Exactly how many ears do frogs have?
Ask any child what sound a frog makes, and you’ll get an adorable, enthusiastic croaked “Ribbit” probably matched with a hopping motion. Your kid pretty much got it right, except for the annoying reality that in nature there always are loads of exceptions. And frogs, like fish and insects, really have explored exceptional boundaries of what natural selection can produce! So, before moving on, I’ll just add the caveat to your child’s performance that some frogs make no sound, and the diversity of sounds frogs produce is staggering. I have done field work in places unfamiliar to me and had to be convinced by colleagues leading me directly to the source, that some odd sound in the forest actually emanated from a frog. Check out this fun clip to get a sense of this diversity.
On the other hand, the frogs that simply do not produce a mating call do manage to somehow communicate. I recall very well one late night in a cloud forest in Puebla, Mexico, encountering a breeding congregation of frogs that was absolutely silent. To be honest, it was creepy and far beyond my years of experience with frogs. I’m not entirely sure how they locate one another, or how females choose from among the hundreds of competing males. A cool project for someday …
But today’s blog is about hearing, not vocalizing, even if the two are closely related. First, let’s get oriented. In many frogs, the eardrum, or tympanum, is external and quite obvious as a big flat disc right behind the eye (see image). One on each side = two ears. Easy-peasy, right?! Nope, not so fast—let the exceptions begin! Firstly, lots of frogs have the tympanum buried underneath the skin so you can’t see it. They can hear just fine: you just can’t see the parts involved. Or, in some cases, the eardrum (and associated tiny ear bones, similar to your incus or “anvil”) are absent. Whoa!
If a frog has no ears, how can it hear the mating call of the species? Well, they can hear it just fine because they have an entirely separate set of internal ears that are unlike anything else among all animals. At the base of their skulls (so not visible externally) are two small openings covered with a tiny disc-shaped bone called an operculum. These are situated right next to the opening where the bone connected to the tympanum enters the skull. There is no tympanum associated with the internal opercular system, but rather a muscle that extends between the disclike operculum and the shoulder blade. Low frequency vibrations in the air or through the ground transfer up their arms to the shoulder blade and travel across this permanently taut muscle (known as a tonic muscle) to the operculum at the base of the skull and thus enters in same inner ear organs that convert physical sound waves to neurological signals. In some senses, their hands are acting like an eardrum. Wow! I told you this was going to get weird!
The result of this (and how I teach this topic to younger students) is that most frogs have not two ears, like us, but four ears! Salamanders do not have an external tympanum, so they only have the two opercular ears. Frogs that lack the tympanum only have the two opercular ears—and this is why frogs that appear to be earless can actually hear. We can hear in stereophonic sound. Many frogs, then, can hear in quadraphonic sound.
Another oddity is the weird frog that has no tympanic system, yet still manages to have quadraphonic hearing. My colleagues discovered that the tiny Gardiner’s frog, from the Seychelles, manages to use the unusually thin bones on the roof of its mouth, the palate, as a functional tympanum that receives sound vibrations and transfers it to the inner ear. Here’s a link from 2013, when this remarkable discovery was announced.
We have yet to find a frog that lacks the opercular system and the tympanic system, so we have not yet found an entirely earless frog. I bet it exists out there somewhere, however! I learned long ago never to presume anything with frogs because there always is an exception out there waiting to be discovered. Maybe I’ll discover it in my wanderings? That would be very exciting.
Until next time, pay attention to the sounds you hear frogs making. Take a closer look at the frogs at Zoo Atlanta. Do they have external tympani? Consider what their reality might be like with an additional set of ears connected to the arms and shoulder blades! Frogs are so cool!
Joe Mendelson, PhD
Director of Research