Whale song hints at new pygmy subspecies

By Melissa Coade

Thursday June 10, 2021

pygmy-blue-whale
A new subspecies of pygmy blue whales may have been discovered in the Indian Ocean. (Image: Adpbne/Richard Carey)

Scientists believe they have discovered a new subspecies of pygmy blue whales in the Indian Ocean, whose ‘powerful singing’ was picked up using underwater bomb detectors.

Historic whaling in the Southern Hemisphere is estimated to have shrunk the number of blue whales to 0.15% of their population size before the twentieth century. So when underwater bomb detectors recorded the sound of an ‘enormous’ number of song calls, believed to be a new population of the pygmy blue whale, marine scientists became very excited.

Marine ecologist Professor Tracey Rogers said although it was unknown just how many whales were in this newfound population, the number of recorded whale calls in the middle of the Indian Ocean suggested there were a lot. 

“Blue whales in the Southern Hemisphere are difficult to study because they live offshore and don’t jump around – they’re not show-ponies like the humpback whales,” Professor Rogers said.

“Without these audio recordings, we’d have no idea there was this huge population of blue whales out in the middle of the equatorial Indian Ocean.”

A new paper published in the journal Scientific Reports outlines how the distinctive song of blue whales means that their populations can be monitored acoustically and their movement through the oceans tracked. Although the pygmy whales are the smallest in the blue whale family – and notoriously elusive – they are mammoth creatures that can grow to be up to 24 metres long.

A team led by researchers from UNSW Sydney used passive acoustic monitoring to listen for the ‘distinct vocal signature’ of different blue whale populations in the Indian Ocean – a region where the distribution and migration routes of the whales are little understood.

Scientists can identify whale songs by analysing data from advanced underwater microphones (called ‘hydrophones’) ordinarily used to detect the soundwaves created by potential nuclear bombs. The recordings pick up many other detailed ocean sounds and have been available for marine science research since 2002.

This time, a previously unidentified whale song appeared with a ‘strong signal’ in the data – analysis of the song’s structure, frequency and tempo led researchers to conclude it must belong to a new group of pygmy blue whales previously unrecorded in the area. 

“I think it’s pretty cool that the same system that keeps the world safe from nuclear bombs allows us to find new whale populations, which long-term can help us study the health of the marine environment,” Rogers said.

“Without these audio recordings, we’d have no idea there was this huge population of blue whales out in the middle of the equatorial Indian Ocean.”

The latest discovery suggests a possible sixth blue whale population in the Indian Ocean (fifth of the pygmy subspecies) that can only be confirmed with a visual sighting. Scientists have named the population ‘Chagos’, which they believe travel as far north as the Sri Lankan coastline and as far east in the Indian Ocean as the Kimberley coast in northern Western Australia.

“If it isn’t a blue whale, it definitely sings like one,” Lead author and bioacoustician Dr Emmanuelle Leroy said. 

“Thousands of these songs were being produced every year (of the 18 years of data that was analysed). They formed a major part of the ocean’s acoustic soundscape.

“The songs couldn’t have just been coming from a couple of whales – they had to be from an entire population,” she added.

The only discernible difference between the blue pygmy populations are their songs; with one group communicating via the Madagascan type-9 song, another using the Australian type-8 song, another group possibly singing a Sri-Lankan type-7 song, and another very recently discovered group with its own special call too. Together with the Antarctic blue whale population, which also has its own distinctive calls, that makes five confirmed groups of the whale subspecies in the Indian Ocean.

The Chagos population promises to be the sixth blue whale group also travelling through the region.

Professor Rogers explained that where different whale species have a totally different style to their singing – think of the difference between jazz and classical music – the different songs within the blue whale species was comparable to generational slag between humans. 

“Blue whales are more traditional. They sing very structured, simple songs,” Rogers said.

“We still don’t know whether they’re born with their songs or whether they’ve learnt it.”

Scientists estimate that low frequency whale songs can be heard between 200 and 500 kilometers away. 

“It’s fascinating that within the Indian Ocean you have animals intersecting with one another all the time but whales from different regions still retain their distinctive songs. 

“Their songs are like a fingerprint that allows us to track them as they move over thousands of kilometres,” Rogers said.


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