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Scientists discovered that fewer humpback whales made the singing noises, as their population grew. “It was getting more difficult to actually find singers,” marine biologist Rebecca Dunlop in Brisbane said. She added, “When there were fewer of them, there was a lot of singing-now that there are lots of them, no need to be singing so much.”

Eastern Australia’s humpback whales came close to disappearing in the 1960s. With the end of commercial whaling (捕鲸), the population began to regrow, climbing to about 27,000 whales by 2015. That number is near estimated pre-whaling levels. As the density (密度) of whales increased, their singing behaviors changed. While 2 in 10 males made crying noises in 2004, 10 years later the number had dropped to 1 in 10, Dunlop said.

The team’s study appeared in a recent issue of Nature Communications Biology. Dunlop said she thinks singing played a big part in bringing in mates when populations severely declined. When humpbacks live in denser populations, males looking for mates also have to deal with competing whales.

Boris Worm, an ocean biologist, was not involved in the research. “As animal populations recover, they change their behavior-they have different cries,” Worm said. The research suggests the seas are still noisy with humpback whale sounds.

Many humpbacks seek to bring in mates with a combination of singing and physical movements, the study notes. The large increase in the humpback population during the study period provided valuable data about changes in the animals’ behavior and they must have been singers long before whaling reduced their numbers, said Simon Ingram. But the new study demonstrates (证明) how necessary their complex and beautiful songs were to their survival and recovery, he added.