Risso’s Dolphins Found to ‘Meal Plan’ With Dives

A pair of Risso’s dolphins. (Photo: Monterey Bay Aquarium)

(CN) – For dolphins, hunting is a balancing act. Since they’re mammals they’re under pressure to find their prey and make it back to the surface before running out of oxygen.

So how do dolphins optimize these missions?

By using information gathered on previous dives to plan subsequent sorties, according to a study published Wednesday in the Journal of Experimental Biology.

“Lab experiments that test the memory of animals for the location of food show that they have a similar ability to that of humans,” said first author Patricia Arranz, a research fellow at the University of St. Andrews in Scotland.

However, it is not clear whether animals that are actively searching for food adjust their hunting strategy in response to the varying conditions of their surroundings.

Arranz’s studied Risso’s dolphins, which dive hundreds of feet to catch squid, to determine whether the marine mammals plan their missions.

Tracking the dolphins was challenging, according to Arranz.

“It is really difficult to approach them and attach something to their backs; you need to be very patient,” she said.

Arranz recalled how two of her co-authors – Ari Friedlaender from the Marine Mammal Institute and John Calambokidis of the Cascadia Research Collective, both in the United States – carefully monitored the animals as they slowly moved into position to attach data loggers to the animals using a 16-foot-long pole.

The data loggers tracked the dolphins’ movements, depths and the sounds they emitted.

Co-authors Kelly Benoit-Bird and Brandon Southall, from the Monterey Bay Aquarium Research Institute and the University of California, Santa Cruz, respectively, then tracked the location of squid using echosounders – devices that use sonar technology to map underwater biological and physical components – mounted on remotely operated underwater robots as the dolphins hunted.

“In one of the experiments, we were extremely lucky as the group that the tagged animal was in stayed in the same area, allowing us to track the dolphin every time it was at the surface and observe the prey with the echosounder right where and when the dolphin was foraging,” said Arranz.

In the lab, Arranz and Benoit-Bird then analyzed the recording from 37 dolphin dives while tracking the positions of squid prey and found that the animals began echolocating shortly after leaving the surface.

“Probably to gain information on the depth distribution and availability of prey and to respond swiftly to rapid changes in habitat structure at different depths,” Arranz said.

The plunging dolphins appeared to sync their echolocation range – emitting sounds and listening to reflected noises to determine the location of objects – to the depth at which they had found the most squid during their prior dive, according to Arranz.

“Which can be interpreted as dolphins recalling information to plan the next foraging dive,” she said.

The researchers also noticed that the animals continued to echolate as they retreated to the surface, even though they were no longer hunting, which suggests they were planning ahead and scouting the ideal location for a subsequent dive.

The dolphins appeared to be able to tailor their driving strategy as the conditions adjusted, occasionally targeting a shallow layer of squid at the beginning, but shifting their focus to deeper areas with more squid later.

It was clear the animals were planning ahead, using information that they had gathered during previous dives and combining this knowledge with their present experience to optimize their missions, according to the report.

The research was funded by the U.S. military’s Chief of Naval Operations Environmental Readiness Division, the U.S. Navy’s Living Marine Resources Program and Office of Naval Research Marine Mammal Program, the U.S. Department of Defense’s Strategic Environmental Research and Development Program, and the U.K.-based Marine Alliance for Science.

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