"This is the first study that has been able to study the calls by directly observing the animal while it is calling and gathering key information such as depth and body orientation—getting a sense of what the animal is doing underwater," said [Erin] Oleson [one of the lead researchers]. "Once you understand the context of specific types of sounds, then you can use those sounds to infer something about what they are doing when you are not there to actually see them doing it."
The researchers found that only males produced sounds known as "AB" calls while "D" calls were heard from both sexes, typically during foraging. The researchers note in the paper, published in the January 25 issue of the Marine Ecology Progress Series journal, that the sex bias evident in AB callers suggests that those calls probably play a role in reproduction.
Oleson hopes such call and behavior information will eventually be used for better understanding whale habitats and calculating species abundances.
While the study is certainly not exhaustive, it is a good first step; the blue whale is supposedly the "loudest" animal on Earth (vocalizing at 150+ dB), but the fundamental frequency of its song is on the lowest threshold of our sound perception, requiring extra steps in analysis. The Scripps-sponsored Voices of the Sea site has recordings and spectrograms from eight different cetaceans, including the blue whale. If you turn up your speakers, you can just barely hear its low grumble.
Direct observation of the animal is exceedingly difficult - it spends 90 percent of its life underwater - so by tying song to behaviors (such as mating), researchers can make indirect observations of how blue whales go about their daily, and perhaps, seasonal lives.