Echolocating bats have noise-cancelling genes that may help explain why they don鈥檛 go deaf despite producing very loud ultrasonic sounds as they fly. The finding could help unravel how echolocation evolved and might also lead to treatments for hearing loss in humans.
Most bats produce and hear intense high-pitched sounds that bounce off objects, enabling them to navigate and find food in the dark. These sonar calls are beyond human hearing鈥檚 frequency range, but they are often louder than 100 decibels. Some bats can even produce sounds exceeding 135 decibels.
A normal conversation between people is at about 60 decibels, but prolonged exposure to anything above 80 decibels can cause ear damage in most mammals, including us. Above 120 decibels, it becomes painful. That is because extreme noise can irreparably harm or kill sensitive hair cells in a mammalian cochlea 鈥� a spiral-shaped cavity in the inner ear 鈥� that are responsible for hearing.
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Echolocating bats have a muscle in their ears that dampens incoming sounds, but the effect isn鈥檛 enough to explain how their hearing 鈥� essential for their echolocation abilities 鈥� isn鈥檛 damaged by the constant cacophony they generate when flying.
Seeking an answer, at the Chinese Academy of Sciences and his colleagues attached brainwave-recording electrodes to the heads of anaesthetised mice, a non-echolocating fruit bat species and five echolocating bat species. Sounds within their hearing range were then played to them at 120 decibels for 2 hours. A week later, the researchers performed the same experiment again.
Results showed that the mice and fruit bats experienced hearing loss and had lost a significant amount of cochlea hair cells, but the echolocating bats were unaffected. By comparing the genetics of bats that do and don鈥檛 echolocate, the researchers discovered that the five species of echolocating bats have several genes that over-produce proteins that seem to protect their cochlear hair cells.
How exactly the genes protect the hairs is unclear, but Peng says one gene called ISL1 is an optimal candidate for testing in the clinic. 鈥淥ur study opens a novel window into preventing the damage and loss of hair cells from intense noises.鈥�
鈥淚t would be exciting to understand how these cochlear hair cells function to resist damage,鈥� says at the Australian National University in Canberra. 鈥淭hat way, we might illuminate when this protective benefit originated, adding a piece to the echolocation origins puzzle.鈥�
Journal of Genetics and Genomics
