Global patterns of body size evolution are driven by precipitation in legless amphibians

Abstract

Body size shapes ecological interactions across and within species, ultimately influencing the evolution of large‐scale biodiversity patterns. Therefore, macroecological studies of body size provide a link between spatial variation in selection regimes and the evolution of animal assemblages through space. Multiple hypotheses have been formulated to explain the evolution of spatial gradients of animal body size, predominantly driven by thermal (Bergmann's rule), humidity (‘water conservation hypothesis’) and resource constraints (‘resource rule’, ‘seasonality rule’) on physiological homeostasis. However, while integrative tests of all four hypotheses combined are needed, the focus of such empirical efforts needs to move beyond the traditional endotherm–ectotherm dichotomy, to instead interrogate the role that variation in lifestyles within major lineages (e.g. classes) play in creating neglected scenarios of selection via analyses of largely overlooked environment–body size interactions. Here, we test all four rules above using a global database spanning 99% of modern species of an entire Order of legless, predominantly underground‐dwelling amphibians (Gymnophiona, or caecilians). We found a consistent effect of increasing precipitation (and resource abundance) on body size reductions (supporting the water conservation hypothesis), while Bergmann's, the seasonality and resource rules are rejected. We argue that subterranean lifestyles minimize the effects of aboveground selection agents, making humidity a dominant selection pressure – aridity promotes larger body sizes that reduce risk of evaporative dehydration, while smaller sizes occur in wetter environments where dehydration constraints are relaxed. We discuss the links between these principles with the physiological constraints that may have influenced the tropically‐restricted global radiation of caecilians.

The multiple origins of sexual size dimorphism in global amphibians

“…For anurans and salamanders, we used the snout–vent length (SVL; this is measured as the snout‐to‐urostyle length in anurans and the snout‐to‐edge of cloacae in salamanders) because this is the most widely used body size proxy in these groups (Duellman & Trueb, 1994; Wells, 2007). For caecilians, we used total body length because this is widely documented and is often the only documented proxy for size (Pincheira‐Donoso et al., 2019). However, most records of body size available for caecilians focus on one overall measure per species or on one of the sexes only.…”

Spatial patterns in the size of Chinese lizards are driven by multiple factors

“…Accordingly, body size could also decrease in response to increasing seasonality owing a reduction in the period of activity. The “net primary productivity hypothesis” (NPP, also referred to as the “resource rule”) predicts that the availability of food resources (Rosenzweig, 1968) could be associated with higher productivity, thereby enabling the development of larger body size(Huston & Wolverton, 2011; Meiri et al., 2010; Yom‐Tov & Geffen, 2006). The “water availability hypothesis” (Ashton, 2002) predicts that in arid areas, bodies of a larger size can preserve water more efficiently, owing to smaller surface‐area‐to‐volume ratios, as has been reported in amphibians (Pincheira‐Donoso et al, 2019). …”

“…In animals, body size is one of the most important life‐history traits, and geographical patterns in size distribution and its determinants are among the most intriguing features of macroecology (Pincheira‐Donoso & Meiri, 2013; Pincheira‐Donoso et al, 2019). In this regard, the influence of climate on the body size of endotherm and ectotherm groups has been well studied (Meiri & Dayan, 2003; Slavenko et al., 2019), given that climate has traditionally been considered a major driver promoting body size evolution (Olalla‐Tárraga and Rodríguez, 2007).…”

Not playing by the rules: mixed support of ecogeographic rules in an arid‐adapted African ground squirrel

“…This hypothesis generally holds in endotherms, where larger‐bodied animals exhibit a lower surface area to volume ratio, minimizing heat‐loss and metabolic needs, meaning that larger‐bodied animals can better adapt to cold temperatures (Brown, Kotler, & Porter, 2017). Though often resting on thermodynamic explanations in endotherms, Bergmann's rule also applies to some ectotherms (Ashton & Feldman, 2003; Pincheira‐Donoso et al ., 2019), suggesting other non‐thermoregulatory explanations may apply to these species, including food scarcity, primary productivity and environmental predictability (Tomlinson & Withers, 2008; Meiri, 2011). The fasting endurance hypothesis suggests that larger‐bodied animals are better able to endure periods of food scarcity, which occurs more frequently at higher latitudes (i.e.…”