It’s mating season for frogs in Kansas. On a walk through the Baker Wetlands, the air was full of the sweet nothings frogs sing to each other.
I’m not sure what species they were, but I’m guessing it was mostly cricket frogs.
In Aristophanes’ classic comedy The Frogs, the chorus of frogs sing:
Brekekekex koax koax
Brekekekex koax koax.
Children of the marsh and lake
harmonious song now sweetly make,
our own enchanting melodies
To which Dionysius, god of the vine, replies: “I’m starting to get a pain in the ass/ from all your koax koax.”
Most frogs don’t sing to irritate gods or demigods. Male frogs sing to protect a mating territory, usually near the edge of some water, and to attract females. Females judge males by the quality of their song. Louder songs generally indicate larger males, so frogs have evolved a range of adaptations to increase volume, reaching more females and making each male seem larger.
Once a female enters a male’s territory, he climbs onto her back and hangs on. That stimulates the female in a way that causes her to release a clump of eggs, which the male then fertilizes externally. Sometimes he will remain on her back as a way to prevent other males from mating with her.
The ear (technically a tympanic membrane, since there’s no sound capturing structure) on the side of a frog’s head is about the same size as the tympanic membrane inside a human ear, which is surprisingly similar to the size of the sound collecting organ of everything from moths to elephants. Elephants are specialized on lower sounds, so they have a larger tympanum, and moths specialize on higher pitches to hear bat calls, so theirs is smaller, but the sizes of the tympanums are closer than the sizes of the bodies.
That is a constraint of simple physics, and the body of each organism has to adjust to those constraints. To vibrate at the right frequencies, a frog’s ear and a human ear have to be about the same size, despite a massive difference in body size.