Humans train animals by rewarding them with tasty treats and trainers couple the reward with a sound, such as a buzz or a whistle. Once the animal has mastered the task, the trainer stops dispensing food, relying instead on the whistle or buzzer to inform the animal that it has performed successfully and that it will be rewarded - with food, but later.

Even though there may be no food reward at the time, whales and dolphins still squeal in response to the sound substituted for the food reward. And Sam Ridgway found that when he trained dolphins and beluga whales to switch off a sound after diving hundreds of meters, the animals produced the same squeals of victory when the sound stopped.  

Are the squeals more than just food signals? Ridgway believes could they be genuine expressions of delight because the time delay between dolphins and whales receiving a reward and their squeals is the same as the delay between a pleasant experience and dopamine release – suggesting that dolphins and whales experience pleasure.

Beluga whales. Credit: Duke University

"The [squealing] behaviour had transferred over to another stimulus that wasn't food," says Ridgway.

The behavior reminded him of studies in the 1950s when animals appeared to derive as much pleasure from electrical stimulation of a region of the brain that released dopamine – a chemical that stimulates the sensation of pleasure – as they did when receiving a food reward. Had the trained dolphins and beluga whales transferred the release of dopamine from the brain's pleasure centers from the food reward to the trainer's reward signal?

Going back through decades of recordings of experiments designed to test the abilities of dolphins and beluga whales that he had conducted with Patrick Moore, Don Carder and Tracy Romano, Ridgway then measured the delay between the trainer's signal and the victory squeals. As dopamine release takes 100 ms, Ridgway says he realized that the animals could be expressing pleasure if the delay between the promise of a reward and the animals' squeals was longer then the dopamine release period. 

"Normally we worked in open waters in the San Diego Bay or out in the ocean. Our recordings sometimes have a lot of background noise, so most of the analysis has to be done by hand using the human ear," recalls Ridgway. However, after months of analysis, Ridgway was convinced that the beluga whales and dolphins were expressing pleasure through their squeals.

"The dolphins take an average of 151 ms extra time for this release, and with the belugas…it's about 250 ms delay," says Ridgway. "We think we have demonstrated that it [the victory squeal] has emotional content', before adding that he is keen to find out more about the cognitive abilities of these expressive animals."