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Communicating underwateris challenging.
Light and odorsdon't travel well, soit's hard for animals to see or smell.
But soundmoves about four times faster in water than inair, so in this dark environment, marine mammals often relyonvocalization to communicate.
That'swhy achorus of sounds fillsthe ocean.
Clicks, pulses, whistles, groans, boings, cries, and trills, toname a few.
Butthe most famous parts of this underwater symphony are the evocative melodies, orsongs, composedbythe world's largest mammals, whales.
Whalesongs areone ofthe most sophisticatedcommunication systems in theanimal kingdom.
Only a few species are known to sing.
Blue,fin, bowhead minke whales, and of course humpback whales.
These areall baleen whales which use hairy baleen plates instead of teeth to trap their prey.
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 movingairover 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 chestmoveair 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, theairis recycled back into the lungs, creating soundoncemore.
Onereason 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 hierarchicalstructureis 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.
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 populationhas a different song.
Songs evolveover timeas units or phrases are added, changed, or dropped.
And when males from different populations are feeding within earshot, phrases are often exchanged,maybe because newsongsmake themmore attractive to breeding females.
Thisisone 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 ifhumannoiseis 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.
If the whales can keep singing and we can keep listening,maybeone day we'll truly understand what they're saying.
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