Scientists say they've made a cool discovery about living longer. Mice genetically engineered to have lower body temperatures live substantially longer than normal mice, researchers say in tomorrow's issue of the journal Science. The newfangled mice are only slightly cooler than standard — just 0.5 to 0.9 degrees Fahrenheit (0.3 to 0.5 degrees Celsius), an effect that occurs only during waking hours. But the temperature drop significantly increases the rodents' life spans, scientists say. Altered male mice live 12 percent longer on average, while females live 20 percent longer than regular mice. 'We've demonstrated that a modest but prolonged reduction of core body temperature can contribute to increased longevity,' said lead study author Bruno Conti, a biologist and neuroscientist at the Scripps Research Institute in La Jolla, California. Which animal did the scientists use for their experiments?