The connection between dopamine and a pleased squeal, Marino tells The Dodo, is "dubious at best" and a case of "going way beyond the data." The flaw, she says, is that the researchers never measured dolphin dopamine levels - and, considering the risk to the cetaceans involved, it would be unethical to do so.
"It's a bit of a stretch to say [the squeal] is emotional," Maddalena Bearzi, a marine mammal expert at Ocean Conservation Society not involved with the study, tells National Geographic.
Though Ridgway stands by his conclusion, he agrees that measuring the dolphins' dopamine would be unwarranted. "Why should we have to repeat this invasive work of the ‘50s and ‘60s to move forward with understanding animal cognition and emotion?" he asks The Dodo in an email.
Because Ridgway and his colleagues work with trained animals (the NMMF is affiliated with the U.S. Navy), Marino says these researchers have a vested interest in showing that this sound is "a positive emotion rather than neutral or negative." She doesn't doubt that dolphins and whales are capable of producing feeding buzzes, but she questions whether or not this indicates pleasure.
Although the dolphins have been trained to respond to a reward, Ridgway says in a statement that his research took place in San Diego Bay and the open ocean. "Much of my work has been with trained animals that swim with us miles in the open sea, where they could easily swim away," he writes. "To me in their approach, their posture, their vocal behavior and their willingness to work, to dive to great depth and return to us far away and unseen on the ocean surface, suggests pleasure."
But on Ridgway's last point - that dolphins are intelligent creatures capable of experiencing pleasure - all the scientists seem to agree.
"Based on 25 years in the field," Bearzi tells National Geographic, "I do believe these animals are emotional creatures."