Abstract
Frogs and toads (Amphibia: Anura) display diverse ecologies and behaviours, which are often correlated with visual capacity in other vertebrates. Additionally, anurans exhibit a broad range of relative eye sizes, which have not previously been linked to ecological factors in this group. We measured relative investment in eye size and corneal size for 220 species of anurans representing all 55 currently recognized families and tested whether they were correlated with six natural history traits hypothesized to be associated with the evolution of eye size. Anuran eye size was significantly correlated with habitat, with notable decreases in eye investment among fossorial, subfossorial and aquatic species. Relative eye size was also associated with mating habitat and activity pattern. Compared to other vertebrates, anurans have relatively large eyes for their body size, indicating that vision is probably of high importance. Our study reveals the role that ecology and behaviour may have played in the evolution of anuran visual systems and highlights the usefulness of museum specimens, and importance of broad taxonomic sampling, for interpreting macroecological patterns.
Highlights
Vision is an important, well-studied sensory system among vertebrates
We used residuals from the phylogenetic generalized least-squares (PGLS) fit for log-transformed eye diameter (ED) versus root of mass (RM) as a measure of phylogenetically corrected relative eye investment, with positive residuals indicating larger than average eyes for a given body mass and negative residuals indicating smaller than average eyes; results with eye investment relative to snout–vent length (SVL) are included in the electronic supplementary material
Relative eye size is often used as an indirect measure of the importance of vision to a taxon (e.g. [29,64,77]); our results suggest that vision is highly important in anurans, yet anuran visual ecology is relatively understudied compared with other vertebrate groups
Summary
Well-studied sensory system among vertebrates. The size and dimensions of an eye directly affect the optics of the visual system and, subsequently, the amount and quality of visual information that an animal can extract from its environment [1]. Because many anurans are most active during the breeding season and may be heavily reliant on vision at this time [30], we predicted that (iii) species breeding in complex sensory habitats (e.g. on vegetation or near fast-flowing water) or (iv) exhibiting sexual dichromatism ( potentially related to conspecific signalling [31,32]) would invest in larger eyes for improved visual discrimination during breeding. Because most anurans have a biphasic life history with decoupled larval and adult morphologies and ecologies [33], we predicted that species with (v) free-living larvae and (vi) larvae active in terrestrial or lotic environments (where vision may be crucial to larval survival) would have larger adult eye sizes due to increased larval investment in vision. We (i) determine anuran eye–body allometric relationships with comprehensive familial sampling, (ii) test for correlations between eye size and ecology based on our above predictions, and (iii) compare anuran eye size and eye–body scaling to other vertebrate groups
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