Close your ears

“Close your ears, honey.” Or you may be more familiar with “earmuffs,” popularized by Vince Vaughn’s character in Old School (pictured on right). These are phrases often spoken to the kids in the room immediately before an unflattering string of obscenities starts flowing about some absurdity of the current presidential race or the sudden fumble on the football field. How exceedingly frustrating it must be for the children: not only are they not yet able to be fully involved with the adult world, they are being asked to perform supernatural tricks in order to achieve this exclusion. But what if it were possible? Think about all the sounds you’d love to shut out if only you could only close your ears…

The concave-eared torrent frog (Odorrana tormota), aptly named for its chosen habitat of fast-flowing streams in China, has come the closest of any animal to achieving this amazing capability. A new study led by Albert Feng of the University of Illinois shows that while this frog cannot shut out sounds entirely, it can ‘close its ears’ to alter the range of frequencies it can hear…sort of.

Like most frogs, male torrent frogs produce calls to attract mates. Unlike most frogs, the calls of torrent frogs are produced at many frequencies simultaneously (harmonics and subharmonics) and span into the ultrasonic range, inaudible to the human ear. Even more rare, these frogs appear to be able to hear and process these ultrasonic sounds.

But the most unique quality of the torrent frog’s hearing is its ability to tune in to different frequencies. By closing off the tubes that connect the ears to the mouth (the Eustachian tubes), these frogs shift the range of sound that reaches the eardrum with enough amplitude to be heard. When closed, the frogs dull lower frequency sounds while becoming more sensitive to higher frequency sounds, including those in the ultrasonic range.

While some other vertebrate species have similar abilities to modify the auditory input from the environment before it reaches the inner ear, the method that these frogs use is entirely novel. In addition, researchers believe that these frogs may be able to actively control the opening and closing of the Eustachian tubes and thus the tuning adjustment of their hearing – no other animal has ever been known to have such an ability.

In fact, this finding was so unexpected, Feng and his colleagues discovered it completely by surprise. They were simply attempting to confirm that these frogs do indeed hear the ultrasonic sounds produced during their calls when they noticed that the Eustachian tubes were not permanently open as was previously believed of all frog species. Because it’s extremely high in frequency, ultrasound can penetrate many mediums that audible sounds cannot and therefore has the potential to stimulate a behavioral response in an animal by directly activating the neurons of its brain. Thus, in order to confirm that these animals are hearing the ultrasonic sounds, Feng was measuring the vibrations of the eardrum in response to sounds of various frequencies. That’s when he stumbled upon the frogs’ unique and amazing ability to adjust the tuning of their ears.

The closing of the Eustachian tubes can be viewed by shining a light up at the frog from below. When the tubes are open, the light shines through. (See video.) It is unclear exactly when male frogs close off their Eustachian tubes to affect this frequency shift. They always do so while producing their own calls, which may help them hear over the sounds of their own vocalizations. The dampening of low frequency sounds may also help the frogs drown out the constant racket of the babbling brooks that they call home to allow them to better hear the calls of their fellow frogs.

In addition to the calls of the males that span into the ultrasonic range, females also call, albeit much less frequently, and their calls average even higher frequencies than those of the males. Experiments have shown that males respond to female calls by accurately localizing its source and immediately hopping in that direction. So it is possible that by muffling the low frequency sounds, males are better able to hear and respond to the calls of females. I imagine that if humans evolved this ability, this is the exact opposite of how men would use it.

It must be noted that the frequency shift is not the only benefit of closing the Eustachian tubes, and it is unclear which prompted the evolution of such an extraordinary adaptation. This skill also physically protects the extremely thin eardrum. About 1/30 the thickness of most other frog species, the thin eardrum of the torrent frog is one of the adaptations that allows these frogs to hear such high frequency sounds. However, because it is so thin, the eardrum is now more susceptible to damage from either the air pressure of vocalization or live prey items on which the frogs feed. Closing the Eustachian tubes reduces the amount of low frequency sound that is high in energy that reaches the inner ear as well as creates a physical barrier between the eardrum and the mouth where the prey are consumed.

But regardless of the origin, the fact remains that these frogs have evolved the ability to adjust their hearing in certain situations. While they cannot shut out the world entirely, they can selectively mute sounds of certain frequencies. So if humans had this unique ability, while we may not be able to close our ears, maybe we could at least silence the Fran Dreschers of the world.

Notes

Learn more about Dr. Albert Feng and his research on torrent frogs.

For more on frequency shifting:

Gridi-Papp, M.; Feng, A.S.; Shen, J.X.; Yu, Z.L.; Narins, P.M.; Active control of ultrasonic hearing in frogs. Proc. Nat. Acad. Sci. 2008, 105 (31), 11013-11018.

For more on males’ responses to female calls:

Shen, J. X.; Feng, A. S.; Xu, Z. M.; Yu, Z. L.; Arch, V. S.; Yu, X. J.; Narins PM Ultrasonic frogs show hyperacute phonotaxis to the female’s courtship calls. Nature 2008, 453, 914-917.

Photo credit 1

Photo 2 courtesy of Albert Feng

Video copyright © by the National Academy of Sciences, all rights reserved.

Many thanks to Rob Combs and Yaniv Brandvain for their comments and suggestions and to the entire Wade lab for their advice and support in starting this blog.

Welcome!

My name is Jef (Jennifer) Akst. I am an aspiring science writer interested primarily in animal behavior. Currently, I am a biology graduate student at Indiana University studying mating behavior and sexual selection in seahorses. (Check out my website for more information.)

Science has always been a passion of mine…from junior year in high school, when I was finally freed of those pesky physical education and foreign language requirements and voluntarily enrolled in five (five!) science classes, to a whirlwind bachelor’s degree in biology at the College of William & Mary to my current graduate endeavors at IU, science has dominated my academic life.

But for the past couple of years, I’ve been intermittently nagged with the worry that I was not as interested in biology as I should be. My studies have grown so focused that I feel I have begun to lose perspective on the rest of the biological community and where my work falls within it. And while I still find seahorses fascinating, minor (and not so minor) setbacks in the research can really dull the excitement I once felt for the sciences. While other students in my programs talk about their research with an enthusiasm that, to me, seems warranted for nothing short of winning the lottery, I haven’t felt ‘all jazzed up’ about anything biological in years.

I’ve struggled with whether or not this was a problem. On the one hand, it seems slightly outlandish to dedicate several years of one’s life (and an infinitely greater number of headaches) to something that could be qualified as a mere interest. On the other, it seems equally ridiculous to expect any one topic of study to generate an interminable excitement that would carry one through to retirement.

So while science remains interesting to me, my passion has clearly dwindled, and I justified this with the gross generalization that it’s just a job, and no one actually likes his job anyway, right? Who really cares? More importantly, who cares if I don’t?

But recently I’ve been feeling excited again. I’ve started to take an interest in science writing. Taking a step back from the ever-narrowing scope of my research has allowed me to once again see science as I used to: a captivating and contemporary field that is constantly churning out hoards of remarkable new research. It has restored my passion for the science that was, in all honesty, starting to bore me. I am once again exploring areas of science outside my particular specialty and LOVING it.

So I’ve decided to start a blog. Not only will it keep me in tune with a much broader focus of behavior than my own research, I get to share what I learn with all of you, and the idea of sparking an enthusiasm for science where disinterest and distrust once resided, motivates me beyond anything I could have predicted. Hopefully you will find these tales of the amazing things animals do as remarkable and noteworthy as I do. Please feel free to leave comments about any of my articles and/or e-mail me with additional questions. Happy reading!