Convergent Anuran Middle Ear Loss Lacks a Universal, Adaptive Explanation

Abstract

Shared selection pressures often explain convergent trait loss, yet anurans (frogs and toads) have lost their middle ears at least 38 times with no obvious shared selection pressures unifying ‘earless’ taxa. Anuran tympanic middle ear loss is especially perplexing because acoustic communication is dominant within Anura and tympanic middle ears enhance airborne hearing in most tetrapods. Here we examine whether particular geographic ranges, microhabitats, activity patterns, or aspects of acoustic communication are associated with anuran tympanic middle ear loss. Although we find modest differences between the geographic ranges of eared and earless species and increased diurnality in earless species, we find no universal adaptive explanation for the many instances of anuran tympanic middle ear loss. The puzzling lack of shared selection pressures motivates discussion of alternative hypotheses, including genetic or developmental constraints, and the possibility that tympanic middle ear loss is maladaptive.