My Sasha (the smaller of the 2) is the "humper". She humped her sister Coco (who happily let her while she sat and played with her toys) and now, when she's feeling well, she's humping CeCe,( who makes her chase and doesn't give her much of a chance to get started!) I tell her "no humpin'! she stops right away....but she just can't help herself and starts right up when I'm not looking!