I just wanted to throw in here about my two. They are both altered and never try to "hump" each other EXCEPT when they are playing. Then they both take turns "humping" each other. Neither seems to be bothered when the other is doing it. I do remember how surprised I was when I first saw Tasha do this to Tommy, but I realized it wasn't being done in the sense that humans often view it, it was a form of play. Perhaps setting aside their roles in the pack (usually Tasha is the alpha). When they play they seem to be on equal ground. They switch off showing their belly to the "attacker" and switch off mounting the other.

I always find it fascinating to watch their interactions with each other. It's like watching a wolf pack interacting in your own home.