For 100s of years, naturalists have been documenting gynandromorphs among insects, spiders, lobsters, and birds. More recently, researchers—aided by increasingly sophisticated laboratory tools—have overturned reigning theories of sexual development by studying such hybrids. As has proven true time and again throughout the history of science, the creatures that seem strangest—those that are too odd, too asymmetrical to fit neatly into our presupposed categories—teach us the most about how all living things work. It turns out, for example, that the standard explanation of how a bird becomes male or female is wrong. Scientists came to this realization not by investigating scores of typical birds, but rather by examining a few gynandromorphs. It all started with an odd zebra finch.

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