<P> The changes in care often required by an older dog may lead a non-experienced owner to release the animal to a shelter or rescue organization . Non-profit groups have sprung up in response to the growing need for senior dog rescue . </P> <P> The aging profile of dogs varies according to their adult size (often determined by their breed): smaller dogs often live over 15--16 years, medium and large size dogs typically 10 to 13 years, and some giant dog breeds such as mastiffs, often only 7 to 8 years . The latter reach maturity at a slightly older age than smaller breeds--giant breeds becoming adult around two years old compared to the norm of around 12--15 months for other breeds . </P> <P> The terms "dog years" and "human years" are frequently used when describing the age of a dog . However, there are two diametrically opposed ways in which the terms are defined: </P> <Ul> <Li> One common nomenclature uses "human years" to represent a strict calendar basis (365 days) and a "dog year" to be the equivalent portion of a dog's lifetime, as a calendar year would be for a human being . Under this system, a 6 - year - old dog would be described as having an age of 6 human years or 40--50 (depending on the breed) dog years . </Li> <Li> The other common system defines "dog years" to be the actual calendar years (365 days each) of a dog's life, and "human years" to be the equivalent age of a human being . By this terminology, the age of a 6 - year - old dog is described as 6 dog years or 40--50 human years, a reversal from the previous definition . </Li> </Ul>

Where does the term dog years come from