If you’ve ever noticed your friend’s tiny Chihuahua still bouncing around well into its teens while your neighbor’s Great Dane started slowing down after just a few years, you’re not imagining things.

Q: Among the dogs I've had over the years, my small dogs lived longer than my large dogs. Is this generally true? If so, why? A: Yes, it's true, though no one is completely sure why. When you think ...