This monkey is part of a control group and a bit over 26. (Credit: Kevin Miyazaki/Redux)

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. Though it's too early to make strong claims about the effects of calorie-restriction on these animals, the preliminary results suggest that the dieting monkeys are healthier as they enter old age.

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