Why do humans say "bye"? 👋 Do animals?

The act of saying goodbye is the way that humans regularly bid farewell or express good wishes upon departure or separating. Saying goodbye is a social convention, a polite way to acknowledge the end of a conversation or interaction. 

The etymology of "goodbye" appears to be a short version of "god be with you."

"Farewell" can also be used as a noun, referring to an occasion when goodbye is said, especially when one is leaving for a long period of time or is unlikely to return. See also valediction, departure, send-off, goodbye.

Saying goodbye/farewell is a way for people to acknowledge and show respect or appreciation for the time they have spent with someone. It can also be a way to express sadness or regret that the end has come for a relationship or interaction. It can provide the chance to make things right. In many cultures, it is customary to exchange words of farewell or to perform specific rituals or gestures when someone is leaving, such as shaking hands or hugging.

I wonder if animals have a concept of goodbye or use any specific behaviors to communicate bon voyage. Some animals may have ways of indicating when they are leaving or departing from a group. For example, some primates might use gestures or vocalizations to signal their intention to leave, and some birds may engage in certain behaviors when they are preparing to fly away.

I like at the end of a song when they say "one more time!" and the band plays the last refrain again. It's usually an energetic moment. Apparently this is called an outro. Outros may provide a sense of closure or finality to the song, and they can be a way for the performer to extend the song for a little longer, to engage with the audience, to show appreciation to the audience, or to create a memorable moment at the end of the song. This is different than an encore, which is similar but is on the scale of an entire concert, not verses in a song, as is an outro.