Frogs Make Sounds and Hear in Surprising Ways
Frog Sounds and Listening
As warm weather is setting in the chorus of frogs is starting up once again. Every time I pass a pond or water filled ditch I'm amazed at the cacophony. One pond I like to visit is devoid of any man-made lights or nearby sounds, so when you close your eyes and listen, just sitting in the dark is like being in a sensory deprivation chamber of just frog calls, quite stirring. This all got me to thinking, how do frogs make sound? Also, since they don't have external ears how do they hear those sounds?
Let's start with the sound making. Like birds, it's only the male frogs that call. Females can make short chirpy sounds or peeps. The male's calls advertise their species, general fitness, size, territory, and location to other males and females. This type of acoustic communication is vital to creatures that mate and breed in ponds which can often be quite large. It's also safer because they can avoid daytime/visual predators. It's nearly impossible (especially for humans) to pick out one frog among hundreds of the same kind calling, so there is certainly safety in numbers at night too. From a male frog's point of view, why bother wasting energy with fighting other males when you can out-call them too? Each species of frog also has its own call, which helps female frogs and biologists identify them at night (check out my blog post on resources for learning frog calls).
How does all that big sound come out of a little creature?
In frogs and toads sound is produced when the nostrils are closed and air from the lungs is pushed over the vocal chords (in the larynx) and through the wind pipe (trachea) into the air sac (that sack below their chin that sometimes pokes out to the side and most often to the front). The air sac acts like a resonance chamber, which magnifies the sound and makes it louder. The glottis, or opening into and out of the lungs, modulates the net flow of air and the call production. You can think of it sort of like your lips, which can modulate air flow. Frogs can't pucker and blow, because they don't have lips per say, just bone covered by skin.
Frogs are one of the true circular breathers. When calling they can expand their air sacs using air from the lungs, and then the air can be forced from the air sac back into the lungs in a circular path. This allows them to croak continuously, even if they are under water. Check out this video of a green tree frog calling. You can see the lungs/body inflate, push air into the air sack, and so forth. Fresh air is brought in via the nostrils on the top of the frog's snout.
The smallest frogs work the hardest to produce sound, because so much of their body is taken up with the muscles needed to produce such a large sound. Energetically they also require much more energy to make big sounds than larger frogs.
How does a frog hear?
Just like you, the frog has an ear drum and inner ear. But, unlike you, he doesn't have any earlobes hanging off his head which, like dangly lips, would be very unhelpful or hydrodynamic under water. Instead, their eardrum is directly on the side of a frog's head, acting as a shield and cover for their inner ear. This is called the tympanic membrane or tympanum. It is covered in skin that doesn't have glands (no sweating in the ol' ear cavity), and is usually located right behind the eye.
The tympanum acts much like our own eardrum. Internally there is a bone attached to the tympanum. When sound strikes the membrane then the vibrations travel down the bone to the inner ear where the semicircular canals, cochlea, and papilla are located.. The tympanum works both under and above water to detect sound.
Teachers, if you ever want to show how sound travels through bone, have students plug their left ear with their left index finger, then hold a struck tuning fork to their elbow. The sound will travel through arm and finger bones into the ear, it's quite startling. Some hearing aids work by picking up sound from implants in the bone of a person's skull.
In some species of frog you can tell males and females apart by the size of their tympanum. The males usually have much larger tympanum for picking up vibrations and sound. This is especially useful for detecting the quieter chirps of females or picking up on the sounds of nearby competitors or predators. There are six North American species of frogs that have this sexual dimorphism:
- Green frogs (Rana clamitans, see below)
- American bullfrog (Rana catesbeiana)
- Carpenter frog (Rana virgatipes)
- Pig frog (Rana grylio)
- Mink Frog (Rana septentrionalis)
- River Frog (Rana heckscheri)
In male frogs with this dimorphism the tympanum is usually larger than the eye of the frog, while the female's eye is about the same size as the tympanum. Here's a short video showing a male bull frog calling. You can see that his tympanum is larger than his eye.
Frogs have a very short distance between their tympanum or eardrums, so they don't hear high frequencies very well (this has to do with sound localization and needing more distance between the eardrum and inner ear). They hear lower frequencies much better. Also, because of the way their head is structured sound vibrations actually travel from their outer ear, into their inner ear, across the roof of the mouth, and into the inner part of the other ear, in effect hearing a sound twice. But wait, it gets even weirder! In some species of frogs part of their body wall, near the lungs, actually vibrates in response to high frequency sound (like the eardrum). It's thought that this vibration acts as another form of sound input that can help them hear, shuttling the vibrations from the body all, through the lung and into the inner ear. You have to admit, having chest ears is pretty interesting.
The male gardiner's frog (S. gardineri) is completely deaf, and lacks any tympanum or middle ear. They do have an inner ear. To hear they use their mouth cavities to amplify sounds that travel to their inner ear using connective tissues.
So, how do frogs make sound? Through their throats, lungs, and mouth. How do they hear it? Using an external eardrum, inner ear, and sometimes chest! Most of us will never get close enough to look at a frog's ears to see if they are larger than their eyes, but many will have the joy of hearing the frogs as they call the first sounds of spring. Frog calls and hearing them? Truly a wonder of nature.!