The lifespan of a canine is far too fleeting; ask anyone who has lost of loved companion. In terms of mammal longeivity the lifespan of a dog is relatively short. Although not as short as some mammals; the pocket gopher is the shortest lived with a life expectancy of only 20 months. Some whales and tortoises can live up to 200 years. It is surprising that within the dog species itself there are large differences in longeivity of individual breeds.
The breeds with the shortest lifepans are the Dogue de Bordeaux, Bloodhound and Irish Wolfhound with the average expectancy being 6-7 years. And the longest living breeds are the smaller guys; the chihuahua, Tibetan Terrier and Border Terriers. So in the big picture, some of the largest mammals live the longest and the smallest have the shortest time here on our earth. So why then does it seem that the larger the dog the shorter the lifespan?
There are alot of factors that play a part in lifespan. The skeletal system is a major one and those of very large and giant breeds suffer wear and tear on their joints early on. The body of a very large breed of dog has a great need for oxygen which is pumped through the body via the heart. This puts a great strain on the heart. The small breeds just don't have the stress on their body that the very large and giant breeds do.
There is no other species that has as many variations as the dog. There are many differences in coat type, size and structure. Dogs as a whole evolved from the wolf, but from there we humans designed what we wanted in a dog. It is the morphing that gave our dog more or less life so to speak. The wolf lifespan is approximately 13 in the wild and markedly more in captivity. So to generalize the average lifespan our dogs started off with was somewhere around 13ish.
All of my dogs have left me at the age of 13. This seems to be a good age for our dogs to live to. I have known several giant breeds who were more the large size, not so giant and they lived a good long 12 years. Sizing down seems to be a good idea for our dogs. With great medical care and proper nutrition; we may be able to increase these years if we don't grow'm so big.