By Jason Arunn Murugesu
NEIL BROMHALL / SCIENCE PHOTO LIBRARY
Naked mole rats have poor hearing because, unlike other mammals, they have abnormal outer hair cells which cannot amplify sound. The animals could be used to model human deafness and help develop treatments.
“Naked mole rats are fascinating creatures,” says Sonja Pyott at the University of Groningen in the Netherlands. Not only do they live exclusively underground, they are blind, make loud piercing noises, and have poor hearing.
Pyott and her colleagues focused their research on naked mole rats (Heterocephalus glaber) and Damaraland mole rats (Fukomys damarensis), aiming to determine what causes this poor hearing and how this trait may have evolved.
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The team first measured the animals’ neural responses to various tones they played them. This confirmed that the animals struggled to hear quiet sounds and could only perceive sound between a narrow frequency, between 0.5 and 4 kHz. Humans, by way of contrast, can detects sounds between about 0.02 and 20 kHz.
The researchers then recorded the sound transmitted by the cochlear, which typically shows the ear is amplifying sound information. They found that no such amplification occurred in either species of mole rat.
The team then used a scanning electron microscope to look more closely at the outer hair cells of the mole rats’ ears. They discovered that the hair bundles in a particular section of the ear were abnormal, when compared to other rodents, like mice and gerbils.
Using a genotype library, the researchers were able to link the proteins involved in abnormal hair bundles to human deafness.
“Using this database, we could identify small changes in proteins essential for hearing in both mole rats and humans that give rise to hearing loss and deafness” says Pyott.
Statistical analysis on the evolutionary history of the gene mutations involved in causing these abnormal hair bundles, suggest that the mutations were not random, but positively selected for. This suggests that the mole rats evolved to have bad hearing.
We still don’t know why that might be the case, though. One theory suggests that the creatures lost some of their hearing ability because the sense is not required underground. Another theory suggests that there are a lot of echoes underground, so the mole rats evolved to have bad hearing in order to avoid acoustic overexposure.
“This new study suggests that these creatures represent natural models of certain types of deafness in humans,” says Matthew Mason at the University of Cambridge. “Future studies of the hearing of mole rats and their relatives may shed further light on the causes of hearing loss in humans.”
Journal reference: Current Biology, DOI: 10.1016/j.cub.2020.08.035
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