Behavior might save energy during long migration
Two years ago, a team of marine biologists was fast asleep on a research vessel off the coast of Brazil when the captain woke them with a harried announcement: There was a “huge animal” following the ship. The bleary-eyed scientists made their way to the stern, where they found a trio of humpback whales riding the ship’s wake—behavior never previously documented in such large whales. For more than an hour, the researchers watched as the whales swam less than 10 meters behind their ship.
“I’ve worked with marine mammals for more than 10 years, but I’ve never seen a whale follow a boat for that long and that close,” says Israel Maciel, a marine mammal researcher at the State University of Rio de Janeiro and co-author of a recent preprint describing the encounter.
Scientists have long known smaller marine mammals such as dolphins will ride the waves that emanate sideways from vessels, known as divergent waves. The low-pressure troughs created by these waves enable the agile animals to swim with very little effort. The humpbacks off Brazil, however, were riding waves emanating from the stern, known as transverse waves. These waves also create low-pressure troughs that can pull an animal along—much like a bicycle rider can draft behind a vehicle or another cyclist.
The late-night encounter, which was videotaped, came as the Brazilian scientists were studying how marine mammals react to the noise generated by the sound blasts that energy companies use to map seafloor geology. The trio of humpbacks—a mother, a calf, and a second adult—were only just visible in the ship’s powerful lights. Still, the researchers believe the episode marked the first time scientists have been able to record a large whale “wake riding” behind a ship, they report in a preprint uploaded to ResearchGate. And it’s a rare instance of observing whale behavior at night.
The whales, which can grow to 17 meters long, were likely using the ship’s wake to save energy as they began their annual 4500-kilometer migration from Brazil to their winter waters off of Antarctica, the researchers say. (Their ship was traveling in the same direction.) Whale calves in particular stand to benefit from wake riding, notes biomechanist Frank Fish of West Chester University, who wasn’t involved with the study. Calves need to surface more often to breathe than adults, and thus experience more drag. By hitchhiking on the transverse wave, a tired mother and calf might have to barely move a muscle, Fish says. “From the standpoint of energetics, this makes perfect sense. Animals will do what they have to in order to conserve energy.”
Humpback whales are a well-studied species, so it’s surprising this behavior hasn’t been documented before, says John Long, a researcher at Vassar College who studies the biomechanics of swimming and serves as a program director at the National Science Foundation. He thinks several factors likely lined up perfectly to produce the wake riding. For example, the ship was moving relatively slowly, at 9 kilometers to 11 kilometers per hour, which is at the higher end of a humpback’s potential speed. And winds were relatively gentle, which likely allowed an attractive wake to form.
The Brazilian researchers hope to find out whether wake riding is widespread among humpbacks. The answer could help reveal how the animals perceive and interact with ships, which can pose a substantial collision threat. Riding directly behind a ship might help the whales avoid strikes with other vessels, but it could also cause the animals to venture into high-traffic sea lanes more often.
But spotting more hitchhiking whales—especially at night—could be difficult, notes whale researcher Steven Katona, a managing director of the nonprofit Conservation International. “There’s really nobody looking for whales at night, it’s a pretty thankless task,” he says. “If the Titanic couldn’t see an iceberg, who’s going to see a whale?”
Still, the observation is a reminder of how much there is yet to learn about the ocean’s leviathans, Long says. “There’s the potential that they’re trying to exploit all sorts of things about their environment to help with these long-term migrations,” he says. “Which is absolutely amazing.”