The whales had disappeared, and now they were back. Or were they?

Since 1985, Hal Whitehead had been leading a team to the Galápagos Islands to search for sperm whales, which gather there in the thousands. The researchers tracked the animals with underwater microphones, day and night, for two to four weeks.

Their recordings revealed that the whales belonged to two distinct vocal clans—large groups that each call using their own dialect. The Regular clan makes a train of regularly spaced clicks, while the Plus-One clan leaves a short pause before their last click. The two clans share both genes and oceans—they are distinct only in their vocal culture.

In the 1990s, for some reason, the whales started to vanish. By 2000,