Does the diversity of anuran iris patterns have an ecological function or is it just beauty in the eye of the beholder?

Abstract

Iris patterns in the animal kingdom are incredibly variable, with anurans having some of the most diverse and intricate patterns. Although the shape and colouration of anuran eyes seem to be correlated with ecological factors, the evolution of iris patterns remains unexplored. We used a large-scale phylogenetic comparison with 960 anuran species to examine the evolutionary and ecological correlates of iris patterns. We classified iris patterns into four broad categories: Reticulated, Plain, Dotted, and Lined, and examined whether iris pattern was correlated with diel activity (diurnal, nocturnal, and cathemeral activity) and habit (aquatic, arboreal, terrestrial, and fossorial) or both. Our analysis suggests that reticulated irises are the most common pattern in anurans and are the most likely ancestral character. The evolution of iris patterns across the anuran phylogeny best matched Brownian expectations, with many transitions between the four pattern types. Iris patterns, however, were mostly uncorrelated with diel activity or habit. The only exception was an association between plain irises and diel activity. Specifically, anurans with plain irises were more likely to be diurnal and less likely to be nocturnal; and the evolution of plain irises seemed to have preceded the evolution of diel activity. Overall, iris patterns across anurans are mostly unrelated to ecological factors, suggesting that this trait may be important for other functions, such as inter- or intra-specific interactions, or that the incredible diversity has evolved through neutral processes. Our findings open avenues for further research, especially to understand the potential adaptive value of the striking ornamentation in iris patterns across taxonomic groups.