During the mating season, male humpbacks emit vocalizations that sound to human ears like barks, chirps, and moans. A whale's unique song slowly evolves over a period of years, never returning to the same sequence of notes even after decades.

Joshua Smith, a doctoral student at the University of Queensland, Australia, investigated songs of humpback whales during three seasons. "Singers are joining females with calves more often and singing for a much longer duration with them than with any other group," Smith said.

Some say that whales, like insects and birds, use their songs to either ensure a territory to warn off other males or to attract females.

But Smith thinks it's more likely that the songs are directed to females showing them the males' fitness, based on their song qualities and allowing them to compare the males and to choose the one they consider the fittest. "The way they structure their songs, perhaps using elements like higher or lower frequencies and how they do that, could reflect attributes of that male, such as his fitness or his age," Smith said.

Smith wants to decode the roles played by the songs in their social interactions.

There is still little evidence that females join males that sing even if males sing most often in the presence of a female. Some males sang for up to 23 hours, others sang for just ten minutes. It was noticed that rather than repel rival males, singing appears to attract other males. There may be a potential strategy for male humpbacks to locate a female, since singing males often have a female present. "Song acts as a broadcast signal, and males in the environment around can hear the song as well as the females," Smith said. "But if another male interacts with a singer, then that singer will typically stop."

Smith says that it seems that the songs are truly mating songs. "Less clear, he says, are the types of information relayed in their songs and how female humpbacks respond when interacting with a singing male."

Michael Noad, Smith's academic supervisor, a whale expert at the University of Queensland's School of Veterinary Science, said "There's a lot of concern about the potential harm to marine mammals from underwater sounds".

"We have very little idea of what sounds they do or do not like; [whether] sounds harm them; and if they do, at what level."

Naval exercises using sonar equipment raise concern in Australia that might hurt the whales as they migrate along the coastline. But further investigations are needed to clear the issue. "We need to learn more about how whales interact with the acoustic environment. Acoustics is their primary sense."

"Once that has been worked out, scientists will then be able to look at whether defense exercises, underwater seismic tests, or even commercial shipping activities are disturbing the humpbacks," Noad says.