Why the heck is yawning contagious? I would expect such a feature to evolve if it served a purpose like, say, warning of something coming. See someone give a warning call, pass it on to the rest of the tribe. But why should a sleepy signal do that? Was there a sleep illness people had to worry about catching? Maybe it evolved to signal to everyone that it's nighttime. How many species is yawning contagious for? I'm picturing lions for some reason. I have a feeling the behaviour is common among mammals