Dolphins are extremely playful, but little is known about how they -- and other marine mammals -- communicate during playtime. New research shows that bottlenose dolphins (Tursiops truncates) use the 'open mouth' facial expression -- analogous to a smile -- to communicate during social play. The dolphins almost always use the facial expression when they are in their playmate's field of view, and when playmates perceived a 'smile,' they responded in kind 33% of the time.