Blue whales love to sing. Males sing to attract mates and to communicate across long distances. People are capable of hearing only certain parts of a whale song, so scientists use special audio equipment to record and study the songs in detail. Over several decades, they’ve noticed that the songs have gotten lower in pitch. Pitch describes how high or low a sound is.
Emmanuelle Leroy and a team of scientists now think they know why. They’ve studied more than a million songs from five groups of whales. The songs were recorded in the southern Indian Ocean over a period of seven years. Last fall, the team’s report was published in the Journal of Geophysical Research: Oceans. It sheds light on why the pitch of whale songs has gotten lower: Whales aren’t singing as loudly as they used to.
“If the whale sings louder, that will slightly increase the pitch of its call,” Leroy told TIME for Kids. “When the whale sings less loudly, the pitch will slightly decrease.”
So why are whales singing more softly? Leroy’s study presents two hypotheses. The first is that there are more whales. Before the 1970s, commercial whale hunting brought whale populations near extinction. But starting in 1985, the International Whaling Commission banned commercial whaling. As the whale population has increased, whales have generally begun to live closer together. And if the whales are closer together, they don’t have to sing as loudly to communicate.
The study’s second hypothesis is that the pitch of whale songs has dropped due to climate change. An increase of carbon dioxide in the atmosphere has made ocean water more acidic, changing its acoustics. Now the songs can be heard from farther away, so the whales don’t have to sing as loudly.
“In whale communication, this is good news,” Leroy says. “But globally, for the environment, it’s not good at all.” Water that’s more acidic could be damaging to some marine life.
Leroy says that confirming either hypothesis would require more research. “We would need to understand how the blue whales emit the song,” she says. But that would mean studying individual whales. That’s hard to do. It’s also very expensive.
Scientists continue to study whale songs. Each study adds to our understanding. “Maybe our hypotheses will be proven in 10 years,” Leroy says. “Science is often a journey.”