Dolphins and humans have hunted mullet together in Laguna, Brazil, for generations, increasing the catch for both species. Drone-based video shows that the dolphins coordinate with each other during these hunts, researchers report February 12 in Current Zoology. The analysis also reveals which of the dolphins’ tactics are most likely to result in a successful catch for both mammal species.

The tandem hunting practiced by Lahille’s bottlenose dolphins (Tursiops truncatus gephyreus) and local fishermen often yields more bounty for all. In the murky estuaries in southern Brazil, the dolphins herd schools of mullet toward the shallows where humans wait for just the right moment to cast their nets, signaled by a dolphin’s tail slap against the water or sudden dive. The dolphins can then pull a few fish from the net to eat or more easily prey on the mullet trapped against the outside of the nets.

“This is a rare interaction between two top predators combining their complementary skills,” says Mauricio Cantor, a behavioral ecologist at Oregon State University’s Marine Mammal Institute in Newport.

Previous research from a 40-year-long monitoring program centered around this joint dolphin-human foraging has shown how the humans cooperate with each other to maximize their fish catch. But scientists weren’t sure if the dolphins also cooperated with each other or instead competed for the mullet.

Cantor and his colleagues used drones to film dolphins’ foraging behavior near Laguna over 18 days in 2018 during the peak of mullet migration season. Of the 45 recorded human-dolphin foraging interactions, nine were successful (meaning people caught fish). Researchers tracked three elements of the dolphins’ movements as seen from the surface: the distance between individuals when they surfaced, the alignment of their swimming trajectories and the synchronization of their dives.

Dolphins and people had a higher chance of joint success when dolphins approached humans along several trajectories as they herded fish and when they synchronized dives, the researchers found. 

These behaviors reveal a complex level of coordination that allows the dolphins to “adapt to and capitalize on human activities,” says Bruno Díaz López, a biologist at the Bottlenose Dolphin Research Institute in Pontevedra, Spain, who was not affiliated with the research.

Understanding the tactics that ensure fruitful foraging outcomes is important for prolonging this unique hunting partnership, Cantor says. “This serves as one lasting example that our interactions with nature don’t have to be one-sided. They can be mutual, too.”

Ecologists on Cantor’s team are now working with computer scientists to build an AI model that can more efficiently extract patterns in the dolphins’ surface behavior from drone videos. Cantor and his colleagues are also analyzing four years of physiological data that track the humans’ fine-scale decision-making.

Cantor hopes to one day gather the same type of physiological data on the cetacean hunters. “This will be a bit tricky,” Cantor admits, as he’ll have to “wait for someone to develop a kind of Fitbit for dolphins”.