Blue Whales: Songs of the Deep

As a young child, I learnt that the blue whale (Balaenoptera musculus) is the largest animal in the world and the largest ever to have lived on our planet. Its heart is the size of a small car, and its arteries are big enough for one of us to swim through. Somehow, I presumed this giant, which can grow up to 30 m (98 feet), lived in a far-off ocean. Two decades later, when I got into whale research, I realised that the “pygmy” variant of the blue whale (21-24 m) lives just a few dozen kilometres from where I grew up on the southwest coast of India. And surprisingly little was known about these whales at our doorstep.

Blue whales are found throughout the world’s oceans except in the Arctic. They live up to 80-90 years, giving birth every two to three years to a calf that is 6-7 metres. They usually stay far from the coast, although they can occasionally come close to land. They feed in shallow waters or at depths of up to 300 m, surfacing every 5-15 minutes. Usually only 1-3 blue whales are seen together, unless large amounts of prey bring tens of whales to the same area. 

When out at sea, you may see only part of the animal — a fin, the back, perhaps the tail. It’s hard to grasp the sheer scale of these aquatic giants. The first time a blue whale surfaced right beside me, it was the sound of its breath that gave away its size. It was a force to be reckoned with — mighty and powerful — producing a 9-10 m-high towering blow (vertical column of air and water vapour expelled with force) that made me hold my breath in awe.

Diving deeper into their lives, I learnt about their songs. Yes, blue whales sing. Simple, powerful songs, unlike the complex, constantly evolving, and more famous songs of the humpback whale. Blue whales’ songs are simple (consisting of a few units or phrases they sing repetitively) and do not change from year to year. These songs are among the loudest on the planet and can be heard across great stretches of ocean. With the right equipment and ocean conditions, you can hear a blue whale 1,000 km away. I wonder how blue whales communicate with one another and how that communication shapes their social structure over long distances. Blue whales should be true champions at long-distance relationships. As with humpback whales, only the males sing, which we presume is to impress females or outcompete other males.

Blue whales travel vast oceanic distances, making them difficult to follow and study from boats. To me, it was immediately clear that tracking their songs was the best way to study these crooners. In 2018, our team installed underwater recorders in the hope of detecting whale chatter in the waters of Lakshadweep. We placed these instruments in shallow waters (approximately 25 m), since that was all the resources we could cobble together. Ideally, deep-water hydrophones are best for listening to blue whales. Since no one had ever tried to listen for blue whales in Indian waters before, we had no idea what we would find. That said, it was only a matter of time before our specialised devices picked up the deep, low-frequency pitch songs of the blue whale. Their powerful songs reverberated across Lakshadweep’s waters, unheard by people, as they were low-frequency and at the very lowest end of human hearing.

I immediately recognised the three-part song as belonging to the north-central Indian Ocean population, first recorded off Sri Lanka in the 1980s. This is my favourite of the 13 or so songs believed to be sung by the blue whale. I’ll admit I am biased. Having listened to these songs over and over again, I always wonder what the three parts meant: the low, deep unit, followed by two slightly higher notes, then one even higher note. What selective forces helped the whales settle on this composition for now? As we listened more closely, we noticed that in Lakshadweep they sometimes skipped the last, higher tone, which automatic detectors widely use to identify this population. True to the elusive nature of blue whales, they sometimes pass through areas undetected, even with the best recording setup.

Our very own Indian Ocean is a hotbed of blue whale acoustic diversity. Pygmy blue whales in the Indian Ocean sing multiple distinct song types, representing several “acoustic” populations (i.e., groups with their own distinct songs, which is an unusually rich variety of regional songs). This contrasts with the Atlantic and Pacific Oceans, where blue whale songs tend to be less acoustically diverse. The drivers of this song diversity in the Indian Ocean remain unknown. We also don’t know exactly how often these song-based groups share the same waters, and this is an active area of research.

Of the four globally recognised blue whale subspecies, two occur in the Indian Ocean: the Antarctic blue whale (B.m. intermedia), the largest, and the pygmy blue whale (B.m. brevicauda), the smallest. A third, the northern Indian Ocean blue whale (Balaenoptera musculus indica), is recognised by some experts as a separate subspecies. However, it remains unclear whether B. m. indica and the pygmy blue whale (B. m. brevicauda) are truly distinct or the same subspecies.

Strandings are when whales wash ashore either dead or alive. Occasionally, a dead blue whale, along with a repository of information, washes ashore. In fact, India has a high number of reported blue whale strandings, most of which occur along the west coast. Studying as many of these carcasses as possible can provide crucial information, including age, potential causes of death, diet, genetic relatedness, population connectivity, and other physiological and life-history traits. This helps fill some gaps in our data-deficient waters and supports the conservation of these species. The catch is that Schedule 1 of the Wildlife Protection Act (1972) requires scientists to obtain special permission before collecting tissue samples from whale and dolphin carcasses, a process that can take considerable time, and decomposing whales will not wait. 

In the early days (18th-19th centuries) of commercial whaling, the sheer size and speed of blue whales largely protected them from hunters. Steam-powered ships and explosive harpoons changed that. By the time industrial whaling was finally curtailed in the late 20th century, modern whaling technology had wiped out an estimated 98–99 per cent of the world’s blue whales. Today, scientists estimate that only around 10,000 to 25,000 blue whales remain globally. Since then, following the introduction of protections, some populations have begun to recover in many regions, although slowly. For others, including pygmy blue whales in our waters, we still don’t have a clear picture of how they are faring.

The northern Indian Ocean blue whale (B.m. indica), or parts of its population, is resident in our waters, meaning they live here year-round. This was established through meticulous record-keeping of illegal Soviet whaling ships that operated in the Arabian Sea in the 1960s and 70s. Usually, blue whales migrate to cold, nutrient-rich polar waters during the summer to feed on dense, abundant patches of krill. However, blues in the northern Indian Ocean cannot migrate to the Arctic because the Asian landmass blocks access to the north, and they do not travel to Antarctic waters in the south (over 6,000 km away). It looks like our unique monsoon winds churn the waters, moving nutrient-poor surface water away and allowing nutrient-rich deep water to rise to the surface. This triggers large phytoplankton (microscopic plants) blooms, making the ocean more productive and keeping the whales here year-round.

Studies around the world have shown that blue whales are very good at lunge feeding, a technique in which they take in water exceeding their body weight and filter it to swallow tiny krill and shrimp. To be highly efficient and maintain their size, they track and target very dense pockets of krill, so if krill clump together in large groups, you should find blue whales there! Interestingly, new research shows that these whales fertilise the waters, creating tiny blooms of phytoplankton that also support these tiny animals. Blue whales feed at depth and fertilise surface waters with their nutrient-rich poop, which is exactly what phytoplankton require to thrive. The scale and importance of these processes to marine food webs are still being assessed. How much we benefit from these “ocean farmers” and what impact commercial whaling had on these ecosystem services remain to be fully determined. Even after they die, many whales sink to the seabed and decompose slowly, supporting a deep-sea ecosystem, sometimes for over a century.

Imagine an elusive giant, bigger than any other giant ever to have lived on this planet, able to speak across ocean basins, helping phytoplankton grow, and eating some of the tiniest animals. It’s remarkable that these gentle souls share the same waters with us.

After our first recordings in Kavaratti (Lakshadweep) nearly 7 years ago, we have extended our “ears” to several other islands in the archipelago. In April 2025, we set up a long-term monitoring programme to determine whether additional “acoustic” populations visit the islands and how long they stay. Which areas do they visit? Critically, what brings them here? Only once we know where the whales are can we protect them. Ship strikes, entanglement in fishing gear, warming oceans and underwater noise all impact whales globally. As we learn more about these whales, we can assess the scale of the impact and put conservation measures in place.