Baleen whales communicate with songs and can hear the lowest frequency sounds of any mammal - some of which are so low (less than 20 hertz) that even humans can't hear them.

But until now, researchers didn't know much about when these hearing capabilities evolved.

An analysis of ancient whale fossils has revealed that early baleen whales also heard at low frequencies - and this evolved much before whales evolved huge bodies and a filter feeding system, reports DailyMail.

The modern blue whale is a type of baleen whale which measures 30 meters in length.

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They have a baleen - a filter feeding system inside the mouths of baleen whales.

Early baleen whales were different to the baleen whales of today - they were a lot smaller and had teeth, and looked more like today's dolphins or toothed whales.

The researchers, based at Monash University in Melbourne, Australia, found that the ability of baleen whales to hear at very low frequencies emerged before toothed whales evolved.

To find out the frequency at which both fossils and living whales can hear, the researchers analyzed of the cochlea - the inner ear.

But this can be complicated to do in old fossils because of their fragility.

They used medical research methods such as CT (computerized tomography) scanning to digitally reconstruct the anatomy of the whale fossil - dating back from between 34 to 23 million years ago.

They found that the ancient whales cochlea was very similar to those of modern baleen whales - despite being a lot smaller and more around the size of a modern dolphin.

"This at last reveals that the earliest baleen whales heard low frequency sound," said Travis Park, a recent PhD thesis-submitter and lead author of the study.

"The similarities of baleen whale cochleae to older whales shows us that this ability evolved well before other extreme adaptations such as baleen, filter feeding and gigantic body size.

"Although there's no way of testing the idea, it's tempting to think that mysticetes [baleen whales] have been singing their complex, beautiful whale songs for millions of years."

The researchers also found that the ancient whales couldn't echolate - a system that uses sound waves to determine where objects are in space.

Some animals such as dolphins use echolocation which requires the ability to hear high sound frequencies of more than 20,000 hertz.

This discovery means that the echolocation and high frequency hearing ability possessed by dolphins and toothed whales evolved after the evolution of low frequency hearing - which the researchers say is contrary to the findings of other studied that lacked information on the inner ear of these ancient whales.

But why low frequency hearing evolved then is still a mystery, the researchers say.

WHAT ARE BALEEN WHALES?

Baleen whales are a type of whale characterized by their baleen plates and paired blowholes.

They have long plates of 'baleen' which hang in a row, like the teeth of a comb, from their upper jaws.

Baleen plates act as a filter feeding system for baleen whales.

They are strong and flexible and made of a protein similar to human fingernails.

Baleen whales strain huge volumes of ocean water through their baleen plates to capture food including krill, zooplankton and small fish.

Source: NOAA Fisheries