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.

It's too early to know the answer for certain. The monkeys in Weindruch's lab are only now growing elderly. And with 80 percent of them still alive, "there are too few deaths" to indicate whether the animals on the restricted diet will live longer, says Weindruch. But one thing is already clear: the monkeys on the restricted diet are healthier. Roughly twice as many of the monkeys in the control group have died from age-related diseases, and perhaps most dramatically, none of the animals on the restricted diet have developed diabetes, a leading cause of death in rhesus monkeys.