Finding the right mate is a challenge for any animal. Consider a female green tree frog at a local watering hole, inundated with mating calls from hundreds of males from multiple species. How can she possibly focus on the calls of a single potential mate? Surprisingly, the solution involves a noise-cancelling feature in these frogs’ lungs, as a new study published in Current Biology reveals.
High-Stakes Mating Game
As spring turns to summer in the southeastern United States, the nighttime scene at lakes and streams is dark and steamy. Male frogs croak loudly through the night, hoping to attract as many females as possible. However, a female green tree frog must be picky — most will mate just once a year, according to the University of Michigan’s Animal Diversity Web.
The situation is similar for other frog species, insects and birds, which together produce a nighttime cacophony of noise. In such an environment, focusing on the sounds produced by an individual of interest can be extremely challenging. Scientists have dubbed this the “cocktail party problem.”
Frogs’ Lungs and Hearing
The researchers behind the Current Biology study worked together to understand how the green tree frog — the most common frog in the U.S. — solves the cocktail party problem. The unique, interconnected anatomy of frog lungs and ears would appear to play a role, but scientists didn’t know how.
While human lungs have no direct effect on how the ears detect sound, the structure of the amphibian system suggests something very different. In frogs, the middle ear on one side is connected to the middle ear on the other side through air-filled “Eustachian” tubes. The middle ear contains the ear drum, which vibrates in response to sound so that it can eventually be processed by the brain. In frogs, sound from the lungs can also reach the middle ear through the voice box, mouth cavity and Eustachian tubes.
As the Current Biology study notes, scientists tested how much the ear drums of female green tree frogs vibrated in response to sounds at different frequencies. When a frog’s lungs were inflated with air, the ear drums vibrated less in response to sounds between 1,400 and 2,200 Hz than they did when the lungs were deflated. In fact, the lungs themselves were vibrating in response to sounds between 1,400 and 2,200 Hz, but not other frequencies. Researchers believe that the inflated lungs produce vibrations that are the exact opposite of the incoming vibrations within that specific frequency range, which dampens the vibrations in the ear drums. This is how noise-cancelling headphones work.
Why This Range?
Why would female green tree frogs selectively cancel noise from 1,400 to 2,200 Hz? Each frog species has a specific mating call, which can range from a high-pitched cackle to a deep croak. Male green tree frogs have a two-part call, with one sound at 834 Hz and another at 2,730 Hz, according to the study. Therefore, cancelling noise from 1,400 to 2,200 Hz does not interfere with the ability of female green tree frogs to hear their own species, but it does reduce noise that could make it harder to identify the two parts of the mating call as linked together. Similar strategies to dampen background noise are used in hearing aids and cochlear implants for humans.
To better understand how cancelling noise from 1,400 to 2,200 Hz might benefit green tree frogs in their natural environment, the researchers used data from the North American Amphibian Monitoring Project. Citizen scientists reported nearly 20,000 instances of green tree frogs “co-calling” with other species, which means producing sounds at the same time. A total of 42 co-calling species were identified, but 10 species accounted for 79% of the co-calling instances. Five of these species produce mating calls that fall at least partially within the 1,400 to 2,200 Hz range.
Therefore, if a female green tree frog wants to find an appropriate mate in a large crowd, she should breathe in and listen.
Future Directions
The discovery of how frog lungs influence hearing leads to many new questions. Do male green tree frogs have the same ability? What about the 6,000+ other species of amphibians, including frogs, toads, newts and salamanders? How do the three sources of sound (external, internal via the opposite ear and internal via the lungs) interact to help frogs navigate the world?
The study authors note that vocal communication evolved in frogs around 200 million years ago, probably after the evolution of the frog ear. It will be amazing to discover how the system has evolved — and perhaps use this knowledge to help humans navigate the world.
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