hair cells in the inner ear, whetherdue to injury or to exposure to certain types of antibiotics such as gentamicin, is typically permanent in humans, but not only do frogs regenerate after injury and drug exposure, there is evidence that they continually create some hair cells as old ones wear out. Somewhere along the evolutionary chain between amphibians and mammals, we lost the ability to heal our brains, our cranial nerves and much of our ears. So studying how a tadpole can completely rewire its brain in forty-eight hours and how a bullfrog can regrow its auditory nerve and restore function is not just basic research done for the sheer fun of doing science. These studies are likely to provide the clues that may allow us to create gene or pharmacological therapies that restore this ability to humans.
Chapter 4
The High-Frequency Club
When I was three years old, I went deaf. No maternal exposure to rubella, no overly vigorous toddleresque Q-tip exploration, just an unfortunate case of chicken pox that lesioned my eardrums. I don’t have any explicit memories of the incident, and my hearing returned, the only residual problem being that my eardrums were slightly scarred and thickened. But now I hear bats.
Most people avoid bats if they can. Even on those warm summer evenings when you are outside, most of your interaction with bats is probably limited to seeing their shadowed forms flittering about, the smaller ones saving you from mosquito bites and the larger ones saving your garden from junebugs. But I spent a great deal of time one-on-one with them in the lab, and it never failed to amaze me how an animal whose brain is the size of a peanut actually builds most of its world with sound, creating three-dimensional images from subtle shifts in echoes.
To most people, bats’ auditory world is so far outside of the human hearing experience that they seem like silent shadows.But bats and humans share a lot of genetic heritage just on the basis of our being mammals. And as a mammal, you’re a member of an exclusive evolutionary club: the high-frequency club. If you’ve ever wished you could hear a dog whistle, console yourself with the knowledge that humans, like all other mammals, have a remarkably wide range of hearing. Non-mammalian vertebrate hearing is generally limited to an upper end of about 4–5 kHz (although some specialist birds such as owls and cave swiftlets can hear up to about 12–15 kHz). Of all the other vertebrates depending on sound to let them hunt, mate, define their territory, or avoid predators, we have the broadest hearing range, from the infrasonics of elephants to the 150 kHz natural ultrasound used by dolphins.
This isn’t because we’re more advanced or the new kids on the evolutionary block. Mammals are as old as the dinosaurs, but our ancestors at the time were rather mouse- or shrew-like, and likely to be listening sharply to avoid becoming someone else’s food. It’s just that our ears are more specialized than those of fish, frogs, reptiles or birds. And while ours shares basic features with all other vertebrate auditory systems, ours has two features that none of the others has, and both of which are critical to high frequency hearing—an outer ear and a cochlea.
Even among auditory scientists, there is a tendency to take the outer ear for granted. 13 For humans it’s something to hang a pair of glasses on or to make fun of if they stick out too far. And since an awful lot of our listening these days uses earbuds or earphones that sit inside or cover our external ear to let us hear more clearly in noisy environments such as airplanes or the gym, wetend to ignore the outer ear as a vestigial organ that doesn’t do much (outside of featuring in a memorable scene in the movie Reservoir Dogs ). But our outer ear is actually a fascinating evolutionary development that tells us a great deal not only about the environment in which mammals listen but also about what we listen to. And
Chapter 4
The High-Frequency Club
When I was three years old, I went deaf. No maternal exposure to rubella, no overly vigorous toddleresque Q-tip exploration, just an unfortunate case of chicken pox that lesioned my eardrums. I don’t have any explicit memories of the incident, and my hearing returned, the only residual problem being that my eardrums were slightly scarred and thickened. But now I hear bats.
Most people avoid bats if they can. Even on those warm summer evenings when you are outside, most of your interaction with bats is probably limited to seeing their shadowed forms flittering about, the smaller ones saving you from mosquito bites and the larger ones saving your garden from junebugs. But I spent a great deal of time one-on-one with them in the lab, and it never failed to amaze me how an animal whose brain is the size of a peanut actually builds most of its world with sound, creating three-dimensional images from subtle shifts in echoes.
To most people, bats’ auditory world is so far outside of the human hearing experience that they seem like silent shadows.But bats and humans share a lot of genetic heritage just on the basis of our being mammals. And as a mammal, you’re a member of an exclusive evolutionary club: the high-frequency club. If you’ve ever wished you could hear a dog whistle, console yourself with the knowledge that humans, like all other mammals, have a remarkably wide range of hearing. Non-mammalian vertebrate hearing is generally limited to an upper end of about 4–5 kHz (although some specialist birds such as owls and cave swiftlets can hear up to about 12–15 kHz). Of all the other vertebrates depending on sound to let them hunt, mate, define their territory, or avoid predators, we have the broadest hearing range, from the infrasonics of elephants to the 150 kHz natural ultrasound used by dolphins.
This isn’t because we’re more advanced or the new kids on the evolutionary block. Mammals are as old as the dinosaurs, but our ancestors at the time were rather mouse- or shrew-like, and likely to be listening sharply to avoid becoming someone else’s food. It’s just that our ears are more specialized than those of fish, frogs, reptiles or birds. And while ours shares basic features with all other vertebrate auditory systems, ours has two features that none of the others has, and both of which are critical to high frequency hearing—an outer ear and a cochlea.
Even among auditory scientists, there is a tendency to take the outer ear for granted. 13 For humans it’s something to hang a pair of glasses on or to make fun of if they stick out too far. And since an awful lot of our listening these days uses earbuds or earphones that sit inside or cover our external ear to let us hear more clearly in noisy environments such as airplanes or the gym, wetend to ignore the outer ear as a vestigial organ that doesn’t do much (outside of featuring in a memorable scene in the movie Reservoir Dogs ). But our outer ear is actually a fascinating evolutionary development that tells us a great deal not only about the environment in which mammals listen but also about what we listen to. And
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