Video and news story about the spiny anteater's remarkable penis.
The spiny anteater's four-headed phallus had been puzzling scientists. "When we tried to collect semen by [electrically-stimulated ejaculation] before, not only did we not get a single drop, but the whole penis swelled up to a four-headed monster that wouldn't fit the female reproductive tract, which has only two branches," says Johnston.
"Now we know that during a normal erection, two heads get shut down and the other two fit," he told New Scientist. The heads used are swapped each time the mammal has sex.
Link (Thanks, Pete!)