Variation in adult sex ratios in tetrapods is linked to sex chromosomes through mortality differences between males and females
Fig 2
Relationships between the type of genetic sex-determination (GSD) system, adult sex ratio (ASR), and sex bias in demographic traits across tetrapods.
(A) Phylogenetic distribution of the six variables across 453 tetrapod species. The innermost color circle shows the type of GSD (XX/XY and ZZ/ZW sex-chromosome systems abbreviated as XY and ZW, respectively), whereas the outer circles illustrate sex biases shown as female (red, negative values) or male (blue, positive values) bias, in the following order from inner circle to outer circle: ASR, birth sex ratio (both sex ratios depicted as deviations from 1:1), and sex differences in juvenile and adult mortality and maturation age. Bias values are plotted as standardized (z) scores centerd on zero (i.e., no bias) for ease of visualization on a comparable scale. Extreme positive and negative values are plotted as 3 and −3 to prevent outliers from obscuring the major patterns. Note that male-biased values mean male-skewed sex ratios and males having higher mortality and later maturation. Missing values are indicated by gray color. (B) Differences in ASR and other demographic traits between XY and ZW systems. In each violin plot, the black diamond and the white box represent the median and interquartile range, whiskers extend to 1.5 × interquartile range, and the polygon is a kernel density plot. Sample sizes (number of species) are shown above each plot; asterisks mark statistically significant differences (*: p < 0.05, ***: p < 0.001; see S2 Table). (C) Relationships of ASR with demographic traits. Data points represent species, colored by the four major tetrapod clades as shown by the color of the animal silhouettes around the phylogeny (A). The regression lines are calculated from phylogenetic generalized least squares (PGLS) models (continuous lines: statistically significant relationships, p ≤ 0.01; dashed lines: non-significant relationship; see S2 Table). Light gray polygons illustrate the 95% confidence bands of the regression lines assuming Brownian motion. In both (B) and (C), >0.5 sex ratio values mean male-skewed sex ratios, while positive bias values mean higher male than female mortalities and maturation times. The data and phylogeny underlying this figure can be found in S1 Data and S1 File, 10.6084/m9.figshare.28562399.