For the first time, scientists spot humpback whales riding behind a ship science

Two years ago, a team of marine biologists were fast asleep on a research vessel off the coast of Brazil when the captain woke them up with a hasty announcement: There was a “huge animal” following the ship. The bleary-eyed scientists made their way to the edge, where they found a trio of humpback whales riding behind the ship—behavior never before documented in such large whales. For more than an hour, the researchers watched the whales swim 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 so long and so closely,” says Israel Maciel, a marine mammal researcher at Rio de Janeiro State University and co-author of a recent preprint describing the meeting.

Scientists have long known that smaller marine mammals like dolphins will ride the waves that come off the side of ships, known as diverging waves. The low-pressure troughs created by these waves enable agile animals to swim with very little effort. However, the humps off Brazil were riding waves coming off the edge, known as transverse waves. These waves also create troughs of low pressure that can pull an animal forward – much like a bicycle can move behind a vehicle or another cyclist.

The late-night encounter, which was captured on video, came as Brazilian scientists were studying how marine mammals respond to noise created by sonic booms that energy companies use to map the sea’s geology. The trio of humpbacks – a mother, a calf and a second adult – were visible only in the ship’s powerful lights. However, researchers believe the episode marked the first time scientists have been able to record a large whale “riding its wake” behind a ship, they report in a preprint uploaded to ResearchGate. And it is a rare case of observing the behavior of whales at night.

The whales, which can grow up to 17 meters, were likely using the ship’s wave to conserve energy as they began their 4,500-kilometer annual migration from Brazil to their wintering waters near Antarctica, researchers say. (Their ship was traveling in the same direction.) Whale calves in particular may benefit from riding, notes biomechanist Frank Fish of West Chester University, who was not involved in the study. Calves have to come to the surface more often to breathe than adults, and thus experience more drag. Hitchhiking across the tide, a tired mother and calf may barely have to move a muscle, Fish says. “From an energy point of view, this makes perfect sense. Animals will do what it takes to conserve energy.”

Humpback whales are a well-studied species, so it’s surprising that 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 Endowment for Science. He thinks that several factors likely lined up perfectly to produce the rear 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 hump’s possible speed. And the winds were relatively mild, which likely allowed for an attractive wake to form.

Brazilian researchers hope to find out if wake riding is widespread among humps. The answer could help reveal how animals perceive and interact with ships, which can pose a significant collision threat. Riding directly behind a ship can help whales avoid collisions with other ships, but it can also cause the animals to cross more frequently into high-traffic sea lanes.

But spotting more hitchhiking whales — especially at night — can be difficult, notes whale researcher Steven Katona, a managing director of the nonprofit Conservation International. “No one actually searches for whales at night, it’s a pretty thankless task,” he says. “If Titanic couldn’t see an iceberg, who will see a whale?”

Still, the observation is a reminder of how much there is still to learn about the ocean’s leviathans, Long says. “The possibility exists that they are trying to exploit all sorts of things about their environment to help with these long-term migrations,” he says. “Which is absolutely amazing.”

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