When night falls over the dense tropical rain forest of El Yunque in northeastern Puerto Rico, the air reverberates with animal vocalizations. “It can be deafening,” says Peter Narins, a vertebrate physiologist at the University of California, Los Angeles. “There are all kinds of things calling.”
But when he sticks his geophone in the ground to record the rumbles beneath the soil, he hears a very different sound track—one that’s much quieter, but far from silent.
It was by eavesdropping on El Yunque’s subterranean world that, in the early 1980s, Narins discovered that the rain forest’s white-lipped frogs could communicate by sending vibrations through the ground.
When white-lipped frogs call, they make a series of audible chirps about four times per second by inflating and deflating a vocal sac under their jaws. But because they often call with their rear ends buried in the mud and their heads and forelimbs ...
