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Why Birds Don't Get Ear Infections and We Do

What we gained—and lost—in the evolution of the human ear

This is a little different from my usual blog post. It's a reflection on the evolution of the mammal ear, dating from the time when mammals and birds took off in separate directions on the evolutionary tree. It was written for a science journal, which decided not to publish it because it's an essay rather than a reported piece. But some readers may be as interested as I was to see this confluence of events.

Add one more hearing problem to the fact that 300 million years ago when mammals split from birds, birds in some ways got the better deal. Humans today pay the price in a susceptibility to irreversible hearing loss.

We’ve known for the past decade or so that birds, like fish, can regenerate hair cells, the tiny vulnerable cells in the inner ear that are essential to propelling sound signals to the brain. What does it take to deafen a bird? Perhaps a collision with a glass skyscraper, a tangle with a hawk, a lab assistant who deliberately destroys the bird’s hair cells.

However the cells are damaged, they regenerate within days and the deaf bird can hear again, just as well as it did before the damage. It can happen over and over again. Not so with mammals. Once a mammal loses its hearing because of damage in the inner ear, it’s permanent.

Abigail Tucker and colleagues at Kings College London have now found that humans came out on the short end of that evolutionary divergence in terms of the middle ear as well. Humans are more prone than birds or reptiles to ear infections, specifically to “glue ear,” a buildup of fluid in the middle ear that can lead to chronic ear problems in children.

If human ears could be enticed to regenerate hair cells, it would represent a potential cure for the hundreds of millions of people around the world who have the kind of hearing loss caused by noise, exposure to damaging drugs, disease and old age. If the middle ear could be induced to change the nature of its lining, it would mean far fewer ear infections, including otitis media, which is a major cause of hearing loss in children, especially in developing countries.

Trying to figure out how birds and fish restore hair cells, and then replicating the process in man, has occupied dozens of researchers at universities around the world for much of the past decade. In 2010, Stefan Heller, at Stanford, announced a major breakthrough, when he succeeded in regenerating hair cells in a mammal (a mouse) using stem cell therapy. At the University of Washington, Ed Rubel and his colleagues have successfully produced new functioning hair cells in a mammal, using stem cell therapy. It’s a long way from mouse to man, but the first steps have been taken.

What did mammals gain in this evolutionary tradeoff? Birds have many more hair cells than mammals, and they cover a larger region of the inner ear, in neatly organized patterns. Humans hair cells are arranged in four rows, like soldiers standing at attention. Human hair cells are highly specialized, connecting to specific frequency receptors, which allows for better hearing across a wide spectrum, including high frequency hearing – essential for understanding speech.

As for the middle ear, the tradeoff was also a gain in hearing acuity. The mammal middle ear contains three bones: the malleus, the incus, and the stapes (which is the smallest bone in the body). These three connected bones sit in an air filled cavity, which allows them to pick up vibrations from the eardrum. From malleus to incus to stapes, the vibrations are passed along to the inner ear, through the oval window. The bones give the sound waves an extra push, possibly (some think) also providing better amplification of the sound.

The hair cells in the inner ear pick up the signal and through a series of steps transmit it via the auditory nerve to the brain, which hears it as sound: speech or thunder or a dog barking. If damage occurs in the middle ear, the vibrations never reach the inner ear.

But what’s the culprit that allows this gluey fluid buildup in mammals but not in birds or reptiles? It goes back to that evolutionary split. Bird and reptile ears have just one bone transferring sound from the eardrum to the inner ear. In the mammal ear, the three bones sit in a cavity lined with two different kinds of cells -- some related to those in the Eustachian tube and some related to the neural tube. The cells from the Eustachian tube are hairy, and help clear debris from the ear. The cells from the neural tube are smooth, leaving that part of the middle ear susceptible to infection.

Dr. Tucker hypothesizes that the “flawed” cell lining may have evolved to make room for the malleus, the incus and the stapes. The smooth part of the lining is an “evolutionary glitch,” as she put it, and “fails to provide an effective barrier against infection.” (The restructuring of the inner ear had the corollary effect of creating a jaw joint that allowed early mammals to chew, something not possible in reptiles and birds. This would have permitted them to eat a wide variety of foods.)

At the expense of a greater propensity to middle ear infections, then, mammals gained better conduction of sound (as well as a more efficient arrangement of teeth). And at the expense of the ability to regenerate hair cells, mammals gained better high-frequency hearing.

Most mammals just have to accept the tradeoff, but humans don’t. We know how to treat middle ear infections, and soon we will know how to regenerate hair cells. It’s a win-win situation.

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