The Emei music frog of China advertises the size and suitability of their underground burrow through the acoustic properties of their croaks.
First, an apology. The image above is not a picture of the Emei music frog, but a relative known as Holst’s frog. I could not find any free or CC licensed images of the music frog – but its story is too good to pass up for want of a better picture.
The Emei music frog of central China has a beautiful call: tones that jump up and down like it’s singing:
Both male and female frogs vocalise, which is unusual, but even more strange is why they sing. Male Emei singing frogs are advertising real estate.
Male frogs of this species build nests in the ground – muddy burrows, really. And female frogs are especially interested in two characteristics of those burrows: how deep they are, and how narrow the entrance is. The burrows need to be deep so that there’s enough room to raise a family; the entrances need to be narrow so that predators cannot easy eat that family.
Research has shown that the male frogs use a very specific call when they are inside their burrow trying to attract mates. A deeper burrow echoes the call around and causes it to last for longer; a narrow entrance lowers the frequency of that call. As a result, listening females can evaluate the quality of the burrow purely on the basis of the acoustic properties of the call. That’s how they decide who to hook up with.
In other words, frog real estate advertisements.
The Generalist Academy 
