Whales Might Move in ‘Acoustic Herds’
5 min read
This article was originally published by Hakai Magazine.
From January to May each year, Qeqertarsuaq Tunua, a large bay on Greenland’s west coast, teems with plankton. Baleen whales come to feast on the bounty, and in 2010, two bowhead whales entered the bay to gorge. As the pair came within 100 kilometers of each other, they were visually out of range, but could likely still hear each other. That’s when something extraordinary happened: They began to synchronize their dives. Researchers had never scientifically documented this behavior before, and the observation offers potential proof for a 53-year-old theory.
Baleen whales are often thought of as solitary: islands unto themselves. However, some scientists believe they travel in diffuse herds, communicating over hundreds of kilometers. The legendary biologist Roger Payne and the oceanographer Douglas Webb first floated the concept of acoustic herd theory (or should it be heard theory?) in 1971.
Payne, who had helped discover and record humpback whale song a few years prior, was struck by the fact that many toothed cetaceans, such as killer whales and dolphins, are highly social and move together in tight-knit family groups. These bands provide safety from predators and allow the animals to raise their young communally. Payne speculated that the larger baleen whales might travel in groups, too, but on a broader geographic scale. And perhaps the behemoths signaled acoustically to keep in touch across vast distances.
Webb and Payne’s original paper on acoustic herd theory demonstrated that fin whale vocalizations—low-frequency sounds that carry long distances—could theoretically travel an astonishing 700 kilometers in certain areas of the ocean. However, it’s been easier to show that a whale is making a call than to prove that the recipient is a fellow cetacean hundreds of kilometers away, says Susan Parks, a behavioral ecologist at Syracuse University, in New York, who studies animal acoustics.
For more than 50 years, researchers have shared compelling anecdotes about baleen whales seemingly coordinating behavior over long distances, but the stories have remained just that—stories. It took a multidisciplinary research team studying bowhead diving behavior to stumble onto evidence for acoustic herd theory.
At first blush, bowhead-whale diving behavior looks “pretty chaotic and unpredictable,” says Evgeny Podolskiy, an environmental scientist at Hokkaido University, in Japan, and the lead author of the new research. The cetaceans will engage in hours-long bouts of diving, only to stop for no apparent reason, he says.
Podolskiy and his team wanted to extract some order from all that apparent randomness. Using satellite tags, they pulled together diving-depth and location data from 12 bowhead whales in the Arctic over 144 days. Then they applied complex algorithms based on chaos theory—a branch of mathematics that untangles underlying mechanisms behind seemingly erratic systems. As they crunched the numbers, patterns emerged.
First, the researchers noticed that the whales’ dives tend to follow a 24-hour cycle, starting shallower in the mornings and getting deeper in the afternoon. This lines up with a phenomenon known as diel vertical migration (DVM)—the daily commute by plankton and other small creatures from the ocean’s surface down into the abyss and back. They also found that the whales execute their deepest dives in spring, which is when DVM tends to be most extreme in Arctic regions.
It isn’t surprising to see large ocean creatures following their prey, Podolskiy says, but what did surprise him was the second pattern. Two of the whales in Qeqertarsuaq Tunua—whose relationship is unknown—synchronized bouts of diving for as many as seven days on end whenever they were within 100 kilometers of each other. According to the team’s calculations, that would be about the maximum acoustic range for whales in the area. “This is very, very peculiar underwater behavior,” Podolskiy says. “It was very exciting.”
Podolskiy adds that it’s possible that the synchrony was a mere coincidence—that ocean conditions were conducive to the whales diving simultaneously in different locations. But, given the persistent behavior for days on end, that explanation seems statistically unlikely. “Our current belief is that it’s related to communication somehow,” he says.
The idea that whales could become long-range diving buddies makes perfect sense to Christopher Clark, a bioacoustics researcher at Cornell University, in New York, who rubbed shoulders with Payne earlier in his career. Clark is among the scientists who have numerous anecdotes about whales synchronizing their behavior from great distances. It’s intriguing that Podolskiy could demonstrate the whales diving in synchrony, he says, but the new research may still fall short of confirming that the two whales were acoustically communicating.
Proving that whale calls are meant for other whales is a difficult feat, because low-frequency sound waves travel very slowly through cold water, Clark says. Sound waves could take more than an hour to reach another whale, so it would be hard for scientists to observe the creatures exchanging calls. “It’s operating over a scale that is unobservable to humans,” he says. Not only do researchers need the proper equipment to even notice the exchange; they need sophisticated analysis to connect the dots between one whale’s signal and another’s apparent reaction.
For Parks, what makes Podolskiy’s research so intriguing is the marrying of mathematics with animal behavior. In the future, she would love to see the team run a similar study using recorded sound data in addition to the satellite tags. That, Parks says, would help demonstrate whether an individual whale’s signal is reaching its peers through the Arctic’s haze of acoustic fog. “That would be an amazing next step,” she says.
For now, the two bowheads’ documented behavior is the best clue yet that Payne and Webb were onto something. What the two whales were saying to each other, however, remains a mystery. Messages transmitted over such distance would likely need to be simple: They might have been letting each other in on the local plankton forecast or coordinating to avoid human activity. Maybe they were playing a whale’s version of Marco Polo. Or perhaps, like us, they simply like knowing they’re not alone.
