Meanwhile, toothed whales do use echolocation, and they and other species of baleen whalesmake social sounds, suchas cries and whistles, to communicate.

But thosevocalizations lack the complexity ofsongs.

So how do theydo it?

Land mammalslike us generate soundby movingair over ourvocal chords when we exhale, causing them to vibrate.

Baleen whales have a U-shaped fold of tissuebetween their lungs and their large inflatable organs called laryngeal sacs.

Wedon't know this for sure becauseit's essentially impossible to observe the internal organs of a living, singing whale, but we think that when a whale sings, muscular contractions in the throat and chestmove air from the lungs across the U-fold and into the laryngeal sacs, causing the U-fold to vibrate.

The resulting sound resonates in the sacslike achoir singing in a cathedral makingsongs loud enough to propagate up to thousands of kilometersaway.

Whalesdon't have to exhale to sing.

Instead, theair is recycled back into the lungs, creating soundoncemore.

One reason whalesongs are so fascinatingis their pattern.

Units,like moans, cries, and chirps are arranged in phrases.

Repeated phrases areassembled into themes.

Multiple themes repeated in a predictable pattern create a song.

This hierarchicalstructure is a kind of grammar.

Whalesongs are extremely variable induration, and whales can repeat themover andover.

Inone recorded session, a humpback whale sang for 22 hours.

Andwhy do theydo it?

Wedon't yet knowthe exact purpose, but we can speculate.

Given that the singers are males and they mostly sing during the matingseason,songs might be used to attract females.

Or perhaps they're territorial, used to deter other males.

Whales return to the same feeding and breeding grounds annually, and eachdiscrete population has a different song.

Songs evolveover time as units or phrases are added, changed, or dropped.

And when males from different populations are feeding within earshot, phrases are often exchanged,maybe because newsongs make themmore attractive to breeding females.

This is one ofthe fastest examples of culturaltransmission, where learned behaviors are passedbetween unrelated individuals of the same species.

We can eavesdropon thesesongs using underwater microphones called hydrophones.

These help us track species when sightings or genetic samples are rare.

For example, scientists have been able to differentiate the elusiveblue whale's populations worldwide basedon theirsongs.

Butthe oceans are getting noisieras a result ofhuman activity.

Boating, militarysonar, underwater construction, and seismic surveys for oil are occurringmore often whichmay interfere with whale'scommunication.

Some whaleswill avoidkey feeding or breeding grounds ifhuman noise is too loud.

And humpback whales have been observed to reduce their singing inresponse tonoise 200 kilometersaway.

Limitinghuman activity along migratoryroutes and in other critical habitats, and reducingnoise pollution throughoutthe ocean would help ensure whales continued survival.