Functional implications of ear loss in sand lizards

Area of Science:

• Evolutionary biology
• Comparative anatomy
• Bioacoustics

Background:

• Tympanic hearing, a key tetrapod trait, has been secondarily lost multiple times.
• The evolutionary reasons for hearing loss in North American sand lizards (Callisaurini) remain unclear.
• Previous hypotheses involving environmental shifts or relaxed selection do not fully explain this phenomenon.

Purpose of the Study:

• To investigate the evolutionary reduction and loss of tympanic hearing in sand lizards.
• To analyze the morphological differences in the stapes bone and their functional implications.
• To determine factors influencing stapedial morphology in relation to hearing capabilities.

Main Methods:

• Creation of 3D stapes models for morphological analysis.
• Harmonic response analysis (HRA) to assess stapedial movement.
• Linear discriminant analyses incorporating diverse squamates to compare tympanic and atympanic species.

Main Results:

• Earless lizards (Cophosaurus, Holbrookia) have short stapes with large footplates, typical of atympanic species.
• The eared lizard Uma exhibits a long, slender stapes, characteristic of tympanic species.
• Callisaurus shows an intermediate stapes morphology, suggesting modifications may precede tympanum loss.

Conclusions:

• Stapedial morphology varies significantly between tympanic and atympanic sand lizards.
• Lifestyle and diet do not explain the observed differences in ear morphology.
• Substrate use and vibration transmission are potential drivers for stapes modification and tympanum loss in this clade.
• Stapes and quadrate modifications may precede tympanum evolution or loss, offering insights into fossil hearing origins.