Masturbating animals.

Historically, one of the many arguments used to demonize masturbation was that it's unnatural (i.e., it doesn't occur in nature). If only they'd had the internet, they'd have figured out that they were dead wrong. NSFW - and feel free to add to the collection in the comments section (for a bunch more, follow the link at the bottom):

[as a side note: animals in cages make me sad]

Mochi the masturbating dog

Dirty little dog masturbating. I must say this little fellah got a quite big dick for his size haha.

it was my favorite part of the zoo that day. Like this little guy, I also prefer to have an audience.

Satisfaction Guarantee...

monkey jacking off

self pleasure life in the Australian Wildlife sanctuary. Enjoyment