The question of potential animal longevity is complicated even among pets by the fact that many dogs and cats do not live an entire life span, because of trauma and euthanasia.
The Animal Medical Center in New York asked animal medicine experts from around the nation at what age they consider pets to be geriatric. The consensus of these experts can be used as a rough guide to life expectancy, within a year or two in either direction.
According to the study, animal size tends to make more difference than anything else.
For the giant-breed dogs, like the Great Dane, St. Bernard and Irish wolfhound, the figure is eight years or so.
For large dogs, like the German shepherd, Irish setter and Labrador retriever, the figure is about ten years.
Medium-sized dogs, like the beagle, Scottish terrier and cocker spaniel, might live twelve years.
The smallest dogs, like the toy poodle, Yorkshire terrier and Chihuahua, live about thirteen or fourteen years.
The same thirteen to fourteen years holds true for cats. There is little difference from breed to breed among cats, although the Siamese has long been rumored to live a little longer than average.
Individual pets can differ considerably from the norm, and as every pet owner suspects, there may be more exceptional pets than average pets.
Why size makes such a difference for dogs may have to do with the types of disease the larger breeds get or with chronic stress.
But genetics probably plays more of a role in health and life span than any other factor.
For each breed, susceptibility is absolutely tied to the genetics of the stock they started with, and some pure breeds have a tendency to live a very long time, probably because they have fewer inherited disease patterns.
As for mixed breeds, veterinarians speculate that they might do better than purebreds as a whole because with different types of stock there is less likelihood of getting matching genes for a defect from both parents.