Fig. 1

Variation in the rate of testes size evolution across tetrapods. The branches of the tetrapod phylogeny (N = 1845) are measured to represent the rate of testes mass evolution (time multiplied by the median of the posterior distribution of rates estimated for each branch, methods). Lineages with comparatively fast rates of evolution in testes mass relative to body mass are represented by longer branches. Branches are additionally coloured according to their relative rate of phenotypic evolution (see scale). Silhouettes are added for purely illustrative purposes to indicate lineages with relatively rapid change in relative testes mass; they are not presented at any scale. All silhouettes are in the public domain and are obtained from phylopic.org