Over two years, Ferrara and her colleagues recorded 220 hours of turtle sounds, using microphones on land and a hydrophone dipped into the water off the side of an aluminum boat. Some turtle cries were lower frequency, for long-range communication; others were higher pitched, which travel easier through shallow water.
The river turtles - who grow up to three feet long - used some sounds, the biologists discovered, only during specific behaviors. The most diverse turtle noises occur during nesting: When nestlings make noise while still in their eggs, the females respond, possibly in order to lead the babies to water after they hatch. (This parental turtle talk, it turns out, sounds sort of like a squeegee dragged across a car window.)
The turtles and hatchlings stay together for over two months traveling in groups of up to thousands. "The most exciting aspect about the fact that turtles are communicating underwater," the scientists write, "is that they appear to possess a much more complex social behavior than previously documented."