Reply #15: I'll second that one -- it's dominance behavior.

[Philostopher]

Neutering may reduce the incidence of it -- and good on you, for fostering cats! I have a house full, I can't really do it, but I respect people who do.

I have one male and three females, and the male used to do that to the younger females in the household, until they got big enough to swat the daylights out of him.

Interestingly, he's been neutered since months before we brought him home. He was an adult before he was neutered, though -- the behavior already was fixed.

Sexual behaviors doesn't explain the fact that the dog, who also was neutered as an adult but before we brought him into the menagerie, also attempts to mount the cats on occasion. Again, I'm pretty sure that's an authority/dominance gesture.

Now, the neutered male cat humps a blanket. He doesn't get swatted, the girls don't get molested, and everybody's happy. Except blankie.