Researchers still don't know what the open-mouth facial expression means or whether it's akin to smiling in humans—but several animals make a similar face during play

Dolphins are extremely playful, but little is known about how they—and other marine mammals—communicate during playtime. New research published October 2 in the journal iScience shows that bottlenose dolphins (Tursiops truncates) use the "open mouth" facial expression—analogous to a smile—to communicate during social play.