The multiple origins of sexual size dimorphism in global amphibians

Abstract

Aim Body size explains most of the variation in fitness within animal populations and is therefore under constant selection from ecological and reproductive pressures, which often promote its evolution in sex‐specific directions, leading to sexual size dimorphism (SSD). Several hypotheses have been proposed to explain the vast diversity of SSD across species. These hypotheses emphasize: (a) the mate competition benefits to larger male size (sexual selection); (b) the benefits of larger female size for fecundity (fecundity selection); (c) the simultaneous benefits of niche divergence for males and females to reduce intersexual competition for ecological resources (natural selection); and (d) the underlying impact of geographical variation in climatic pressures expected to shape large‐scale patterns of SSD in synergy with the above selection pressures (e.g., intensification of fecundity selection as breeding seasons shorten). Based on a new, global‐scale amphibian dataset, we address the shortage of large‐scale, integrative tests of these four hypotheses. Location Global. Time period Extant. Major taxa studied Class Amphibia. Methods Using a > 3,500 species dataset spanning body size, ecological, life‐history, geographical and climatic data, we performed phylogenetic linear models to address the sexual, fecundity, ecological and climatic hypotheses of SSD. Results Evolution of SSD is discordant between anurans and salamanders. Anuran SSD is shaped by climate (male‐biased SSD increases with temperature seasonality) and by nesting site. In salamanders, SSD converges across species that occupy the same types of microhabitats (“ecodimorphs”), whereas reproductive or climatic pressures have no effects on their SSD. These contrasts are associated with latitudinal gradients of SSD in anurans, but not in salamanders. Main conclusions Amphibian SSD is driven by ecological and climatic pressures, whereas no roles for sexual or fecundity selection were detected. We show that macroevolutionary processes determined by different forms of selection lead to latitudinal patterns of trait diversity, and the lack of them.

Global gaps in age data based on skeletochronology for amphibians

“…In addition, samples were highly sex-biased in some studies. As a consequence, we cannot assess sex-specific life history strategies such as body size, reproductive rate and survival, despite the fact that the sex differences have implications for ecological and evolutionary processes (Zhang & Lu 2013a,b;Lemaître et al 2020;Pincheira-Donoso et al 2021).…”

The global macroecology of brood size in amphibians reveals a predisposition of low‐fecundity species to extinction

“…Then, for the macroecological analyses, we performed phylogenetic multiple regressions containing all four environmental predictors (temperature, precipitation and the seasonality in these two variables). Given that distinctive continent‐specific patterns of macroecological variation have been reported for different traits among ectotherms (Meiri et al., 2020; Pincheira‐Donoso et al., 2021), we investigated whether the differences in climatic pressures across continental regions drive distinctive macroecological patterns of amphibian brood size evolution that could remain concealed under the global models. To conduct these analyses, we repeated the same multivariate climatic models for anurans and salamanders separately, but separately for the Americas, Eurasia, Africa and Australia.…”

General patterns of sexual dimorphism in graylings (Thymallus), with a comparison to other salmonid species

“…The general mechanisms that drive the evolution of sexual dimorphism are well studied in several taxonomic groups including insects (Wilhelm et al 2011), birds (Berns and Adams 2012), mammals (Swanson et al 2013), reptiles (Agha et al 2017), fishes (Oke et al 2019) and amphibians (Pincheira-Donoso et al 2021). Among fishes, salmonids (family Salmonidae) have become one of the most frequently studied groups of species used to address questions on the evolution of sexual dimorphism and associated selective forces (Fleming and Reynolds 2004).…”