Water constraints drive allometric patterns in the body shape of tree frogs

Kathleen M S A Castro,Talita F Amado,Carlos A Navas,Sidney F Gouveia,Pablo A Martinez,Miguel Á Olalla-Tárraga

doi:10.1038/s41598-020-80456-1

Abstract

The origin of morphological diversity is a critical question in evolutionary biology. Interactions between the environment and developmental processes have determining roles in morphological diversity, creating patterns through space and over time. Also, the shape of organisms tends to vary with increasing size as a result of those developmental processes, known as allometry. Several studies have demonstrated that the body sizes of anurans are associated with hydric conditions in their environments and that localities with high water stress tend to select for larger individuals. However, how environmental conditions alter those patterns of covariance between size and shape is still elusive. We used 3D geometric morphometric analyses, associated with phylogenetic comparative methods, to determine if the morphological variations and allometric patterns found in Arboranae (Anura) is linked to water conservation mechanisms. We found effects of the hydric stress on the shape of Arboranae species, favouring globular shapes. Also, the allometric patterns varied in intensity according to the water stress gradient, being particularly relevant for smaller frogs, and more intense in environments with higher water deficits. Our study provides empirical evidence that more spherical body shapes, especially among smaller species, reflect an important adaptation of anurans to water conservation in water-constrained environments.

Highlights

Similar to body size, the shape of an organism can affect the rate of water loss through evapotranspiration[4,28]
We explored whether species exposed to environments with low water vapor pressures tend to have a more spherical shape, and analysed if their allometric patterns in shape become stronger in arid environments where small species show more spherical body shapes as compared to the larger ones (Fig. 1)
PC2 was more associated with body shape, with species showing negative values for PC2 having more globular bodies than those on the positive side of the axis, which had more elongated shapes (Fig. 2)

Summary

Introduction

The shape of an organism can affect the rate of water loss through evapotranspiration[4,28]. The rounded shape of these animals can be explained, at least in part, by the fact that more globular organisms have smaller surface areas exposed to the environment, reducing water loss[4]. The shape is associated with the rate at which processes such as the diffusion of matter and thermal conduction occurs[17] In this case, the area of evaporative surface (e.g., the area exposed to the external environment) will be inversely related to the distance that the material or energy must traverse across the body[17,31,32]. Arboranae comprises approximately 45 genera and more than 800 species distributed in the Americas, Eurasia, Australia, and Papua New G uinea[28] Their broad taxonomic and environmental diversity makes them an ideal group for analysing the effects of water availability on the covariance of shape and size. We explored whether species exposed to environments with low water vapor pressures tend to have a more spherical shape (the shape hypothesis), and analysed if their allometric patterns in shape become stronger in arid environments where small species show more spherical body shapes as compared to the larger ones (the differential allometry hypothesis) (Fig. 1)

Methods

Results

Conclusion