Disentangling the interrelations of body mass, egg deposition site, climate and microhabitat use in frogs and salamanders

Abstract

Amphibians exhibit a large diversity in reproductive and developmental strategies, which in turn are linked to their body size, life history and habitat. Here, we explore why terrestrial egg laying frogs are on average smaller than aquatic egg laying ones and whether this pattern also exists in salamanders. We hypothesized that egg deposition site and body mass are not linked directly across species, but that terrestrial egg layers occur in climates and use microhabitats that favor small masses. To test this, we compiled a dataset on egg deposition site (terrestrial or aquatic), development mode (biphasic with larvae or direct development without larvae), body mass, microhabitat use (water-dependent, ground-dwelling or arboreal) and climate within their distribution area (temperature, precipitation and seasonality in both) of 3091 frog and 244 salamander species. We analyzed the interrelations between these traits and environmental factors by using a cross-species approach and phylogenetic generalized least squares analysis. Body masses increased along a gradient from warm, humid and unseasonal climates to cold, dry and seasonal climates in frogs and salamanders. Terrestrial egg deposition was constrained to warm, humid and unseasonal climates only in frogs. Terrestrial eggs and an arboreal microhabitat use were linked in frogs and salamanders, and arboreal frogs were smaller than non-arboreal ones. We confirmed that frogs with terrestrial eggs had smaller average body masses than those with aquatic eggs, irrespective of their development mode, but this difference disappeared when we corrected body masses for the effects of climate and microhabitat use. In salamanders, however, egg deposition site and development mode were neither directly related to body mass, nor indirectly via the effects of climate and microhabitat use. Our results suggest that thermal and hydric environmental conditions determine the geographical distribution of body mass and reproductive strategies in amphibians and set the framework for their evolution.