True story that just happened today. My wife, daughter and I were driving down a rural road. Out in the yard of a house were two good sized dogs. One of the dogs had mounted the other one and was really going at it. However, upon closer inspection we noticed that the dog was on the wrong end...he was humping the head of the other dog! We were cracking up because it looked so comical...the poor dog underneath probably had quite a headache.
LOL, that's what I was talking about. Animals do that to establish dominance. The dog on top was probably the "topdog" and the other dog was the "underdog". :-) I crack myself up sometimes. LOL
It's very common in the animal world but really has nothing to do with sex. You won't see two male dogs setting up house together or two roosters building a nest and sitting on eggs.