In 1989, researchers at the University of California at Davis invented the first birth-control vaccine for (nonhuman) animals. By causing a female's immune system to block sperm from her eggs, the drug, called PZP, offered a humane method to keep elephant, donkey and deer populations in check. The compound worked, but it had a troubling side effect: The animals stayed in heat longer than normal. In one trial, deer were in heat for six months instead of one. That's a problem because an amorous deer is a dangerous deer. When pursued by stags, females often run into traffic.
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