Birds' selective fall hearing may hold lessons for humans

November 2009

Editor: Selective hearing, you say? How many times has someone accused you of having that? Well, if you accuse a bird of having selective hearing, it may well be true! Here's the story from the folks at Purdue.

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It appears that some birds have found a simple solution when they are not looking for a mate in the fall - they just ignore love's call by muting their hearing.

Purdue University biologists studying how both birds and humans adapt to noise have found that some bird species have degraded hearing ability in the fall - when it's not mating season - as well as in other select situations. The findings have potential implications for hearing loss in humans, said Jeffrey Lucas, a Purdue professor of biological sciences.

"We've been thinking a lot about human hearing," Lucas said. "The world is getting noisier as the environment gets more urbanized. Noise becomes much more important to understand."

In ongoing research, Lucas is looking at how birds adapt the precision of their hearing to seasonal changes as well as to disturbances in their environment. His work even goes so far as to suggest hearing ability differs between the sexes.

Birds serve as a good model for hearing research because of how they learn vocalizations and adapt their hearing to behavioral changes induced by the changing of the seasons.

In the fall, for example, the reproductive activity of some birds comes to a standstill. This behavior causes certain birds to "down regulate," or decrease the precision of their hearing, Lucas said. Females, in particular, invest less energy in maintaining their auditory system in the fall because they are not looking, or more aptly, listening, for a prospective mate.

Components of this work will be published next year in the Journal of Experimental Biology. A previous paper describing this research was published in 2007 in the Journal of Comparative Physiology A.

While it's not known how birds alter their hearing ability, there are two factors believed to help explain the seasonal decrease in auditory performance.

Scientists know that messages, or impulses, sent spontaneously by nerves decrease in frequency during the fall. It also appears that the hormone estrogen plays a role because of how it alters certain parts of the auditory system, like hair cells in the inner ear that convert sound to impulses.

Another dimension of Lucas' current research program is how noise affects auditory processing in birds.

Birds in the forest, for example, adapt their song to the reverberation caused by trees. In contrast, birds in open areas, such as sparrows, adapt their song to noise generated by the wind.

"By understanding in the broadest sense how organisms adapt their hearing to their environment, we can get a better understanding about how our own hearing is adapted to our environment and how that hearing might change either for the good or for the bad," Lucas said.

The next phase of Lucas' research is to find out if hearing differences exist between the sexes.

To determine this, graduate student Megan Gall is studying the hearing system of brown-headed cowbirds, a substantial pest to more than 200 species of birds. The role of hearing differs substantially between male and female cowbirds.

"The female lays eggs in the nest of other birds and then flies away," Lucas said. "The female uses the song of the host to get some information about, first of all, what the species of that host is, and also to get some information about where the nest is."

Males, on the other hand, use hearing primarily for interacting with female cowbirds.

What they have found so far shows that females, because they have to deal with a broad range of sounds produced by the host, make a greater investment in hearing than males do.