This is a really good question. I don't know any stats on this, though in my experience female pure-breeds do live longer than male pure-breed dogs. I'm not even sure how you can find out, we don't normally collect stats on dogs and cats or birds for that matter, especially since there are within-breed differences (a German shepherd will live less than a small terrier). The supposed reason for longevity is the female body is made for pregnancy (hormones!), so it's able to withstand more stress. That said, dogs and cats give birth to many young at once, and then breast feed them all, and that can be really stressful on the organism in the long run. Additionally, we do sterilize our pets usually so the female/male hormones don't even play a role anymore. And, there are differences in the households of pets - table-food, skinny dog, fat cat, mean owners, lots of children, adopted vs puppy, etc etc.