NEIL BROMHALL / SCIENCE PHOTO LIBRARY
Naked mole rats have poor hearing because, unlike other mammals, they have abnormal outer hair cells that can’t 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, have poor hearing and make loud, piercing cries.
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 played to them. This confirmed that they struggled to hear quiet sounds and could only perceive sound between a narrow frequency, between 0.5 and 4 kiloHertz. Humans, by contrast, can detects sounds between about 0.02 and 20 kHz.
The researchers then recorded the sound transmitted by the cochlea, a part of the inner ear typically shows the ear is amplifying sound information. They found that no such amplification occurred in either species of mole rat.
The team used a scanning electron microscope to look more closely at the outer hair cells of the mole rats’ ears. The hair cell bundles in a particular section of the ear were found to be abnormal compared with those of other rodents, like mice and gerbils.
Using a genotype library, the researchers were able to link the proteins involved in abnormal hair cell 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 of the evolutionary history of the gene mutations involved in causing these abnormal hair cell bundles suggests that the mutations weren’t random, but instead positively selected for. This indicates that the mole rats evolved to have bad hearing.
We still don’t know why that might be the case. One idea is that the creatures lost some of their hearing ability because the sense isn’t required underground. Another suggestion is 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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