It's rare that Darby has possession of a ball when Bonnie doesn't have one, too.
When she does have one, as in this photo, she guards it very carefully--but also seems to tease Bonnie with it just a bit.
Wish I understood canine body language a little better. I need to re-read Patricia McConnell's books. A recent post on her blog talks about play styles of dogs. You can read about it here.