I don't really know, but I'll tell you what mine do.
Princess (Mimi's daughter) is 9 months old & spayed, she'll occasionally hump her loofa toy. She hasn't done it in a while, so I'm thinking even tho she's spayed & never experienced a heat cycle, she's having some sort of instinctual heat or claiming the loofa as her toy.
Maybe it's something that will pass (hopefully). I also read somewhere that the dog's dominance is constantly changing, so maybe for now... Wicket is the top dog! Lol