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In 2001, a pair of physicists turned whale researchers noticed something puzzling in their data. John Hildebrand and Mark McDonald were trying to build a system that would allow them to automatically detect blue whale songs off the coast of southern California. But their algorithm kept crashing.
Blue whale songs fall below the range of human hearing. If you want to listen to one, to actually hear its ethereal patterns of wobbly pulses and haunting moans, you have to speed it up by at least two-fold. But according to Hildebrand and McDonald’s instruments, the tonal frequencies of the songs had been sinking to even greater depths for three straight years.
“This is weird,” Hildebrand thought. To figure out if it was just an anomaly or something more, Hildebrand and McDonald embarked on a quest to find some really old songs. Eventually they got their hands on some of the earliest known recordings, created by the Navy in the 1960s and stored on analog cassettes. They were floored. The frequencies had declined by 30 percent over 40 years.
“You could really see, ‘oh my God, this thing has shifted a lot,’ ” says Hildebrand, who heads the whale acoustics lab at Scripps Institution of Oceanography in San Diego. Then he and McDonald, who runs a private ocean acoustics consultancy, listened to other populations of blue whales in the Antarctic and the Central Pacific, each of which sings a different song. The trend held.
Together, they had stumbled onto what would become one of the biggest unsolved riddles of blue whale research for decades to come. Blue whales are not only the world’s largest animals, over 75 feet long and weighing around 300,000 pounds; they are the world’s loudest, whose 180-decibel songs—as loud as a jet plane—can be heard 500 miles away by properly-attuned ears. (If it seems strange that their songs are so loud yet imperceptible to us, consider that our ears barely register 100-decibel dog whistles.) But now their voices have inexplicably shifted from bass to basso profundo, Elvis to Barry White. And that shift is consistent around the world—even though the local anthems are not.
“It’s just kind of tormenting all the whale scientists that we can’t figure it out,” says Ally Rice, a researcher in the whale acoustics lab at Scripps.
In late 2009, Hildebrand, McDonald, and Sarah Mesnick, an ocean ecologist at Scripps, formally described the falling song frequencies in an Endangered Species Research journal article.1 In it, they floated numerous hypotheses to explain the phenomenon: climate change related fluctuations in ocean acidity, whaling-related shifts in average whale size and population density, rising ocean noise. None, however, satisfied them. For one thing, the rates of change in song frequency—basically a straight linear progression across 40 years—didn’t mathematically match up with rates of change for ocean acidity, whale size, or population. Also, in an ocean more polluted with noise, lower frequency calls are harder, not easier, to hear. And while the deeper frequencies were found in blue whale populations worldwide, the Indian Ocean had actually gotten quieter.
They had stumbled onto one of the biggest unsolved riddles of blue whale research.
Over the past decade or so, the mystery has only deepened as other whale researchers have attempted to resolve the riddle without success. Ocean acoustics physicist Alexander Gavrilov of Curtin University in Australia noticed that blue whale song frequencies vary with the seasons.2 His lab also discovered that the “spot” calls of what they believe are southern right whales off the coast of Australia declined for many years—even more rapidly than the calls of blue whales—and then suddenly popped back up by a few hertz.3 Other scientists have found that it’s not just the songs of blue whales and possibly right whales that are declining, but fin whales4 and bowhead whales,5 as well.
More recently, Hildebrand and colleagues at Scripps, including Rice, noticed that, at least in California, blue whales aren’t changing their tune quite as much each year—the decline in frequencies has leveled off. They presented these findings in PLoS ONE in April 2022.6 But they are divided about what the plateau means. Hildebrand argues that it may support what’s known as the population recovery hypothesis: As post-whaling population recovery levels off, the declines in song frequency are expected to plateau, too.
The population recovery theory is complex. Following a 1968 moratorium on commercial whaling, a rebound in populations should mean that more blue whales can now be found in any given territory; as they would need to communicate across shorter distances than before, they could afford to sing at lower and lower pitches. Because the songs in question are thought to be sung only by males and used to attract mates and discourage rivals, and a deep voice suggests a big body, males might choose to sing at lower pitches in order to advertise their fitness. But as populations plateau and the distance between whales stops shrinking, there would come a point at which it no longer makes sense to sing any lower. Too-low songs just wouldn’t carry very far.
Or so the explanation goes—but Rice and others are doubtful. “None of us really believes that hypothesis,” says McDonald. Many of the supporting details are still speculation rather than certainty. It’s possible that the songs are used not just for mating but also to monitor the environment, like sonar. And we don’t know whether the whales can even perceive the small frequency changes recorded every year.
“It’s just kind of tormenting all the whale scientists that we can’t figure it out.”
Perhaps more importantly, the math doesn’t neatly line up: Rates of frequency decline have not correlated with trends in population growth across geographies. The song changes were linear and constant in all groups of blue whales around the globe through 2009, yet population recovery rates have varied significantly from one region to the next. Arctic blue whale populations are still growing quickly, for example, while blue whale populations in the northeast Pacific are not. “I would say on my list of questions about blue whales that are unanswered, this is number one because I have not seen a good explanation,” says Trevor Branch, who studies fisheries and whales at the University of Washington and whose approach involves applying novel statistical modeling to historical data sets.
Still, Hildebrand hopes other researchers will look for similar plateaus in blue whale song outside of California, to see whether they follow the stabilization in global populations since 2009. If the theory proves correct, song frequency could be used as a metric for population recovery in blue whales. Current techniques for assessing their numbers are difficult and imprecise, relying on extrapolations from visual surveys that inevitably miss many whales.
It “would be sort of stunning,” says McDonald, to have such a simple formula for estimating whale populations. But he’s not sure the question of what is causing blue whale songs to change will ever be answered. “It’s just not like quantum mechanics and particle physics, you know. Biological systems are just too complicated,” he says. “Physicists like to have it all make sense. Biological systems, they can be crazy.”
Rice, on the other hand, is hopeful. Maybe in another 10 years, she says, someone will do another survey, an update to the update, and find something new that solves the riddle. In the meantime, she respects that the blue whales still hold unfathomable secrets. It’s what drew her to study them in the first place. “I like that the whales get to keep some of their mysteries, right?” she says. “We don’t just get to know everything about them.” 
Lead image: Nina Vetrova / Shutterstock
References
1. McDonald, M.A., Hildebrand, J.A., & Mesnick, S. Worldwide decline in tonal frequencies of blue whale songs. Endangered Species Research 9, 13-21 (2009).
2. Gavrilov, A.N., McCauley, R.D., & Gedamke, J. Steady inter and intra-annual decrease in the vocalization frequency of Antarctic blue whales. The Journal of the Acoustical Society of America 131, 4476-4480 (2012).
3. Ward, R., Gavrilov, A.N., & McCauley, R.D. “Spot” call: A common sound from an unidentified great whale in Australian temperate waters. The Journal of the Acoustical Society of America 142, EL231 (2017).
4. Weirathmueller, M.J., et al. Spatial and temporal trends in fin whale vocalizations recorded in the NE Pacific Ocean between 2003-2013. PLoS One 12, e0186127 (2017).
5. Thode, A.M., Blackwell, S.B., Conrad, A.S., Kim, K.H., & Macrander, M. Decadal-scale frequency shift of migrating bowhead whale calls in the shallow Beaufort Sea. The Journal of the Acoustical Society of America 142, 1482 (2017).
6. Rice, A., et al. Update on frequency decline of Northeast Pacific blue whale (Balaenoptera musculus) calls. PLoS One 17, e0266469 (2022).
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