[ Earth’s largest animals are singing in ever-lower tones, and
nobody knows why.]
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THE MYSTERY OF THE BLUE WHALE SONGS  
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Kristen French
November 23, 2022
Nautilus
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_ Earth’s largest animals are singing in ever-lower tones, and
nobody knows why. _

, Nina Vetrova / Shutterstock

 

n 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.

_To listen to a Blue Whale song, click here_
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“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.

BASSO PROFUNDO: For the past 20 years, whale researchers have
struggled to understand why the songs of blue whales keep steadily 
sinking to lower registers. _Photo by Chase Dekker / Shutterstock._

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.

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.”

Kristen French is a contributing editor at _Nautilus_.

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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