One of the earliest recorded assertions that large animals live longer than small ones comes from Aristotle in 350 BC. This has subsequently been supported by more rigorous scientific study. As this graph shows, there is a positive correlation between body size and lifespan in many different groups of mammals.
Why bigger is better for lifespan, however, is less clear. Aristotle’s theory that larger animals contain more fluid and heat hasn’t held up very well. More scientific hypotheses about the relationship between metabolic rate, production of free radicals, or ecological niche and longevity have proven more robust, but there is no absolute explanation for the relationship between size and longevity.
A species’ maximum lifespan is influenced by many factors, some intrinsic and some extrinsic, and the general relationship between longevity and size can be exaggerated or weakened by the net effect of many variables. As a broad generalization, though, larger species do tend to live longer than smaller ones.
This relationship often gets turned on its head, however, when we look at the effect of size on lifespan within species. It is well established, for example, that larger dogs age faster and have shorter lives than smaller dogs. One study showed that body size alone explained almost 50% of the variance in life expectancy between breeds.
So why do old dogs age faster and die younger than small dogs? Again, there is no single, simple answer. However, genetic differences are probably the key. When the degree of inbreeding is added to body weight, these variables explain almost 95% of the difference in lifespan between breeds. The enormous variation in body size across breeds is due to differences in only a few genes, and these genes contribute to shorter lifespans as well as larger size. The Healthspan Study and other research we are conducting at Loyal will hopefully one day allow us to clearly understand the genetic differences between breeds that influence lifespan and to intervene to give dogs and their human companions more time together.