In the race of life, some animals are hares, running at top speed, producing lots of young and dying quickly. Others are tortoises, maturing slowly, producing few offspring, and living a long time. There must be physiological mechanisms that regulate these differing life strategies, and researchers studying bird hormones have found a clue. Corticosteroids, found in many animals, are stress hormones released in the blood. If an animal is threatened by a predator or rival, the hormones allow the individual to survive by fighting or fleeing. The strange thing about corticosteroids is that, just like life strategies, they vary dramatically from one species to the next. The question is: Why?