Dolphins often squeal when they get a fish treat, sounding much like happy children.
US researchers said Wednesday they believe these calls are not just ways of signaling to others in the group that there is food around, but expressions of pure delight.
The reason they think the dolphins' sounds indicate pleasure is they match the amount of time it takes for the brain to release the hormone dopamine.
The study in the Journal of Experimental Biology was led by Sam Ridgway of the National Marine Mammal Foundation in San Diego, California.
Ridgway and colleagues work with bottlenose dolphins and beluga whales, training them to repeat behaviors and rewarding them with fish.
Previous research using electrical stimulation and brain chemistry has shown that rats and primates have reward systems involving dopamine neurons.
Experiments on a dolphin in the 1950s showed that dolphins also have dopamine neurons in reward areas of their brains and that dolphins vocalize after their brain is stimulated.
Ridgway and colleagues went back over years of recordings of their own research with dolphins and beluga whales.
They found that the time delay between the pleasant experience -- the reward, or expectation of reward -- was just a bit longer than the time it takes for the dopamine release, usually around 100 milliseconds.
"The dolphins take an average of 151 milliseconds extra time for this release, and with the belugas it's about 250 milliseconds delay," said Ridgway.
"We think we have demonstrated that it (the squeal) has emotional content."
Since dolphins and belugas "are highly vocal animals... the character, timing and context of their sounds may reveal more about their emotional states and about the function of their sounds in communication," concluded the study.