Convergent Anuran Middle Ear Loss Lacks a Universal, Adaptive Explanation

Area of Science:

• Evolutionary biology
• Bioacoustics
• Herpetology

Background:

• Anurans (frogs and toads) exhibit recurrent loss of tympanic middle ears, a trait crucial for airborne sound reception in most tetrapods.
• Acoustic communication is prevalent in anurans, making middle ear loss particularly puzzling.
• At least 38 independent instances of middle ear loss have been documented in anurans.

Purpose of the Study:

• To investigate potential shared selection pressures associated with anuran tympanic middle ear loss.
• To examine correlations between ear loss and factors like geographic range, microhabitat, activity patterns, and acoustic communication.

Main Methods:

• Phylogenetic comparative methods were employed to analyze the evolutionary patterns of middle ear presence or absence.
• Data on geographic distribution, microhabitat, activity patterns (diurnal/nocturnal), and acoustic communication were compiled and analyzed.

Main Results:

• While some differences in geographic ranges and a higher prevalence of diurnal activity were observed in earless anurans, these factors did not universally explain middle ear loss.
• Significant overlap in geographic distributions between eared and earless species was found.
• No single, unifying adaptive explanation was identified for the numerous cases of anuran middle ear loss.

Conclusions:

• The repeated loss of tympanic middle ears in anurans lacks a universal adaptive explanation.
• Alternative hypotheses, such as genetic or developmental constraints, are proposed.
• The possibility that middle ear loss may be maladaptive in some anuran lineages is considered.