A stickleback is a small fish with sharp spines on its back. These spines normally lie flat, but when the fish feels threatened, it raises them up like tiny swords, making itself difficult for predators to swallow.

Sticklebacks live in freshwater streams, ponds, and coastal areas throughout the Northern Hemisphere. They rarely grow longer than a few inches, but what they lack in size they make up for in fascinating behavior. Male sticklebacks are dedicated fathers who build intricate nests from plant material and algae, gluing everything together with a sticky substance from their kidneys. After a female lays eggs in the nest, the male guards them fiercely, fanning fresh water over them with his fins until they hatch.

Scientists love studying sticklebacks because they've adapted to many different environments. Some populations lost their spines entirely after moving to waters without predators, showing evolution in action. For such a tiny fish, the stickleback has taught researchers enormous amounts about animal behavior, evolution, and how creatures adapt to their surroundings.