Scientists have been sneakily eavesdropping on the songs of blue whales voyaging through the oceans off the coast of Southern California, hoping to catch some insights into their lives.

As part of new research, presented today at the 2018 Ocean Sciences Meeting, scientists have recorded over 4,500 sound samples from blue whales tagged with underwater microphones and pressure sensors around Southern California’s Channel Islands from 2002 to 2016.

Despite measuring 30 meters (100 feet) long and weighing up to 172 tonnes (190 tons), we know surprisingly little about these gentle giants. So, this team of oceanographers turned to the blue whales’ songs, to see whether the patterns within their vocalizations could unveil any insights into their behavior.

“Understanding the context under which blue whales make calls is a critical step in developing non-invasive, non-lethal acoustic methods to study population trends and recovery status of this endangered species,” Ana Širović, an oceanographer at the Scripps Institution of Oceanography in California, said in a statement. “And as a species on top of the food web, understanding their status and contribution to the ecosystem is important for understanding the status of the ecosystem as a whole.”

^{_{Credit: Ana Širović / Scripps Institution of Oceanography}} 

You can listen to some of the blue whale vocalizations from the research in the audio player above. Each of the vocalizations appears to sound dramatically different, at least to human ears, as the songs comprise complex patterns of wails, groans, and hit-pitched shrieks. 

One of their central discoveries was that vocalizations varied depending on the whale’s sex. Males seem to be a lot noisier than females, suggesting the calls might play a role in mating. The singing was most common at dusk or throughout the nighttime and then lowest during the daylight hours when the whales feed. It isn't clear why exactly, however, it's speculated that they don't want to be loud when prey is around or they are conserving their energy to optimize their hunting. 

As well as singing, the males also use a variety of single sounds, which might be to do with pair-bonding when they are foraging with females. Other mammals, such as bonobo chimpanzees, sometimes call to nearby females when food is present, hoping to up their chances of mating.

Singing was more common in the near-surface dives, with 69 percent of shallow dives containing calls compared to 23 percent of the deeper dives. The calls also seemed to be higher and more regular during the fall months of September and October.

Still, there's much about the blue whale's songs that we don't know – a testament to the complexity and flexibility of the vocalizations. Just last year, researchers noticed that the frequency of whale calls are lower today than they were just 20 years ago. They still have no idea why.