By Katherine Bourzac
Scientists know that rats fed a nutritionally adequate diet of 30 percent fewer calories than normal tend to live 30 percent longer. Similar effects have been observed in organisms from yeast to fruit flies but not, as yet, in primates. At the University of Wisconsin, researchers led by Richard Weindruch have been testing a calorie-restricted diet in a group of rhesus monkeys since 1989. The monkey in this photograph, a dieter, is 25 years old.