Humans are not alone in getting cancer. In fact, it seems to be a pretty inescapable risk of being an animal. As cells divide and mutate, some mutations may make cells ignore the needs of the body and multiply madly. That’s too bad for other animals, but there’s a silver lining for us: by studying other animals, scientists can get some clues to how cancer evolves in us.

The delicate swordtail (Xiphophora cortezi) is particularly prone to getting melanomas (the bottom picture here shows a fish with a tumor in its tail). When Andre Fernandez and Molly Morris of Ohio University went fishing for delicate swordtails in mountain streams in Mexico, they found six fish with melanomas in a single day’s