Giant breeds like Danes, Saint Bernards, Newfoundlands, Mastiffs and Pyrenees live under 10 years. Smaller breeds live up to 15 years, why?
IANAVeterinarian, but have had 2 St. Bernards. Both died of things that a smaller breed either would not have had, or could have survived.
The first got cancer in a bone in her leg. The vet told us that a huge dog like that just cannot get around on 3 legs, and usually will not tolerate a prosthesis. The second one got a twisted stomach. The vet said large-chested breeds are more prone to this twisting since there is so much room inside for the stomach and intestines to move around in. By the time we knew what was wrong, it was too late.
I’m sure there are other ailments as well.
Someone will probably smack me down for this, but I think heart trouble is a big part of the equation. Larger breeds are more prone to it, as are larger people - the heart just has to do more work.
There are generalizations about lifespans by size, which should give you some idea about the probable size of the ancestor of domestic dogs - most of them, anyway. I’m not the one who invented them; I’m quoting what others have said about those.
First of all, the dog that has a healthy diet, regular exercise, and regular preventive health care (including teeth!) has the best chance of living to the limits of his genetic potential. All too many dogs in the U.S. die before their time because they’re overfed and under-exercised. It’s only in the last 25 years or so that vets began to urge good dental care (brushing teeth & scaling tartar, extracting teeth that are bad) on dog owners. I remember when one of my dog buyers told me about her vet’s orders on care for her dog’s teeth. It was quite a surprise, but when I thought about it, I realized the merits of it.
Another thing an owner can do that will help ensure a longer life for his/her dog is to spay or neuter. Dogs are vulnerable to cancer just like people are, and older ones seem to be particularly vulnerable to cancers in their gonads. Elderly female dogs are also very vulnerable to infections of the sex organs, which used to be known as “pyometra”, and can kill them. I don’t know what they call it now, but I do know that it’s nasty, and very smelly. They are also prone to breast cancers, if they haven’t been spayed. It’s not a guarantee that it will keep it from happening, but it surely helps! Since certain bloodlines of Great Danes (which I used to breed, and including “my” bloodline) are known to be more susceptible to cancer than average, I paid close attention when that was first announced.
Many of the ones that live longest are in the 12-45 pound range (as adults), but there are some which are larger, but still live longer.
I believe that many of the toy breeds that live very long lives do so because they are exposed to fewer of the risks that larger dogs are. They are rarely let outdoors unless someone is with them, and that protects them from many things that might otherwise make an early end of them. I double-checked, and the largest dogs in the toy group (where weight limitations were given) must weigh no more than 18 pounds. Most weigh less (if they’re at a healthy weight) than that. And nearly all of the toy breeds have been around for hundreds - even thousands - of years. During that period, the breeds’ genes have been cleaned, if I can put it that way, by the early deaths of those with less than rugged genomes.
The only dogs that people mostly cared whether they lived very long or not were the pets, the ladies’ lap dogs. The dogs with other purposes were expected to sometimes die young, because most of them were doing risky things. The owners of such dogs had more than a few of them, and so long as a particularly valued dog had reproduced, their deaths weren’t seen as the painful loss that they are to us because they were viewed as a class of livestock; no more, no less.
Small dogs, which I define as being in the size range of 20 inches, plus or minus a few inches, and weight range of 25-40 pounds, seem to have some of the longest lifespans, if they are well-kept, including observing the rules I set out at first. It’s not that unusual to hear of 14-15 year old dogs in this range, with good health care and protection from hazards (kept from running free, kept mostly or entirely indoors, etc.).
Most medium dogs, which I’m defining as being up to 25 inches, with weight ranges of 40-60 pounds or so, can be expected to live to 12 years or more, with good health, blah, blah
Large dogs are mostly defined as being up to 28 inches tall, and wieghing 60-100 pounds, usually have lifespans of around 10 years, with good health, blah, blah
Giant breeds are generally defined as being 30 inches or taller, and 100 pounds or more. Their weight varies wildly according to structure and ancestral purpose. The average lifespan of most of them is usually given as 7 years, with good health, blah, blah. It doesn’t seem fair that the large and giant breeds live shorter lives, and nearly everybody that’s ever thought about it tends to agree on that. One reason why it happens is, as I stated at the beginning, that I believe the ancestors of most of them were smaller than wolves, even quite a bit smaller.
The canid genome, i.e., that of dogs, wolves, foxes, dingoes, coyotes, and various other kinds of wild dogs, is said to be very “plastic”. What this means is that you can reshape the animal through selection of breeding animals. And that’s how we’ve got everything from Chihuahuas to Irish Wolfhounds and Saint Bernards.
I won’t go into why (we suppose) various kinds of exaggerations were done, as we don’t really know. We know what has been said, but very few breeds have been created de novo since literacy became widespread. We have some degree of knowledge about why Herr Dobermann created that breed, and ditto for a very few of the terriers (those interested may explore for themselves). I’m just trying to explain my reasoning about lifespans.
I stated at first why I think the toy breeds live so long. The problem is to account for why larger dogs live “so short”. I believe that part of the problem is the genetic plasticity. Those breeds which have been exaggerated in some way - all of the giant breeds have exaggerated size.
The Bulldog, Boxer, Pekingese and others have a head that’s been exaggerated by being squashed so that their lower jaws protrude. The gazehounds, or coursing hounds, were exaggerated by stretching them out and refining them to slender shapes that have better aerodynamics than other canids. Both of those selection factors made them bettter suited to running really fast for a very long time to chase prey animals for their humans. The scenthounds were selected for their superior sense of smell, with apparently little attention paid to other characteristics. And so on…
It seems to me that structural alterations impose a penalty on health and longevity. The health deficits may be in the form of genetic defects that came along with the qualities that the breed’s creators desired; this is frequently true in any form of selective breeding. However, if the alterations were accompanied or followed by stringent selection for physical soundness (as the gazehounds were), it would tend to offset the effects of the previous selection for non-natural functions.
Deficits to longevity, I believe, are a temporary thing - so long as they are viewed as something that can be eliminated in the long term (where long term is hundreds of years; i.e., I don’t think you or I will live to see the end result). Deficits that are the result of alterations of proportion (e.g., Bulldog-type remodeling of the head) are probably not solvable without genetic engineering (also a long time off).
I believe that other kinds of deficits can be eliminated if more breeders begin to study genetics, and more of them apply it across the board to all aspects of their breeding programs. I’m not too hopeful about this, because there are some bloodlines - in many breeds - that would have to be abandoned, and I don’t think many people are willing to do that.
Those two factors that I mentioned in the fifth paragraph (talking about toys) - exposure to hazards and age of the breed as a reasonably stable gene pool - are two of the most important factors, as I see it. A dog’s owner can control many of the risks; what you can’t control is the content of your dog’s genome, nor can you know how stable the breed’s gene pool is. Earlier this evening, I mentioned collies (another thread), and I said the breed here had been ruined. It took some decades, and an awful lot of bad breeding, to do that.
Herding dogs related to Collies - especially the several herding breeds that were developed in Australia - are physically and mentally sound, and have excellent longevity. It’s just a pity that they don’t make very good pets; they need too much exercise and - even more important - they need constant mental stimulation - i.e., a job of some sort. They’re great working dogs. They aren’t old breeds, but a lot of culling has taken place in their gene pools (at least part of which is likely thanks to the harsh environmental conditions in which many/most herding dogs in Australia exist). As I said, they have excellent longevity.
I mentioned near the beginning of this that Great Danes are one of the breeds in which you can find dogs that are susceptible to cancer. However, those bloodlines also have excellent longevity by comparison with most other Dane bloodlines. It’s not a contradiction, because the majority of them don’t get the cancers until they’re 9 or 10 years old - or later. In other words, it’s much the same as when a doctor says that any human being will eventually get cancer - if they live long enough. This is also true of specific bloodlines in some other dog breeds that have inferior longevity.
Another poster mentioned heart disease. It’s uncommon in Danes unless they’re overweight, but I believe it’s fairly common in some of the other giant breeds - mostly the ones that are heavy, like Saints or Great Pyrenees. The other poster also mentioned bloat (AGD, or acute gastric dilatation). That can attack any dog, although it’s rare in toys. It does tend to be more common in the “deep chested” breeds. I don’t think that’s the reason why, but I don’t think I want to expound two different (and unrelated) hypotheses in the same thread. 
My sister had a newfoundland who lived to be 13, way past the expected age for a dog of his size. One of the things her vet suggested kept him alive was that he was a small dog for a Newfoundland, only about 110 pounds. Compare that with his sire who was 140.
Our Saint died of bone cancer, like MLS said. She couldn’t be amputated, and those long long bones were (we were told) especially prone.
They’re Replicants.
“If you could see the things I’ve seen. Ships on fire off Sirius. Nebulas in Canis Minor…”
The layman’s explanation I got from my sister, who’s a vet, was that the original dogs were medium-dog size. When you breed to increase the size, you introduce a larger circulatory system than the heart was originally designed to maintain, so you wear the heart out faster.
Not nearly as thorough as Tygerbryght, and probably very simplified to avoid getting into an hours-long discussion.
One quibble: The original “dog” was a wolf, and wolves are on the larger size when compared to all the dog breeds, but still smaller than some of those mentioned in the OP.
Not all wolves are really large, and nobody knows in which part of the Old World that the ancestors of domestic dogs were found, if indeed all dogs do originate from the same ancestral canid stock, as opposed to being domesticated in multiple sites. Some claim that the modern day dog is descended from the Indian wolf or from the Arabian wolf, both of which are considerably smaller than the North American wolf that you no doubt have in mind when you think of wolves.
Unless you’ve got a time machine so you can take us back and prove to us that the wolf ancestor to dogs was a large canid, I suggest that you consider whether you can support your claim. In every part of the world where wild dog strains have arisen from animals gone feral, the size of the mongrels stabilizes in or near the 40-60 pound range, and greatly resembles the Australian dingo , or the Carolina dog. Just because the North American wolf is a big animal, in the 80-110 pound range across the northern part of the continent doesn’t mean it’s the same root stock. European wolves are smaller, and so are the wolves in Mexico and the southwestern U.S. I refer you to Bergmann’s Rule, which may help. The dingo, BTW, seems to be definitively descended from the Indian wolf (makes sense; it’s in the “neighborhood”).
I don’t know where Frelling Tralk’s sister got her information, but as a vet, I presume it was either from a textbook, a seminar, or from coursework back in vet school. It sounds right to me.
Where did I get mine? It just popped up when I thought about all the data I’ve collected over the years about dogs. Before I thought it all through, I used to think of the dogs’ ancestor(s) as big wolves, too. I don’t think that way anymore, because the evidence taken from a survey of longevity by breed weight/size and the kinds of diseases that are prevalent are both consistent with that conclusion.
I’m not really sure where she got her info. My husband and I were debating replacing our beagles with Great Danes when the beagles eventually pass on, and she brought up the Great Dane’s reduced lifespan vs. the beagle as a factor we might want to consider. This led to a conversation about the correlation between size and mortaility, at which point she gave me the spiel I mentioned above. She’s been a vet for almost 20 years and is chief of staff at her clinic, so I pretty much take what she says at face value and don’t grill her on sources. 
