Many mammals suffer hearing loss as they age, but bats were thought to be immune because of the importance of hearing for echolocation. However, Israeli scientists have discovered that bats, like humans, lose their hearing as they age. According to a study published in the journal Life Science Alliance, bats may have evolved some innate ability to limit this age-related hearing loss because they roam extremely noisy colonies that would quickly damage the hearing of humans and other mammals.
Many species of bats have extremely long lifespans more than 40 years compared to other mammals of similar size, such as mice. “While high-frequency hearing provides a survival advantage to many animals, it is essential for the survival of echolocating bats, which rely on it to navigate their environment,” explains Yossi Yovel, a neuroecologist at Tel Aviv University.
PhD student Yifat Tarnovsky of Yovel’s lab and colleagues from the universities of Tel Aviv and Maryland assessed the age of 47 wild Egyptian bats (Rousettus aegyptiacus) by measuring the accumulation of chemical markers on the animals’ DNA. The researchers then tested the bats’ hearing by monitoring the electrical responses of their brains to sounds of varying pitch and volume.
The recordings revealed clear age-related hearing loss, which, as in humans, was particularly pronounced at higher sound frequencies. Moreover, the rate of hearing loss, ~1 dB/year of age, is very similar to that observed in aging humans.
Bats experience hearing loss
Further tests suggest that, similar to humans, bats experience an age-related decline in the structure and function of the cochlea, as well as a decrease in the processing speed of the auditory nerve. This second symptom, known as neuronal presbycusis, can impair speech understanding in humans and could make echolocation difficult in old bats.
“The bats we studied rely on echolocation for various tasks, but they also rely heavily on vision when possible,” says Tarnovsky. “It will therefore be important to repeat our tests in bats with poor vision, where echolocation is almost the only orientation mechanism.”
One possible contributor to the bats’ hearing loss is their cumulative exposure to high noise levels in their environment. Like many other bat species, Egyptian bats live in large colonies and emit frequent, loud, social calls.
Tarnovsky and colleagues placed several microphones in the bat’s cave and found that the animals were continuously exposed to noise of more than 100 dB, roughly equivalent to the noise of a motorcycle or chainsaw. Surprisingly, however, the loudest sounds were at lower frequencies, at which bats show little to no hearing loss as they age.
“Taken together, the very high noise levels bats are exposed to and the mild (human-like) levels of age-related hearing loss suggest that bats may have some special adaptations to cope with their very noisy environment.” says Yovel. The researchers hope that understanding these adaptations could provide insight into the mechanisms of age-related hearing loss in humans.
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