The Asian gibbons were migrating across the Yellow River and had to carry light loads--just their bodies--in order to swing across to the branches on the other side.
Upon landing, they realized they had abandoned their children and spouses behind on the other side of the river.
Now, they are empty-hearted and lonely. So, hoping their loved ones would hear their sadness out, these gibbons got onto to tree tops and screamed the saddest, most beautiful sounds in any jungle in the world.
Those sounds have become their tradition, even after having mated and bred with other primates on that side of the river.
https://www.youtube.com/watch?v=zgq2Mr6P6r8