When I was growing up, my mother worked at a dolphin rescue center. I spent a whole lot of my early childhood there.

Some dolphins would be make friends with specific other dolphins, and enemies out of others (bullying / attacking) and needed to be kept apart. Not all of them would actually recover enough to live to old age.

I distinctly remember times when a dolphin had a friend die, they would display what I would consider depression symptoms. Not wanting to play with toys, less interest in eating, way less activity in general.