Sexual dichromatism in frogs: natural selection, sexual selection and unexpected diversity

Abstract

Sexual dichromatism, a form of sexual dimorphism in which males and females differ in colour, is widespread in animals but has been predominantly studied in birds, fishes and butterflies. Moreover, although there are several proposed evolutionary mechanisms for sexual dichromatism in vertebrates, few studies have examined this phenomenon outside the context of sexual selection. Here, we describe unexpectedly high diversity of sexual dichromatism in frogs and create a comparative framework to guide future analyses of the evolution of these sexual colour differences. We review what is known about evolution of colour dimorphism in frogs, highlight alternative mechanisms that may contribute to the evolution of sexual colour differences, and compare them to mechanisms active in other major groups of vertebrates. In frogs, sexual dichromatism can be dynamic (temporary colour change in males) or ontogenetic (permanent colour change in males or females). The degree and the duration of sexual colour differences vary greatly across lineages, and we do not detect phylogenetic signal in the distribution of this trait, therefore frogs provide an opportunity to investigate the roles of natural and sexual selection across multiple independent derivations of sexual dichromatism.

Figure 1.

Examples of frog species showing (a,b) dynamic sexual dichromatism and (c,d) ontogenetic dichromatism. (a) Litoria leseueri (Hylidae): males turn yellow for several days during the breeding season (Photo credit: Stewart Macdonald); (b) Rana arvalis (Ranidae): males turn blue for several weeks during the breeding season (Photo credit: Lars Iversen); (c) Rhinella icterica (Bufonidae): at sexual maturity males are yellow and females are mottled brown and tan. Females retain the juvenile coloration (Photo credit: Célio F. B. Haddad). (d) Hyperolius ocellatus (Hyperoliidae): at sexual maturity males are green with white dorsolateral lines and females are rusty red to silver with small spots. Males retain the juvenile coloration (Photo credit: Rayna C. Bell).

Figure 2.

Phylogenetic distribution of (a) dynamic sexual dichromatism and (b) ontogenetic sexual dichromatism. Families are shown in bold and subfamilies in regular print. Branches are coloured according to the percentage of dichromatic species in each clade and the proportion of dichromatic species is shown in parentheses for each tip. The phylogeny is modified from [13].

Figure 3.

Of the frogs that exhibit dynamic sexual dichromatism, males undergo a temporary colour change to become yellower or brighter than females in 75 per cent of species (grey bar), while in the remaining 25 per cent of species, males become bluish or darker than females (hashed bar). Of the frogs that exhibit ontogenetic dichromatism, males are more conspicuously coloured than females in 13 per cent of species (grey bar), females are more conspicuously coloured than males in 11 per cent of species (hashed bar) and males and females are different coloured but equally conspicuous in 76 per cent of species (black bar).