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, the whales had completely gone, and Whitehead ceased his annual expeditions.

Then, in 2011, a colleague in the Galápagos told the team that the sperm whales had apparently returned. Whitehead’s team, including Mauricio Cantor, Shane Gero, and Luke Rendell, went back in 2013 to listen for themselves.

They did, indeed, find sperm whales, sighting more than 4,400 individuals across two years. But none of these were from either the Regular or Plus-One clan, which were around in the 1980s. Instead, they belonged to two different groups that were heard elsewhere in the Pacific but were previously rare or absent around the Galápagos: the Short clan, which makes a brief train of clicks, and the Four-Plus clan, whose calls have a base of four regular clicks.

There have been cases in which groups of humpback whales have changed their tunes, discarding their old songs in favor of those from neighboring groups. But humpback songs continuously evolve, whereas the calls of sperm whale clans are stable for at least a decade. It wasn’t that the Regular and Plus-One whales had changed their tune; the team couldn’t match photos of any of the new whales to those from the 1980s. Instead, those vanished groups had been replaced by the Short and Four-Plus clans.

It seems that over the last 30 years, the Galápagos sperm whales have gone through a cultural turnover, when individuals that speak one dialect totally replace those that speak another. Until now, such replacements were unknown outside of humans.

The Regular and Plus-One whales haven’t vanished entirely. They can still be heard off northern Chile and the Gulf of California. So why did they leave the Galápagos? “I think it’s really hard to say,” says Gero. “Some kind of significant environmental changes might have led the animals to find greener pastures.”

In the 1980s and 1990s, El Niño events—cycles of warm and cold ocean water—caused massive upheavals in the Galápagos. These oscillations might have slashed the numbers of squid that the sperm whales eat. Rather than stay and adapt, the whales might have fled.

Here’s another possible explanation: Between 1957 and 1981, whalers and pirates devastated the sperm whales of South America. Their actions would have triggered a boom in whale prey, and perhaps the Regular and Plus-One clans left to take advantage of this temporary glut.

Either way, it’s striking that the clans moved together and no whales from either group seemed to stay behind. “Being with animals that behave like you is important,” says Gero. “The clans seem to have a significant impact on the lives of these whales. They take these cultural groups seriously—they define their social relationships and movements, which are critically important to them.”