Queensland, Australia: The head and casque of a female cassowary: Bertha. She is over forty years old and the dominant bird in her rainforest territory. The casque continues to grow throughout the life of a cassowary and is thought to be used in communication between individuals. This image is from Dino Bird.
Flatworms are armed, flexible and dangerously bisexual. These marine hermaphrodites engage in violent sexual sword fights with the loser bearing the physical burden of pregnancy. This image is from Extreme Animals.