http://www.newsweek.com/captive-dolphins-look-forward-play-humans-pavlovs-dogs-946315

Do dolphins enjoy playing with human companions? No one knows for sure, but a new study indicates that captive bottlenose dolphins react when they know they are going to play with familiar trainers, moreso than they do when they expect to play with toys or when they expect to be left alone.

For a study published recently in the journal Applied Animal Behavior Science, researchers tested captive bottlenose dolphins in French theme park Parc Astérix. They played a different sound before offering the animals different stimuli, including adding toys to the water, bringing a trainer to play with them, and a control, where they left the dolphins to do as they wished.