Earless toads sense low frequencies but miss the high notes

Area of Science:

• Zoology
• Bioacoustics
• Evolutionary Biology

Background:

• Anuran species frequently lose tympanic middle ears, despite the importance of acoustic communication.
• The evolutionary persistence of hearing in earless anurans suggests alternative sensory mechanisms may compensate for the loss of middle ears.

Purpose of the Study:

• To investigate whether alternative sensory pathways enable anurans lacking tympanic middle ears to hear airborne sound or sense environmental vibrations.
• To compare hearing and vibrational sensitivity between eared and earless anuran species.

Main Methods:

• Auditory brainstem response recordings were used to assess hearing sensitivity.
• Vibrational sensitivity was measured in 10 Neotropical true toad (Bufonidae) species (six eared, four earless).

Main Results:

• Species lacking tympanic middle ears showed reduced sensitivity to high-frequency sounds.
• However, low-frequency hearing and vibrational sensitivity were equivalent between eared and earless species.
• Extratympanic hearing sensitivity varied among earless species, indicating diverse compensatory mechanisms.

Conclusions:

• Earless anurans possess functional low-frequency hearing and vibrational sensitivity, compensating for the loss of middle ears.
• Ancestral species likely utilized these alternative pathways and developed behavioral strategies to offset high-frequency hearing loss.
• This research sheds light on sensory adaptation and evolutionary resilience in anurans.