Two surprising discoveries: contrary to the popular hypothesis, bats do suffer from hearing loss with age, but they apparently have surprising mechanisms that help them slow down the rate of hearing loss
Urban bat (photo: Yuval Barkai)
Residents and residents of the big cities are already used to the industrial noise, which is considered an integral part of life in the city and is considered harmful to hearing. But it turns out that there are those who benefit from mechanisms that help protect them from noise damage, and these are the bats: a new study by Tel Aviv University refutes the propaganda of many researchers in the scientific community, according to which bats are immune to hearing loss in old age (a phenomenon that characterizes many mammals). In the current study, the team of researchers states that the quality of the bats' hearing is indeed impaired, but since they live in particularly noisy colonies, they may have developed the ability to slow down hearing loss compared to humans and other mammals. Maybe we also have comfort in life next to bus stops and renewable building sites?
"Left at the next tree! do you hear me??"
In the past, there was a widespread misconception that due to the importance of hearing to the sonar system of bats, they maintain a good quality of hearing that allows them to orient themselves in space, even at advanced ages. "However, although high-frequency hearing provides a survival advantage to many animals, and is essential for the survival of bats - to date, no study has systematically examined the effect of age on hearing in bats," says Prof. Yossi Yuval, one of the leaders of the study.
The research was conducted under the leadership of the doctoral student, Yifat Tarnovski from the laboratory of Prof. Yossi Yuval, neuro-ecologist from the School of Zoology and a head The Sagol School of Neuroscience and in collaboration with a dean The Sackler Faculty of Medicine, Prof. Keren Avraham and Dr. Shahar Taber from her laboratory team. Colleagues from the University of Maryland also participated in the study. The study was published in the journal Life Science Alliance.
As part of the study, the researchers first estimated in their study the age of 47 Egyptian fruit bats (Rousettus aegyptiacus), by measuring the accumulation of methylations (a process in which a methyl group replaces a hydrogen atom) of the animals' DNA molecules. The researchers then tested the bats' hearing by monitoring electrical responses in their brains to sounds of varying frequency and intensity. The recordings showed a clear decline in hearing, related to the age of the bats, and as with humans, the decline was particularly noticeable at higher sound frequencies. Also, the rate of hearing loss in relation to the age of the bats was very similar to the rate observed in aging humans.
Further tests indicated that, similar to humans, bats experience a decrease in hearing related to the structure and function of the cochlea, as well as a decrease in the processing speed of the auditory nerve. "This last symptom," Tarnovsky explains, "may impair the understanding of speech in humans, and may make it difficult to use sonar in adult bats. The Egyptian fruit bats we studied rely on echolocation when performing various tasks, but they also rely heavily on vision when possible. Therefore, the tests we carried out in the research must be repeated, this time in bats with poor vision, for which echo-location is almost the only orientation mechanism."
New discoveries about the process of hearing loss
Also, the researchers estimate that one of the possible causes of hearing loss among the Egyptian fruit bats is the cumulative exposure to high noise levels in their environment. Like many other bat species, Egyptian fruit bats live in large colonies and make frequent and loud social calls. Tarnowski and her colleagues placed several microphones inside the fruit bat's cave and found that they were frequently exposed to more than 100 dB, a level equal to the noise of a motorcycle or chainsaw. Surprisingly, the loudest noises were in low frequencies, while the tests conducted showed that the hearing loss is mainly manifested in high frequencies.
"The combination between the very high noise levels to which fruit bats are exposed and the moderate rate (similar to humans) of age-related hearing loss indicates that bats may have special adaptations to deal with their noisy environment," concludes Prof. Yuval. The researchers hope that understanding these adaptations can provide insights into the mechanisms of age-related hearing loss in humans.
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