While many mammals suffer hearing loss in old age, bats were thought to be immune to this phenomenon due to the critical importance of their hearing for echolocation. However, a team of researchers led by Tel Aviv University (TAU) in Israel has recently found that bats often lose their hearing in old age just like other mammals, including humans.
Yet, because bats roost in very noisy colonies which would quickly damage the hearing of humans and other mammals, it is likely that they evolved some innate ability to limit this age-associated hearing loss.
“While high-frequency hearing confers a survival benefit for many animals, it is essential for the survival of echolocating bats, which rely on it for orienting in their environment,” said senior author Yossi Yovel, a neuroecologist at TAU. “However, to date, no study has systematically examined the effect of age on hearing in bats.”
The scientists evaluated the age of 47 wild Egyptian fruit bats (Rousettus aegyptiacus) by measuring the accumulation of age-related chemical markers on their DNA, and tested their hearing by monitoring their brains’ response to sounds of varying pitch and volume. The analysis revealed a clear age-related hearing loss that, as in aging humans, was more pronounced at higher sound frequencies. Moreover, the rate of hearing loss – approximately one dB per year – was also similar to that observed in humans.
Further tests showed that bats experienced an age-related decline in the structure and functionality of their cochlea, along with a decrease in the speed of auditory nerve processing – a symptom known as “neuronal presbycusis,” which can impair speech comprehension in humans and make echolocation more difficult for aging bats.
One possible cause for bats’ hearing loss is their constant and cumulative exposure to high noises in their environments. By placing microphones in their caves, the experts discovered that they are continuously exposed to over 100 dB of noise (the equivalent of the sound produced by a motorcycle or chainsaw). However, the loudest sounds seemed to occur at lower frequencies, at which bats show no sign of age-related hearing loss.
“When taken together, the very high levels of noise that fruit-bats are exposed to and the mild (similar to human) levels of age-related hearing loss suggest that bats might have some special adaptations to cope with their very noisy environments,” Yovel said.
Since Egyptian fruit bats also rely heavily on vision when possible, further research is needed to explore age-related hearing loss in bat species with poorer vision, for which echolocation is the main mechanism of orientation. Better understanding bats’ adaptation to hearing loss could offer new insights into the mechanisms structuring age-related hearing loss in humans.
The study is published in the journal Life Science Alliance.
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