The strongest evidence in favor of the idea linking early life growth rate and aging comes from the analysis of size and mean life span among dog breeds. The squared correlation coefficient indicates that more than half of the life-span variation among breeds is explained by the factors-virtually all genetic-that modulate interbreed differences in body weight. The differences in longevity among dog breeds of different sizes seemingly reflect not only survival per se but also real differences in aging rate in multiple degenerative conditions. Diseases appear earlier in larger breeds compared to smaller ones. Indeed, the age at which clinical veterinarians consider dogs to require "geriatric" care ranges from 6 to 9 years in giant breeds to 9 to 13 years in smaller breeds. Life span of rodents can be extended by at least two classes of nutritional manipulations, those that diminish total caloric intake and those that restrict the levels of essential amino acids, such as methionine or tryptophan. © 2005 Elsevier Inc. All rights reserved.