Have you ever placed your iPhone inside a glass bowl to make the music louder? You're not alone. A species of pearlfish from French Polynesia uses oyster shells to amplify the sounds they make, which are vibrations produced by their swimbladders for communication purposes. As described by Loic Kever and his research team, the pearlfish take up lodgings inside oysters and other mollusks; in the study area, 70% of oysters had a fish tenant. Once inside, their calls are louder and-- the researchers hypothesize-- propogate farther, aiding communication.

This is reminiscent of a species of tiny bat called Spix's disk-winged bat, which has been said to use rolled-up leaves (where it roosts, for safety) as "trumpets" to amplify their calls. For these bats, though, it is a double-edged sword (or more accurately, a double-edged leaf); sounds produced from within the funnel-shaped leaf are indeed amplified (imagine a bullhorn), but the roosting bats have more trouble distinguishing incoming calls when they are inside the leaf. The sloped surface of the leaf distorts and garbles social calls from outside visitors, which means that these bats probably miss a few visits from their in-laws.

In both cases, sound amplification occurs in conjunction with a structure that also serves as shelter-- for the bats, protective leaves, and for the fish, hard oyster shells. Are their instances in nature of animal-initiated sound amplification that does not also serve another purpose?

Ask the red-headed woodpecker that eagerly drills on the aluminum siding right outside my room every morning at 6:00.