A common growth-promoting hormone used worldwide in the cattle industry has been found to affect the sexual behaviours of fish at a very low concentration in waterways – with potentially serious ecological and evolutionary consequences. Researchers from Monash University, in collaboration with researchers from Åbo Akademi University in Finland,  have found that the steroid 17β-trenbolone – used on livestock to increase muscle growth – alters male reproductive behaviour in guppy fish (Poecilia reticulata).