Humans and most other vertebrates experience yawns as a result of various physiological and mental states, including sleepiness or emotional arousal, such as fear or excitement. But despite their different sources, these yawns often look identical and are difficult to distinguish by onlookers.
Gelada baboons, native to Ethiopia, are unique among yawning animals in that they have three distinct types of yawns that are easy to identify. These types include yawns with covered teeth; yawns with uncovered teeth; and yawns with uncovered gums and head movements. The more teeth and gums revealed through a yawn, the more intense researchers consider it to be. [See Photos of Cute Gelada Monkeys]