Are humans the only animals that masturbate?

I know dolphins have sex for pleasure just like humans do, but does any other species masturbate besides humans?
Answers:
Many animals stimulate their genitals. Higher apes masturbate in very much the same way we do. Like the post above says, Bonobos do. Bonobo chimpanzees are highly sexual and not only engage in masturbation, they have oral sex, missionary position sex (amongst other positions) and they even kiss each other on the lips.
I think bonobo's do too.
No.
monkeys like to finger their butt and play with themselves
Wayne J does it too and baboons
dogs know how to, hence the humping of things like stuffed animals and legs, big hugs
My rabbit did
No .Hamsters ,monkeye, cats, dogs,and the majority of mammals ..
MONKEYS-APES
My rabbit likes to hump my dogs leg!
no , graham norton is a huge w*nker
Most Mamals do. both sexes.
my dog used to lick his willy (when he was still alive) i thought it was disgusting! i was just a kid at the time and dint know much about the birds and the bees. (and i had no brothers so i never saw a real willy until i saw my male cousins playing with themselves for a laugh!!) i threatened to tell their dads even though i had no clue what they were doing, i was about 10.
well, i daresay when i've seen horses on heat, their 1-2 foot whoppers dangle about and they look all hot and bothered, sort of trying to spot the nearest horse fanny? or did they manage that with a hind leg, i always feel embarrassed staring, but its just SO HUGE!
Lots of 'em do, in lots of different ways.

http://en.wikipedia.org/wiki/animal_sexu.
yes - other animals mastrubate. Think about it - the dog that likes to hump everything in sight, what do you think he's doing?!?! That is just one example - but it's seen throughout the animal kingdom.
Interesting question.
Honestly, I can see why, with opposable thumbs, we have the advantage there.
Adolescent male whales sort of do. They ejaculate into the water swimming around one another. But since they're in a frenzy with other adolescent males, I'm not sure you can call that masturbating.
The other one is bonobos (they're a primate). They masturbate, but I don't know if they ejaculate when they do, so I don't know if that counts.
I've also seen male dogs hump inanimate objects, but never seen one ejaculate on an inanimate object. Dunno if that counts either.
dogs do masturbate when you see them licking their willy that is how they do it.
Yup, many animals do. People have already listed primates, dogs and rabbits, but birds do as well! My male cockatiel is definately no exception..LOL