Geographic variation of body size in New World anurans: energy and water in a balance

Abstract

The validity of Bergmann's rule, perhaps the best known ecogeographical rule, has been questioned for ectothermic species. Here, we explore the interspecific version of the rule documenting body size gradients for anurans across the whole New World and evaluating which environmental variables best explain the observed patterns. We assembled a dataset of body sizes for 2761 anuran species of the Western Hemisphere and conducted assemblage-based and cross-species analyses that consider the spatial and phylogenetic structure in the data. In accordance with heat and water-related explanations for body size clines, we found a consistent association of median body size and potential evapotranspiration across the New World. A relevant role of water availability also emerges, suggesting the joint importance of body size for thermoregulation and hydroregulation in anurans. Anurans do not follow a simple Bergmannian pattern of increasing size towards high latitudes. Consistent with previous regional findings, our Hemisphere-wide analyses detect that the geographic variation in anuran body sizes is highly dependent on a trade-off between heat and water balance. The observed size-climate relationships possibly emerge from the interplay between thermoregulatory abilities and the benefits inherent to reduced surface-to-volume ratios in larger species, which decrease the rates of evaporative water loss and favour heat retention. Our results also show how temperature becomes important for species that are directly in contact with the substrate and water, like burrowing and terrestrial anurans, while arboreal species exhibit a body size cline linked with potential evapotranspiration.

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

“…Macroecological studies on amphibians have revealed highly conflicting evidence for a role of temperature as an agent of spatial gradients in body size (Feder et al 1982, Ashton 2002, Olalla-Tarraga and Rodriguez 2007, Adams and Church 2008, Cvetkovic et al 2009, and the only known study on caecilians, on one species, showed a link with elevation (Measey and Van Dongen 2006). In contrast, the role of water-deprivation as a source of selection for larger body size as an adaptation to reduce rates of evapotranspiration has increasingly gained support (Olalla-Tarraga et al 2009, Gouveia and Correia 2016, Amado et al 2019. Our evidence, stemming from a complete coverage of caecilians, strongly supports the hypothesis that increases in body size are promoted by aridity -in particular among nonaquatic species, which reinforces the functional role of water conservation.…”

“…The leading rule, Bergmann's rule -increases in body sizes toward colder climates as greater body mass, relative to surface area, reduces heat loss (Bergmann 1847) -has set the theoretical benchmark for research on large-scale patterns of animal size (James 1970, Blackburn et al 1999, Meiri and Dayan 2003. However, evidence from across the animal kingdom reveals that Bergmann's rule tends to hold in endotherms (Freckleton et al 2003, Meiri and Dayan 2003, de Queiroz and Ashton 2004, Olson et al 2009, but see Riemer et al 2018), while its validity is inconsistent in ectotherms (Ashton and Feldman 2003, Olalla-Tarraga et al 2006, Olalla-Tarraga and Rodriguez 2007, Pincheira-Donoso et al 2007, Adams and Church 2008, Pincheira-Donoso and Meiri 2013, Feldman and Meiri 2014, Moreno-Azocar et al 2015, Amado et al 2019, Slavenko et al 2019). These discrepancies have discredited temperature as a primary driver of body size clines (Pincheira-Donoso 2010, Meiri 2011, Olalla-Tarraga 2011.…”

“…For example, while heat production implies high metabolic expenditure of energy for endotherms, the dependence of ectotherms on environmental heat neutralises such pressures (Brown et al 2004, Angilletta 2009); finally 3) the 'seasonality (or 'fastingendurance') rule', predicts that increasing seasonality selects for increased body size to enhance tolerance to unstable environments (Lindsey 1966, Boyce 1979, Calder 1984. Given the contrasting mechanisms that these hypotheses offer to explain the same phenomenon, evidence supporting them has been conflicting across lineages (Meiri et al 2005, Yom-Tov and Geffen 2006, Olalla-Tarraga and Rodriguez 2007, Olalla-Tarraga et al 2009, Oufiero et al 2011, Pincheira-Donoso and Meiri 2013, Gouveia and Correia 2016, Kelly et al 2018, Amado et al 2019). Furthermore, our understanding of body size macroecology has fundamentally been advanced based on above-ground organisms, while analyses on fossorial lineages remain anecdotal (Meiri and Dayan 2003, Measey and Van Dongen 2006, Feldman and Meiri 2014.…”

Anuran 3D models reveal the relationship between surface area‐to‐volume ratio and climate

“…These differences between predictors highlight the discrepancy of constraints that differently affect organismal body length and body geometry. Previous works already stressed that geographical size variation in amphibian species may have multiple causes (Adams & Church, ; Amado et al., ; Gouveia & Correia, ; Olalla‐Tárraga & Rodríguez, ; Sinsch et al., ). More specifically, for E. calamita , geographical variation in size was suggested to be a result of the impact of temperature, type of microhabitat for hibernation and habitat constraints on growth rates.…”

“…Even though the variation in SA:V has been recurrently suggested as the cause of body size responses to climatic gradients, most inferences on organismal size clines stem from single linear measurements of body length as a size metric instead of direct SA:V estimates (Amado, Bidau, & Olalla-Tárraga, 2018;Gouveia & Correia, 2016;Olalla-Tárraga & Rodríguez, 2007;Slavenko & Meiri, 2015). This can be mainly explained due to the inherent technical limitations in the capture of geometric measurements of animal's bodies.…”