Diving back two hundred million years: yawn contagion in fish.

Yawning is a widespread and automatic behavior in vertebrates. Yawn contagion, responding with a yawn to others' yawns, helps synchronize motor activities, particularly in social animals, promoting coordination within groups. While primarily observed in social, endothermic species, yawn contagion remains unconfirmed in ectotherms. We discovered yawn contagion in zebrafish (Danio rerio). Using a deep learning model to distinguish yawning from breathing, we found that fish not only yawn but also "catch" yawns from others. The presence of yawn contagion in fish raises important evolutionary questions, particularly regarding its origin. According to evolutionary biology theories, on one hand, it could be a shared trait among vertebrates, with the secondary loss of this phenomenon in some taxa. On the other hand, it may be a result of convergent evolution, emerging independently in different evolutionary lineages as a response to the need for synchronization of motor actions within social groups.