it's natural...it also shows dominance.....the one dog i have , when he was no bigger than a minute, used to do the side hump, he would lay on his side and go to town...i used to laugh til i criedhe was about the same age....he finally got it right though, lol....females will do it too....m sister's 2 dogs do it all the time when they are playin with thier bird(toy)....