Mammals, reptiles, fish, birds, and frogs yawn, however, the reasons for yawning may be different. Yawning may be related to circadian rhythms (like when we’re tired), feeding behavior, sexuality, or social interactions.
Insects do not yawn because they don’t have lungs.
This website has a ton of info on yawning: http://www.baillement.com/english/cicumstances.html
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