Sciencemag.org and the American Association for the Advancement of Science (AAAS) sought to answer this very question. Although there are many theories, they feel that size has quite a bit to do with a species’ longevity.
Larger species, like Elephants, live much longer in the wild than smaller ones, like Field Mice. Larger animals tend to have fewer predators and therefore evolve to be much heartier than the little guys. Smaller species must dash to eat, reproduce and just get by. Therefore, they tend to “burn out” faster.
However this is flipped in the case of cats vs. dogs. The average dog lives to about age 12, while the average kitty survives to age 15. According to scientists, this could simply mean that feline ancestors were better at avoiding prey than their canine counterparts, and therefore evolved with heartier bodies.
The massive variation of breeds within the canine species also seems to play a role with larger breeds having complications from faster growth rates.
Interestingly, feral and outdoor cats can have a lifespan as short as 2 – 3 years, showing that care from humans definitely makes a big difference!