By Helen Briggs and Victoria Gill for BBC News
A humpback whale. Photo: konart/123RF
Scientists have worked out how some of the largest whales in the ocean produce their haunting and complex songs.
Humpbacks and other baleen whales have evolved a specialised "voice box" that enables them to sing underwater.
The discovery, published in the journal Nature, has also revealed why the noise we make in the ocean is so disruptive for these ocean giants.
Whale song is restricted to a narrow frequency that overlaps with the noise produced by ships.
"Sound is absolutely crucial for their survival, because it's the only way they can find each other to mate in the ocean," said Professor Coen Elemans, of the University of Southern Denmark, who led the study.
"[These are some] of the most enigmatic animals that ever lived on the planet," he told BBC News. "They are amongst the biggest animals, they're smart and they're highly social."
The humpback whale is big, smart and highly social, Professor Coen Elemans says. Photo: Michelle Fournet
Baleen whales are a group of 14 species, including the blue, humpback, right, minke and gray whale. Instead of teeth, the animals have plates of what is called baleen, through which they sieve huge mouthfuls of tiny creatures from the water.
Exactly how they produce complex, often haunting songs has been a mystery until now. Prof Elemans said it was "super-exciting" to have figured it out.
He and his colleagues carried out experiments using larynxes, or "voice boxes", that had been carefully removed from three carcasses of stranded whales - a minke, a humpback and a sei whale. They then blew air through the massive structures to produce the sound.
In humans, our voices come from vibrations when air passes over structures called vocal folds in our throat. Baleen whales, instead, have a large U-shaped structure with a cushion of fat at the top of the larynx.
This vocal anatomy allows the animals to sing by recycling air, and it prevents water from being inhaled.
The researchers produced computer models of the sounds and showed that baleen whale song is restricted to a narrow frequency which overlaps with noise produced by shipping vessels.
"They cannot simply choose to, for example, sing higher to avoid the noise we make in the ocean," Elemans said.
The head of the study says it has been "super-exciting' to solve the mystery. Photo: Aotearoa Festival of the Arts
His study demonstrated how our ocean noise could prevent whales from communicating over long distances. That knowledge could be vital for the conservation of humpbacks, blue whales and other endangered giants of the sea.
It also provides insight into questions that researchers have been asking for decades about these eerie songs, which some sailors used to attribute to ghosts or mythical sea creatures.
Whale communication expert Dr Kate Stafford, from Oregon State University, called the study "groundbreaking".
"The production and reception of sound is the most important sense for marine mammals, so any studies that elucidate how they make sounds has the potential to move the field forward," she said.
The research also paints an evolutionary picture - of how the ancestors of whales returned to the oceans from the land, and the adaptations that made it possible to communicate underwater.
The way so-called toothed whales produce sound is better understood, because the animals are easier to study. These marine mammals, which include dolphins, orcas, sperm whales and porpoises, blow air through a special structure in their nasal passages.
Dr Ellen Garland, from the Sea Mammal Research Unit at the University of St Andrews, said: "I've always wondered exactly how baleen whales - especially humpbacks, which my research is focused on - actually produce the variety of sounds they do.
"Studying large whales is extremely challenging at the best of times, but trying to uncover how they produce sound when you may not even be able to see them underwater while vocalising is an added level of difficulty, so these researchers have been very creative."
Dr Stafford added that the mammals' ability to make such complex vocal signals was "remarkable" and highlighted "how special these animals are".
This story was first published by the BBC.
