Think you know all about the birds and the bees? Most male birds--97 percent--don't have penises. Instead, they rely on internal plumbing and gravity to deposit sperm. Stranger still, biologists at the University of Sheffield in England discovered that the red-billed buffalo weaver has a penis, but it's a fake.

During the buffalo weavers' half-hour-long copulation ritual, the male stimulates the female with his phony phallus. Meanwhile, he transfers sperm through the adjacent sex organ. The additional stimulation may encourage the female to take up more sperm.