My 3 love to puppy a stuffed "Puppy So Real" laborador toy ( http://images-p.qvc.com/is/image/t/30/t10830.001?$uslarge$ ). They will actually form a humping line to that they can take turns abusing "Humper-Nutter!" I find it to be hysterical and since they never hump anything else- I don't see it as a problem. Cozy will even put it away in my room when people come over- It is her guilty pleasure

All are spayed now. Cozy started to do it when she was pregnant with her first litter and have never stopped. Her daughters just learned it from her!
Poor Buddy (Golden) leaves the room when he seeing them doing it. If they are using "Humper-Nutter" and he comes it to the room and high tails it out of there