Geographical clines of body size in terrestrial amphibians: water conservation hypothesis revisited

Abstract

Aim Faced with the dispute regarding spatial gradients of body size in ectotherms, we build upon their long‐known allometric relationship with water economy, which scales with thermal and hydric regimes, to revisit and refine the water conservation hypothesis (WCH). We provide a brief description of the WCH, including its physiological basis and geographical predictions for body size clines in terrestrial amphibians, and test it against heat‐based hypotheses in four amphibian clades. Location The Americas. Methods We employ phylogenetic comparative analyses to examine relationships between body size and both temperature, as a descriptor of the effect of heat alone, and potential evapotranspiration, which describes the water constraint along evaporative gradients. We assess these relationships in four amphibian clades: the subfamily of leaf frogs (Phyllomedusinae), genera of gladiator frogs (Hypsiboas), salamanders (Plethodon) and the family of glass frogs (Centrolenidae). Results Three clades did not show phylogenetic signals in body size variation. In addition, three clades showed a positive relationship with potential evapotranspiration as predicted, and all of them were unrelated to temperature. When present, however, the explanatory power of evaporative energy on body size variation was relatively weak. Main conclusions The conservation of water across evaporative gradients is both a more comprehensive explanation and a more pervasive driver of spatial clines in body size among terrestrial amphibians than is the balance of heat alone. However, the relatively low predictive ability of evaporative energy and its dependence on specific climatic configurations both emphasize and elucidate the non‐universality of the phenomenon.

Body size variation and sexual size dimorphism across climatic gradients in the widespread treefrog Scinax fuscovarius (Anura, Hylidae)

“…Several studies have shown that moisture can be more correlated with intraspecific variation in body size of animals than temperature, and anurans usually follow this trend (McDiarmid 1968;Nevo 1972Nevo , 1973Yom-Tov & Geffen 2006;Castellano & Giacoma 1998;Olalla-T arraga et al 2009;Gouveia & Correia 2016;Oromi et al 2012;Oyamaguchi et al 2016; this study). Among anurans, it is expected that because they need to keep their skin moist in order to breath a simple mechanism to achieve this is decreasing the surface-to-volume ratio by increasing body size, a mechanism analogous to the conservation of heat.…”

“…BIO1 = Annual Mean Temperature, BIO4 = Temperature Seasonality (standard deviation 9 100), BIO12 = Annual Precipitation, BIO15 = Precipitation Seasonality (Coefficient of Variation). In addition, potential evapotranspiration (PET) for each point was obtained from CGIAR-CSI Global Soil-Water Balance Database (Trabucco & Zomer 2010) following Gouveia and Correia (2016) and Normalized Difference Vegetation Index (NDVI) of each site using images from 2000 (satellite Terra, MODIS sensor) with a spatial resolution of 1 km (https:// reverb.echo.nasa.gov/reverb/#utf8=%E2%9C%93&spatial_ map=satellite&spatial_type=rectangle). The NDVI provides values that are highly correlated with photosynthetic mass and primary productivity.…”

“…(2009) and Gouveia and Correia (2016) link body size variation to environmental water deficit, mainly in tropical regions. Boaratti and Da Silva (2015) compared different ecogeographic hypothesis between three Brazilian species and suggested that intraspecific variation in anuran body size is more dependent on species-specific response than on the region (i.e.…”

“…On the other hand, several authors have demonstrated a geographic trend in body size and SSD within species, some with some environmental variables as explaining factors, other with no relationship (Nevo 1973;Sch€ auble 2004;Laugen et al 2005;Cvetkovic et al 2008;Yu et al 2010;Oromi et al 2012;Hsu et al 2014;Feng et al 2015;Valenzuela-S anchez et al 2015;Davenport & Hossack 2016;Gouveia & Correia 2016). Also, differences in age at maturity, longevity, growth rates, or substrate could also account for population divergence (e.g.…”

“…These competing hypotheses being: (i) heat balance: this hypothesis predicts a negative relationship between body size and temperature (Olalla-T arraga & Rodr ıguez 2007). The lower surface-to-volume ratio of larger animals implies higher heat conservation capacity; (ii) starvation resistance: this hypothesis predicts that in more seasonal environments, large-bodied species should be able to ride out longer periods of unfavorable conditions because energy stores increase with size faster than metabolic rate (Cushman et al 1993); (iii) growing season length: predicts that season length has a considerable influence on body size because of constraints on resource availability, resulting in an increase in body size in less seasonal environments (Bidau & Mart ı 2007); (iv) water availability: predicts that a large body size is associated with dry climates because of higher desiccation tolerance (Ashton 2002); (v) Converse water availability: predicts large body sizes are associated with areas with greater precipitation because the activity of amphibians is often associated with high water availability or wet periods (Zug et al 2001); (vi) Primary productivity: areas with higher productivity provide more food supply and thus permit the attainment of a larger size (Rosenzweig 1968); (vii) the water conservation: predicts that body size increase towards more dehydrating environments which does not necessary means warmer environments (Gouveia & Correia 2016); and (viii) the energy-water conservation: a large body provides greater heat conservation in cold macroclimates and greater water conservation in dry areas (Olalla-T arraga et al 2009).…”