Natalie Angier in The New York Times:
Every night during breeding season, the male túngara frog of Central America will stake out a performance patch in the local pond and spend unbroken hours broadcasting his splendor to the world. The mud-brown frog is barely the size of a shelled pecan, but his call is large and dynamic, a long downward sweep that sounds remarkably like a phaser weapon on “Star Trek,” followed by a brief, twangy, harmonically dense chuck. Unless, that is, a competing male starts calling nearby, in which case the first frog is likely to add two chucks to the tail of his sweep. And should his rival respond likewise, Male A will tack on three chucks. Back and forth they go, call and raise, until the frogs hit their respiratory limit at six to seven rapid-fire chucks.
The acoustic one-upfrogship is energetically draining and risks attracting predators like bats. Yet the male frogs have no choice but to keep count of the competition, for the simple reason that female túngaras are doing the same: listening, counting and ultimately mating with the male of maximum chucks. Behind the frog’s surprisingly sophisticated number sense, scientists have found, are specialized cells located in the amphibian midbrain that tally up sound signals and the intervals between them. “The neurons are counting the number of appropriate pulses, and they’re highly selective,” said Gary Rose, a biologist at the University of Utah. If the timing between pulses is off by just a fraction of a second, the neurons don’t fire and the counting process breaks down.