Journal Club: Beluga and Bowhead whale cochlea - anatomical analyses

Today's journal article

Peacock J, Thewissen JGM. Intraspecific variation of cochlear morphology in bowhead and beluga whales. 

Why I picked this article

This research aims to understand the variability in the size and shape of the bowhead whale and the beluga, two species of whales. 

The sense of hearing is different between different animals. Small animals, such as mice, hear mostly higher-frequency sounds compared to humans. Hearing frequency and sensitivity of the animal naturally correlate with the animal's presumed dependency on using sounds, especially for communication. Unique frequency range and sensitivity of hearing are achieved by the arrangement of cells in the cochlea, and hence, sometimes one can speculate on the animal's hearing by comparing cochlear anatomies from other species. While laboratory animals such as mice and guinea pigs cochleae are well studied, the cochleae of other animals, particularly those of rare wildlife, are not easy to study due to the rare opportunity to acquire their specimen. 

According to Wikipedia, the Bowhead whale is approximately 4 m long, a whale living in cold polar seas, of which almost 1/3 of the body length is the mouth (big mouth!). They can live 200 years (I wonder how they know this?). The Bowhead whale is highly vocal and communicates with low-frequency sounds less than 1000Hz; this will be similar to our low voice.(https://en.wikipedia.org/wiki/Bowhead_whale)

Beluga whales are cute, white whales approximately 3.5-5.5 m long, that often appear in aquatic zoos. According to Wikipedia, their life expectancy is probably up to 30 years. They use "echolocation" and can hear sounds from 1.2 - 120 kHz (wow!). The sound is believed to be received by bone-conduction through the jaw; conversely while Beluga has an ear drum, if they are functional is not known.  (https://en.wikipedia.org/wiki/Beluga_whale). 

Beluga (thank you Pixabay.com) 

Some of the research findings

Animal specimen: 

  • Apparently, these specimens were collected as part of "indigenous subsistence
  • harvest" with "petrosal bones being collected by the Department of Wildlife Management of the North
  • Slope Borough".
  • Bowhead whale (Balaena mysticetus) - 10 individual bones, aged between 1.5 - 20.4 year-old and near-term. Mixed sex. 
  • For Bowhead, age is estimated from baleen plate length. 
  • Beluga (D. leucas) - 6 - 21 year-old, Mixed sex. 
  • For Beluga, the age is estimated from their teeth. 
Anatomical measurements: 
  • Specimens were fixed within several hours of death. 
  • The cochlea was scanned using microCT to obtain 3D anatomical data. 
  • The cochlea is a fluid-filled cavity that spirals around like a snail shell. Along the cochlear spiral, they characterised: 
    • "Lamina gap" - this corresponds to where the sensory area of the cochlea usually lives. 
    • "Rosenthal's canal"- this is where auditory nerves are found in clusters.
    • "Spiral lamina" - this is a bony bridge that partitions the cochlear spiral, where the fluid compartments in the cochlea are.
    • "Cross-sectional area of SVM"- this is the cross-sectional area of cochlear compartments filled with fluid.

Findings:

  • The length of the basilar membrane (= the part that vibrates and detects sound) along the cochlea was an average of 53mm in bowheads and 48 mm in Belugas. The basilar membrane length is often used as an estimate of "cochlear length" compared across species. This is about ~3+ cm in humans, so both whales have cochlear spirals that are longer! 
  • The laminar gap of the Bowhead whale and Beluga were very different, with it being wider and changing slowly along the cochlear turn for the Bowhead whale. 
  • Rothenthal's canal area of the Bowhead whale and Beluga showed similar trends (and perhaps not as varied as the laminar gap). 
  • Spiral lamina width and thickness were overlapping (researchers divided them into primary and secondary). 
  • The cross-sectional area of the SVM was larger for bowheads than for belugas. 

Haruna's takeaway

The main great point of this literature is providing variability between individuals within the same species by having a reasonably large sample size, n = 10. This is a sample size that is very challenging for rare animals. Some parameters had variability within species of 20%+, and this raises caution about getting size measurements for a species from a small sample size (e.g. often is the case for archaeological findings of the cochlea). 

The study also shared the histology of auditory neurons. The Rosenthal's canal cross-section of Beluga looked so round! They look distinctly different from sheep. 

Beluga was my favourite creature in the Disney movie "Finding Nemo 2".... What would be interesting to quantify is the more details on the spiral lamina, to see the density of the bone /upper lamina and where plates of the spiral lamina are, as that's likely to have implications on hearing frequency. I wonder if they would allow access to the histology/or microCT data. 

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This is Haruna's 26/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.