Journal Club: Sense of hearing in bats and dolphins

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Today's journal article

Simmons JA, Ketten DR, Simmons AM. Cochlear representation of wideband biosonar sounds and the emergence of neural oscillations. 

Why I picked this article

Hearing in some animals is even more amazing than ours; bats and dolphins can hear much higher frequencies than we can. They can hear beyond 100kHz ultrasounds and use them for echolocation. For echolocation to work, these animals transmit signals, detect the reflected sounds and process them as cues about surrounding structures. Insight from the sense of hearing of these animals is fascinating. This research is not biomedical research using animal models, but instead is a review of research available about the special senses of bats and dolphins, and how speculation into how their special hearing is enabled. 

Summary from the research article

Bats (brown bats)

  • can hear 23-100kHz, 
  • broadcasting ultrasonic sweep - wavelength of about 0.7 cm
  • broadcast forward with a beamwidth of 45◦- 90◦
  • Conduction through air (higher energy)
  • Discrimination threshold for object location: 1 cm 
  • Conduction: through outer and middle ears.
  • Cochlea: 8.7 cm long, slightly over 2-turns
  • Stapes at the part-way up the first turn of the cochlea. 
  • No auditory nerve recording ever made = neural response remain unknown. 
  • There is a study that showed the brainstem can respond to 10-100kHz 


Dolphins (bottlenose dolphins)

  • can hear 40-140kHz
  • Clicks broadcast - 2-3 cm wavelength. 
  • broadcast forward with a beamwidth of 20◦- 30◦
  • Conduction in water
  • Discrimination threshold for object location: 1 - cm
  • Conduction: through the jaw and the fat-filled channels. 
  • Cochlea: 39.2 mm long, 2-2.5 turns
  • Dense otic capsule with no air space. 
  • Stapes at the very end of long, narrow tube in the hook region of the cochlea. (hook dimension seems unique)
  • No nerve recording ever made = neural response remain unknown. 

Part of Figure 1. Showing transmitted sounds for echolocation by bats and dolphins. Simmons et al. 2025

Researchers suggest two theories on bats' and dolphins' hearing: 

  1. High-frequency sounds arriving in the cochlea generate low-frequency interference patterns in travelling waves along the basilar membrane, which are detected and cause a neural response. 
  2. High-frequency sounds trigger neural responses as seen in the bat’s brainstem, which generates echo interference patterns at the neural level. 

Haruna's takeaway

Both bats and dolphins are protected species, and hence, the available data on these animals is very limited. Available data points to a very different cochlea and auditory signal system processing compared to ours, or those of common laboratory animal models. It is very exciting to think about the diverse hearing sense and the wonder of evolution! 

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This is Haruna's 64/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.