What drives the evolution of body size in ectotherms? A global analysis across the amphibian tree of life

Abstract

Aim:The emergence of large-scale patterns of animal body size is the central expectation of a wide range of (macro)ecological and evolutionary hypotheses. The drivers shaping these patterns include climate (e.g. Bergmann's rule), resource availability (e.g. 'resource rule'), biogeographic settings and niche partitioning (e.g. adaptive radiation).However, these hypotheses often make opposing predictions about the trajectories of body size evolution. Therefore, whether underlying drivers of body size evolution can be identified remains an open question. Here, we employ the most comprehensive global dataset of body size in amphibians, to address multiple hypotheses that predict patterns of body size evolution based on climatic factors, ecology and biogeographic settings to identify underlying drivers and their generality across lineages. Location: Global. Time Period: Present. Major Taxa Studied: Amphibians. Methods: Using a global dataset spanning 7270 (>87% of) species of Anura, Caudata and Gymnophiona, we employed phylogenetic Bayesian modelling to test the roles of climate, resource availability, insularity, elevation, habitat use and diel activity on body size. Results: Only climate and elevation drive body size patterns, and these processes are order-specific. Seasonality in precipitation and in temperature predict body size clines in anurans, whereas caecilian body size increases with aridity. However, neither of these drivers explained variation in salamander body size. In both anurans and caecilians, size increases with elevational range and with midpoint elevation in caecilians only. No effects of mean temperature, resource abundance, insularity, time of activity or habitat use were found.Main Conclusions: Precipitation and temperature seasonality are the dominant climatic drivers of body size variation in amphibians overall. Bergmann's rule is consistently rejected, and so are other alternative hypotheses. We suggest that the rationale sustaining existing macroecological rules of body size is unrealistic in amphibians and

Body size and trophic structure explain global asymmetric response of tetrapod diversity to climate effects

“…However, they do not affect amphibian's body size. Our findings support the Bergmann's rule for mammals (Meiri & Dayan, 2003), the reverse pattern for squamates (Ashton & Feldman, 2003), no support for amphibians (Adams & Church, 2008;Johnson et al, 2023), although it is opposite the expected pattern for birds (Meiri & Dayan, 2003;Salewski & Watt, 2017). These nuances of Bergmann's rule are highly debated and other explanations for Bergmann-type clines, like precipitation, primary plant productivity, trophic level, and competition, have a key role acting simultaneously with temperature to determine the latitudinal body size pattern (Alhajeri & Steppan, 2016;Hantak et 2021).…”

“…As a result, the conclusions of Bergmann's rule tend to be either ambiguous or limited, particularly in the case of some taxonomic groups such as ectotherms. This ambiguity arises due to the existence of either positive, negative, or nonlinear patterns in body size-latitude correlation, or sometimes no discernible pattern at all (Ashton & Feldman, 2003;Johnson et al, 2023;Meiri & Dayan, 2003;Rapacciuolo et al, 2017). This lack of generality may be caused by the fact that the body size is determined by multiple simultaneous pressures, which are linked with other traits such as body size variation and trophic level.…”

Macroecological correlates of richness, body size, and species range size in terrestrial vertebrates across the world

“…We obtained the range maps of birds from BirdLife International (2021) and the other three vertebrate classes from IUCN (2022) and calculated the range size for each species. The body size (mass, g) data were obtained from Etard et al (2020), Wilman et al (2014), andJohnson et al (2023). We divided the globe into 300 km × 300 km grid cells based on the Mollweide (equal-area) projection (Fig.…”

Risk of extinction increases towards higher elevations across the world's amphibians

“…To address the role of life-history traits as drivers of extinction risk towards higher elevations, we employed our global dataset spanning body size, fecundity and parity mode, collected from the same sources described above. For both frogs and salamanders, snoutvent length (SVL) is the most used measure of body size (Amado et al, 2021;Johnson et al, 2023;Pincheira-Donoso, Harvey, Grattarola, et al, 2021;Wells, 2007), whereas total body length is used for caecilians (Pincheira-Donoso et al, 2019). To make analyses involving body size comparable, we converted maximum SVL and maximum total body length, respectively, into body mass using the approach presented by Pough (1980) based on order-specific allometric formulas (Pincheira-Donoso & Hodgson, 2018;Ripple et al, 2017).…”