Eyes wide open: Frogs with reduction of auditory communication ability show lower eye, but higher corneal, investment

Abstract

Vocal communication is the primary mechanism of mate attraction in frogs, but some species have lost the auditory structures needed to hear airborne sounds, raising the question of how they communicate in their absence. We hypothesized that frog species with auditory system reductions have compensated through enhanced visual ability, including through enlarged eyes. To test this trade-off hypothesis, we estimated relative eye and corneal investment across 264 species and tested for correlations with auditory reduction. Contrary to our expectations, we found that species with auditory reductions had smaller eye size and lower eye investment. Instead, we found that non-fossorial species with auditory reductions had significantly greater corneal investment, after controlling for habitat-based variation. Enlarged corneas enhance light sensitivity but are expected to have a lower metabolic cost than larger eyes. Relatively larger corneas may, in combination with other changes, allow frogs with auditory reductions to rely more on vision for communication, while at the same time minimizing energetic costs in small animals that are already metabolically constrained. Our findings suggest that the visual system plays an adaptive role in response to the reduction of auditory communication and highlights the condition of sensory systems as an important factor shaping frog evolution.