<P> A major study of dog longevity, which considered both natural and other factors affecting life expectancy, concluded that: </P> <Dl> <Dd> "The mean age at death (all breeds, all causes) was 11 years and 1 month, but in dogs dying of natural causes it was 12 years and 8 months . Only 8 percent of dogs lived beyond 15, and 64 percent of dogs died of disease or were euthanized as a result of disease . Nearly 16 percent of deaths were attributed to cancer, twice as many as to heart disease . (...) In neutered males the importance of cancer as a cause of death was similar to heart disease . (...) The results also include breed differences in lifespan, susceptibility to cancer, road accidents and behavioral problems as a cause of euthanasia ." </Dd> </Dl> <Dd> "The mean age at death (all breeds, all causes) was 11 years and 1 month, but in dogs dying of natural causes it was 12 years and 8 months . Only 8 percent of dogs lived beyond 15, and 64 percent of dogs died of disease or were euthanized as a result of disease . Nearly 16 percent of deaths were attributed to cancer, twice as many as to heart disease . (...) In neutered males the importance of cancer as a cause of death was similar to heart disease . (...) The results also include breed differences in lifespan, susceptibility to cancer, road accidents and behavioral problems as a cause of euthanasia ." </Dd> <P> In general, dogs age in a manner similar to humans . Their bodies begin to develop problems that are less common at younger ages, they are more prone to serious or fatal conditions such as cancer, stroke, etc., they become less mobile and may develop joint problems such as arthritis, and in old age often become less physically active . Additionally, they become less able to handle change, including wide climatic or temperature variation, and may develop dietary or skin problems or go deaf . In some cases incontinence may develop and breathing difficulties may appear . </P>

Where did the concept of dog years come from