Natural and sexual selection act on different axes of variation in avian plumage color

Abstract

The bright colors of birds are often attributed to sexual selection on males, but in many species both sexes are colorful and it has been long debated whether sexual selection can also explain this variation. We show that most evolutionary transitions in color have been toward similar plumage in both sexes, and the color of both sexes (for example, bright or dull) was associated with indices of natural selection (for example, habitat type), whereas sexual differences in color were primarily associated with indices of sexual selection on males (for example, polygyny and large testes size). Debate about the evolution of bird coloration can be resolved by recognizing that both natural and sexual selection have been influential, but they have generally acted on two different axes: sexual selection on an axis of sexual differences and natural selection on both sexes for the type of color (for example, bright or dull).

Keywords: comparative analysis; dimorphism; plumage; sexual selection; spectrometry.

Fig. 1. Brightness (A) and hue (C) of males and females are strongly correlated in phylogenetic reduced major axis (RMA) regressions of PC scores (red lines).

Males increased in brightness (r² = 0.74, slope: 1.13, t₇₁₅ = 7.4, P < 0.001) and hue (r² = 0.40, slope: 1.52, t₈₁₆ = 16.9, P < 0.001) at a greater rate than females did (equal rate of change is indicated by the blue dashed line). (B and D) Most evolutionary transitions in brightness (B) and hue (D) were from sexually dichromatic to monochromatic for both males (blue arrows) and females (red arrows). For clarity, arrows are only shown where at least 3% of transitions occurred [median (range) for both PC1 and PC2: 1.0% (0 to 3.7%); n = 72 possible transitions from nine states]. The percentage of evolutionary time in each of the nine states is indicated inside each box. Analysis was based on stochastic character mapping of three categories of brightness and hue for each sex.

Fig. 2. Sexual dichromatism in brightness (PC1) and hue (PC2) in relation to mating system categories.

Mean (squares) and SE (lines) values are based on full phylogenetic generalized least squares (PGLS) models (table S3). Note that polygynous males were duller than females because many species had extensive black plumage (fig. S3).

Fig. 3. Plumage brightness (PC1) for each sex in relation to morphological, ecological, and behavioral traits.

Mean and P values are based on full PGLS models (table S3). Regression lines from PGLS models are shown for each sex (males, blue; females, red) plotted against the original body and testes mass data. P values for nest height refer to interactions between nest height and male parental care [coded yes (solid line) or no (dashed line)]. Dichromatism is the sum of PC scores for males minus the sum for females.

Fig. 4. Plumage hue (PC2) for each sex in relation to morphological, ecological, and behavioral traits.

Mean and P values are based on full PGLS models (table S3). See Fig. 3 legend for more details.