These orcas have dolphin sidekicks. Scientists have untangled why

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

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

“Seeing them dive and hunt in sync with dolphins completely changes our understanding of the significance of these encounters,” said Sarah Fortune, Canadian Wildlife Federation Chair in Great Whale Conservation and assistant professor in the department of oceanography at Dalhousie University. Fortune was the lead author of the study published Thursday in the journal Scientific Reports.

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

Their footage showed the killer whales moving towards the dolphins and following them to the surface. Underwater footage revealed that the killer whales also followed the dolphins as they dived down to 60 meters (197 feet), where the orcas could prey on Chinook salmon.

Although light levels are low at these depths, Fortune said cameras captured the killer whales catching salmon, with clouds of blood leaking from their mouths, and hydrophones picked up the crunching of prey.

To better understand what was happening, the researchers also listened to the echolocation clicks made by dolphins and orcas, which allow the animals to navigate and sense their surroundings by listening to the echoes of the noises they make. “We can look at the characteristics of these clicks to infer whether a whale is actively pursuing prey for a fish and also whether it was able to catch the fish,” Fortune said.

Researchers recorded 258 instances of dolphin and orca interactions between August 15 and 30, 2020.

They found that all the whales that interacted with the dolphins also killed, ate, and searched for salmon.

Together, the data collected by Fortune and his colleagues suggest that killer whales, fearsome predators capable of preying on great white sharks and whale sharks several times their size, primarily used dolphins as scouts.

“By hunting with other echolocating animals like dolphins, they could increase their acoustic field of view, providing a greater opportunity to detect where the salmon are. That’s kind of the dominant idea here,” she explained. Using dolphins in this way would also allow orcas to save energy, with salmon often hiding in the depths to try to avoid predators such as orcas.

But what do dolphins get from these interactions?

The video collected by Fortune and colleagues shows that once the orcas caught their prey and shared it with the group, the dolphins quickly ate the remains.

But salmon is not an essential part of the dolphins’ diet, so better access to food probably wasn’t the only motivation, Fortune said. By associating with the orcas, the dolphins likely gain protection from other pods of orcas that pass through the area and prey on the dolphins.

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

Interactions between northern residents and dolphins have been occurring on northeast Vancouver Island for at least three decades, according to Brittany Visona-Kelly, senior director of the Canadian conservation group Ocean Wise’s Whales Initiative, who was not involved in this research but has studied interactions between dolphins, porpoises and the same population of orcas.

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

“After several years of observations, we concluded that it is the dolphins and porpoises – not the killer whales – that benefit most from these encounters. The dolphins and porpoises likely benefit from protection from their main predator,” she said by email.

“We suggest that Northern Resident Killer Whales do not derive any clear benefits from these interactions, but that actively avoiding or resisting them may impose higher energetic costs than tolerating them,” she added.

Fortune, however, said his team’s findings upend the prevailing view among scientists about the interactions.

“In this paradigm, the dolphins should just be hanging out on the surface, picking at the remains, without expending any time, energy and effort in the process, which they certainly are,” she said, adding that her team found no evidence of antagonistic or avoidant behavior on the part of the orcas towards the dolphins.

Additionally, the research by Fortune and colleagues marks the first time underwater images have been used to understand behavior, she added.

Cooperation between different species is relatively common in nature, but rarer among mammals and generally does not involve predators, said Judith Bronstein, professor emeritus in the department of ecology and evolutionary biology at the University of Arizona, who studies interspecific cooperation. However, she noted that coyotes have been observed hunting with badgers and opossums with ocelots.

Many species feed together, Bronstein said, noting that “mixed flocks of birds, mixed schools of fish, for example, are all looking for predators.”

“What’s interesting about this example is that each of the species has different capabilities,” she said, “and when you’re considering cross-species collaboration, you’re always looking for the benefit that outweighs the cost.”

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