Arguably, the most effective 'engineer' of long life spans and anti-aging mechanisms thus far has been evolution by natural selection. Senescence theory predicts that organisms with effective protections against mortality will evolve delayed reproduction, efficient mechanisms of long-term somatic maintenance, and slow aging. A substantial body of literature supports this prediction. Vertebrate species now known to have remarkably slow or negligible rates of aging-related fitness declines in nature include certain fishes, turtles, birds, and flying, gliding or subterranean mammals. While obviously not as well-developed in many respects as standard laboratory animal models, a number of these species have great potential as tools for understanding the metabolic and molecular correlates of delayed aging. Birds, for example, age slowly for their small body sizes, high metabolic rates and body temperatures; data gathered to date in our laboratory and others suggest they accumulate products of glycoxidation more slowly and defend against oxidative stress more effectively than do similar-sized mammals. With judicious use of the comparative method and careful selection of species, the feasibility of productive collaborative studies between biogerontologists and zoologists studying longevity in natural populations is growing. Studies of naturally long-lived animals hold great promise for revealing common aging and anti-aging mechanisms in a wide range of species.