Sam Ridgway of the National Marine Mammal Foundation in San Diego, Calif., and colleagues recently published a study that suggests that dolphins and beluga whales squeal out of excitement when given rewards.

The scientists said they believe the sounds that are emitted by the marine mammals are expressions of pure delight and not just a method of telling others in the group that there is food around. They have assumed that the sounds coming from the dolphins are sounds of pleasure as they matched the length of time until the hormone dopamine is released from the brain.

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Ridgeway and his team regularly works with beluga whales and bottlenose dolphins, teaching the creatures to repeat behaviors and tasks and rewarding them with fish upon completion.

Research conducted on brain chemistry that used electrical stimulation has shown that primates and rats possess reward systems that involve neurons for dopamine.

In the 1950s, experiments were conducted on a dolphin that proved the creatures have similar dopamine neurons in their brains' reward areas and that dolphins emit sounds after the brain is stimulated.

The team of researchers reviewed their accumulated recordings of sounds from their previous studies on beluga whales and dolphins. Ridgway and his colleagues found out that the time delay between the pleasurable activity, which could be the expectation of a reward or the reward itself, was about a little bit longer than  the 100 milliseconds required for the feel-good hormone to be released.

"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 beluga whales and dolphins are animals that are heavily dependent on members of their pods, the timing, context and character of their squeals may uncover more about the emotional states of the individual and the specific functions of the sounds used for communication.

TagsDolphin, Science, scientific research, Scientists, Research, marine mammals, california, dopamine, Hormones