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Hearing

Frogs’ lungs help them find a mate

Male frogs call to attract females, but how can females tell that it is a male of the same species calling? Green tree frogs use the same principle as in noise-cancelling headphones – and they are far better at it.

By Birgitte Svennevig, , 3/4/2021

Anyone who has been near a lake or a waterhole with frogs knows that frogs can make a real racket. The males croak to attract females and to defend good calling spots against other males.

Often several different species are calling at the same time, and there is also noise from wind, water, traffic, people, machines etc.

So how can the coveted females hear the males calling? And distinguish between the different species? And, ultimately, assess whether the call comes from a male that is worth leaping for?

Frogs have a special ear

Frogs do not have external ears, but pick up sounds with an eardrum, just like we do. A distinctive feature of frogs (and lizards and birds) is that the two eardrums can affect each other acoustically – the sound literally goes in one ear out the other.

This coupling of the ears makes them directionally sensitive, and frogs are unique in that sound can also reach the eardrums through the lungs and amplify or attenuate the vibrations of the eardrums.

So, one could say that frogs also hear with their lungs. But what exactly can they hear with their lungs?

Ancient and super sophisticated

Researchers now believe this mystery has been solved in a new study of American green tree frogs (Hyla cinerea), published in Current Biology.

One of the researchers behind the study is biologist and expert in frog hearing, Jakob Christensen-Dalsgaard from the University of Southern Denmark.

One theory has been that pulmonary hearing is intended to locate sound sources. But according to Jakob Christensen-Dalsgaard, the purpose is something completely different: to sort out certain sounds – just like when we put on noise-cancelling headphones in an open-plan office.

Although the frogs are utilising an ancient mechanism, it’s a sophisticated solution to a problem that human technology is currently working on

Jakob Christensen-Dalsgaard, Associate Professor

– Although the frogs are utilising an ancient mechanism, it’s a sophisticated solution to a problem that human technology is currently working on, he says and adds:

– There’s a lot of research into noise-cancelling headphones and hearing aids, and it cannot be ruled out that the principles we describe can be applied in both headphones and hearing aids.

The cocktail party problem

The research team believes that the special ‘pulmonary hearing’ can be traced all the way back to the age when the first animals moved ashore. Over time, some of them developed ears and vocal communication.

Their studies show that the lungs of the female frog mainly respond to sound frequencies that are NOT found in calls from males of the same species as the female herself. In other words, they respond to sounds that are irrelevant to the female.

The mechanism thus filters out unwanted sounds so that the female can concentrate on the sounds that matter to her: calls from males of the same species.

It can be compared to the cocktail party effect in us humans: Our ability to hear what a particular person is saying at a cocktail party and filter out the background chatter.

Photo: Shutterstock

About the study

  • The researchers’ studies are based on laser measurements of vibrations of the eardrum and lungs during acoustic stimulation, on recordings of frog calls and data from the citizen science project The North American Amphibian Monitoring Program, which has recorded calls from frogs and other amphibians during an 18-year period in the eastern and central United States
  • The research team consists of Norman Lee from St. Olaf College, Minnesota, USA, Jakob Christensen-Dalsgaard from the University of Southern Denmark and Lauren A. White, Katrina M. Schrode and Mark Bee from the University of Minnesota, Twin Cities
  • The work is supported by the National Science Foundation.
Meet the researcher

Jakob Christensen-Dalsgaard is an Associate Professor and expert in animal hearing at the Department of Biology.

Contact

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  • Corporate Communication University of Southern Denmark
  • Campusvej 55
  • Odense M - DK-5230
  • Phone: +45 65 50 90 00
Editing was completed: 04.03.2021