 200
 µm) and a PX-2 pulsed xenon light source oriented perpendicular to the
 measured surface. Data consisted of the percentage of light reflected at
 each wavelength from 300 to 700 nm, calibrated against a Spectralon white
 reflectance standard and averaged into 10 nm bins, collected using
 OOIBASE32 software. We averaged the reflectance data across three
 individuals for each feather patch, separately for each sex within each
 species. We calculated color differences (ΔS) by first calculating
 receptor quantum catch values for each feather patch using the equation
 Q<sub>i</sub> = ∫<sub>λ</sub>
 R<sub>i</sub>(λ)S(λ)dλ, where λ denotes wavelength,
 R<sub>i</sub>(λ) is the spectral sensitivity of each cone cell
 of type <em>i,</em> S(λ) is the reflectance spectrum of a
 given feather patch, and integration is over the entire range of avian
 visual sensitivity (300-700 nm).
 Q<sub>1</sub>–Q<sub>4</sub> therefore represented
 the receptor quantum catch of the ultraviolet sensitive cone, the
 short-wave-sensitive cone, the middle-wave-sensitive cone, and the
 long-wave-sensitive cone, respectively. We used Maximum Likelihood
 reconstructions of continuously valued characters on a molecular phylogeny
 for the grackles and allies to estimate ancestral values at each node on
 the phylogeny for each of the four quantum catches
 (Q<sub>1</sub>-Q<sub>4</sub>), separately for both
 males and females. We summed the ∆S values for all 22 feather patches to
 calculate all-patch-∆S differences between the sexes, reflecting overall
 levels of sexual dichromatism, and changes in males and in females along
 each phylogenetic branch.
description: # Losses of sexual dichromatism involve rapid changes in female plumage
 colors to match males in New World blackbirds
 [https://doi.org/10.5061/dryad.fbg79cp28](https://doi.org/10.5061/dryad.fbg79cp28) The dataset includes all-patch-deltaS measurements used in calculating evolutionary changes in dichromatism, in male plumage, and in female plumage among 37 New World Blackbird taxa. ## Description of the data and file structure There are four CSV files: "All\_Patch\_DeltaS\_male\_vs\_female\_ML" includes measures of sexual dichromatism in extant taxa and reconstructed on the phylogeny, including 73 nodes altogether. The file includes DeltaS measures of 22 plumage patches, as well as their sums as total All-Patch values. "All\_Patch\_DeltaS\_Males\_ML" and "All\_Patch\_DeltaS\_Females\_ML" include measures of color change in males and females, respectively, on each of the 72 branches of the phylogeny. Each file includes measures of the 22 plumage patches, showing DeltaS measures from one node to the next on the phylogeny, plus their sums as total All-Patch values. The fourth file, "Changes\_In\_Dichromatism", includes calculations of changes in sexual dichromatism, color changes in males, and color changes in females on each of the 72 branches of the phylogeny. For each branch, a change in dichromatism was calculated as the difference between the All-Patch DeltaS value at the more recent node and at the older node. These dichromatism changes were then sorted by whether they were positive (scored as an "Increase"), negative (scored as a "Decrease"), or zero (scored as "NA"). Branches with zero change in dichromatism were not included in further analyses. The remaining 64 values allowed us to compare rates of evolutionary color change between the sexes when sexual dichromatism was either increasing or decreasing.