For the better part of two decades, Richard Weindruch, a professor of medicine at the University of Wisconsin-Madison, has fed half of a colony of 78 rhesus monkeys a diet adequate in nutrition but severely limited in calories -- 30 percent fewer calories than are fed to the control group. Scientists have known for nearly 70 years that such calorie restriction extends the life span of rodents, and Weindruch is determined to find out whether it can extend the life span of one of man's closest relatives, too.

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