It is common knowledge that large dog breeds tend to have shorter lifespans (generally around 10-13 years; for the wolfhound and the great dane, as low as 7-8 years) than smaller dog breeds (some of which can go on for as long as 15-20 years). In fact, one study found that within 74 different breeds, dogs lose about one month of life expectancy per 4.4 pounds (2 kg).
So why does this happen?
The short answer is that the answer is not definitively known. There are currently a number of theories, none perfectly satisfying, and the fact of the matter is that there are probably a number of different reasons why large dogs tend to live shorter lives than small dogs.
Let’s start with probably the most generally accepted explanation for this phenomenon in science today.
First, it should be noted that dogs are not the only species that has this trend. In general, smaller individuals within a species have a tendency to live slightly longer than the larger ones. This is even true of humans (but don’t worry if you’re very tall- remember, these data are generalizations across massive samples of people and there are plenty of other factors that affect longevity).
Dogs have some of the most extreme differences in body form within a single species (as a matter of fact, not only are all dogs in the same species, they’re a subspecies– of the wolf). It’s not a big surprise that within these different forms ages can vary so drastically.
Then why is increased size related to shorter lifespans*? Well, one theory states that it is because the larger animals in a species have more growing to do in the same amount of time as smaller ones do. This means that their bodies have to work harder to build that extra mass- obtain more energy, create more cells- and this leads to faster aging.
The authors of the dog study I mentioned earlier pointed out that many large breeds are more susceptible to cancer than smaller ones are. and Cancer is caused by abnormal cell growth- the rapid growth of large dogs may be what makes them more vulnerable to cancers.
However, it should be noted that this explanation isn’t perfect. There are a number of exceptions to the higher weight = shorter lifespan correlation. Case in point, some of the longest-lived dogs ever are medium-sized dogs, not small dogs.
In fact, even though the trend in dogs is that lifespan decreases as weight increases, there is a great deal of variability.
Now, if you look at the list of the world’s oldest known dogs, you’ll notice something else: well over half of them are cross-breeds or mutts. This is not quite an anomaly- on average, mixed-breed dogs tend to live longer than purebred dogs. The reason may be that mixed-breed dogs are protected from genetic disorders caused by inbreeding or overbreeding.
Larger breeds of dogs may also be more susceptible to genetic disorders than smaller dogs- at least, the disorders that cause more rapid death or euthanasia.
For example, one of the major reasons owners put their dogs down is because of difficulty or inability to walk. Large dogs, with their higher weights, have a harder time with genetic disorders such as hip and elbow dysplasia, luxating patella, and arthritis.
They are also more likely to die from gastrointestinal problems and developmental disorders than small dogs, likely due to stress on organs from rapid selective breeding for size.
(On the flip side, smaller dogs are more likely to die from endocrine problems than larger dogs. It’s just that the endocrine problems tend to come up later in life than the others.)
Note that overbreeding and inbreeding can have detrimental effects on breeds of any size: just look at the English bulldog, a medium-sized dog at around 40-50 pounds with an average lifespan of 6-7 years.
An interesting point of fact- while few studies have compared the lifespans of large and small mixed-breed dogs, at least one has found that large mixed dogs still have a shorter lifespan than small mixed dogs (albeit with a smaller difference than between large and small purebreds). So, again, no one factor can explain this phenomenon.
I know this is a slightly depressing topic. Here’s a fact that may cheer you up: thanks likely to modern veterinary science, overall dog lifespan is on the rise right now. Fido lives!
*Increased size WITHIN a species is correlated with shorter lifespans. Between species, however, increased size is related to longer lifespans. Compare a mouse’s lifespan to an elephant’s.
Fleming, J. M., Creevy, K. E., & Promislow, D. E. L. (2011). Mortality in North American Dogs from 1984 to 2004: An Investigation into Age‐, Size‐, and Breed‐Related Causes of Death. Journal of Veterinary Internal Medicine, 25(2), 187-198.
Kraus, C., Pavard, S., & Promislow, D. E. (2013). The Size–Life Span Trade-Off Decomposed: Why Large Dogs Die Young. The American Naturalist, 181(4), 492-505.
Patronek, G. J., Waters, D. J., & Glickman, L. T. (1997). Comparative longevity of pet dogs and humans: implications for gerontology research. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 52(3), B171-B178.
List of Oldest Dogs via Wikipedia
Dog Longevity website by Dr. Kelly M. Cassidy