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

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 exists in
salamanders, too. 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.