On a crisp spring morning in 2008, Shane Gero overheard a pair of whales having a chat. Gero, a Canadian biologist, had been tracking sperm whales off the Caribbean island nation of Dominica when two males, babies from the same family, popped up not far from his boat. The animals, nicknamed Drop and Doublebend, nuzzled their enormous boxy heads and began to talk.

Sperm whales “speak” in clicks, which they make in rhythmic series called codas. For three years Gero had been using underwater recorders to capture codas from hundreds of whales. But he’d never heard anything quite like this. The whales clicked back and forth for 40 minutes, sometimes while motionless, sometimes twirling their silver bodies together like strands of