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A pod of Pacific white-sided dolphins off the coast of British Columbia have been observed cooperating with orcas, a traditional enemy that’s better known for taking out great white sharks than friendly interaction.

Scientists say they have documented the dolphins and a local population of killer whales known as Northern Resident orcas teaming up to hunt the orcas’ staple food: salmon. Though other groups of orcas feast on dolphins, Northern Residents do not. Still, it is the first time this type of cooperative behavior has been documented between the two marine mammals, researchers reported.

“Seeing them dive and hunt in sync with dolphins completely changes our understanding of what those encounters mean,” said Sarah Fortune, Canadian Wildlife Federation chair in large whale conservation and an assistant professor in Dalhousie University’s oceanography department. Fortune was the lead author of the study, which published Thursday in the journal Scientific Reports.

To witness the dolphins and orcas interacting, the researchers captured drone footage as well as underwater video by attaching suction tags to the orcas that were equipped with cameras and hydrophones.

Their footage showed that the killer whales traveled toward the dolphins and followed them at the surface level. The underwater footage revealed that the killer whales were also following dolphins on their dives of up to 60 meters (197 feet), where the orcas were able to prey on Chinook salmon.

Though light levels are low at those depths, Fortune said cameras picked up the killer whales catching salmon, with clouds of blood billowing from their mouths, and hydrophones picked up the crunch of a kill.

To understand better what was happening, the researchers also eavesdropped on the echolocation clicks made by dolphins and orcas, which allow animals to navigate and sense their environment by listening to the returned echoes of the noises they make. “We can look at the characteristics of these clicks to infer whether a whale is actively chasing a prey for a fish and also whether it may have caught the fish,” Fortune said.

The researchers recorded 258 instances of dolphins and orcas interacting between August 15 and 30, 2020.

They found that all the whales that interacted with dolphins also engaged in killing, eating and searching for salmon.

Put together, the data Fortune and her colleagues collected suggested that the killer whales, fearsome predators able to take on great whites and whale sharks several times their size, were essentially using the dolphins as scouts.

“By hunting with other echolocating animals like the dolphins, they might be increasing their acoustic field of view, providing greater opportunity to detect where the salmon are. That’s sort of the prevailing thought here,” she explained. Using dolphins in this way would also allow the orcas to conserve energy, with salmon often hiding at depths to try and avoid predators such as orcas.

But what do dolphins get out of the interactions?

The video Fortune and her colleagues collected showed that once the orcas caught their prey and shared it with the pod, the dolphins were quick to eat the leftovers.

But salmon isn’t a core part of a dolphin’s diet, so greater access to food likely wasn’t the sole motivation, Fortune said. By hanging out with the orcas, dolphins likely gain protection from other orca pods that pass through the area and hunt dolphins.

In addition to the salmon-eating Northern Resident killer whales, the region is home to a distinct type of orca known as the Bigg’s or transient killer whales that specialize in eating marine mammals such as dolphins.

Interactions between Northern Residents and dolphins have occurred off northeastern Vancouver Island for at least three decades, according to Brittany Visona-Kelly, a senior manager at Canadian conservation group Ocean Wise’s Whales Initiative, who wasn’t involved in this research but has studied the interactions between dolphins, porpoises and the same population of orcas.

In her experience, it was the dolphins that initiated interaction with the killer whales, not the other way around, and she said she was skeptical that the two were genuinely engaging in cooperative foraging. Instead, she said, the orcas may have viewed the dolphins as an annoying pest that was easier to put up with than get rid of.

“Over several years of observations, we concluded that dolphins and porpoises — not killer whales — benefit most from these encounters. Dolphins and porpoises likely gain protection from their primary predator,” she said via email.

“We suggest that Northern Resident killer whales derive no clear benefits from these interactions, but that actively avoiding or resisting them may impose greater energetic costs than tolerating them,” she added.

Fortune, however, said her team’s findings upended the prevailing view among scientists of the interactions.

“Under that paradigm, the dolphins would need to be just kind of hanging out at the surface, grubbing scraps, not exerting time and energy and effort in the process, which they certainly are,” she said, adding that her team found no evidence of antagonistic or avoidant behavior by the orcas toward the dolphins.

What’s more, the research by Fortune and her colleagues was the first time underwater footage has been used to understand the behavior, she added.

Cooperation between different species is relatively common in nature, but rarer among mammals and typically doesn’t involve predators, said Judith Bronstein, University Distinguished Professor in the University of Arizona’s department of ecology and evolutionary biology, who studies interspecies cooperation. However, she noted that coyotes had been observed hunting with badgers and opossums with ocelots.

Many species feed together, Bronstein said, noting that “mixed flocks of birds, mixed shoals of fish, for instance, all look out for predators.”

“What’s cool about this example is that each of the species has different abilities,” she said, “and when you look at collaboration between species, you’re always looking for the benefit that outweighs the cost.”

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