Why do big dogs have unusually short lifespans compared to small dogs?
Dogs come in an astounding range of shapes and sizes. The smallest breeds, like the Chihuahua and Yorkshire Terrier, have a maximum size of about 6-7 pounds, with many individuals weighing less than this. The largest breeds, such as some varieties of mastiffs, can weigh over 150 pounds even when not overweight.
This variety in size is greater than that of any other species. Dogs vary so much in size and appearance because of intensive selective breeding by humans. The genetics of body mass is complex, and an individual’s adult size is determined by combinations of many genes. However, scientists have identified a small number of genes which account for almost half of the variation in body size between dog breeds. The general effect of selective breeding on these genes has been to make dogs smaller than their wolf ancestors.
Based on Figure 1. Body size and lifespan in dogs in Selman et al (2013)
Dogs breeds vary in more than just body size
These genetic changes don’t only affect body size. Dog breeds differ in many other ways, from body shape and color to temperament and even healthspan and lifespan. The net effect of the many genetic changes that distinguish large and small breeds is that the smaller breeds tend to live longer than large and giant breeds. There is a direct, or linear, relationship between size and lifespan, so the larger a breed is, the shorter the average longevity of dogs in that breed.
These differences in lifespan can be very large. Some of the biggest breeds, such as English Mastiffs and Bullmastiffs, and Great Danes live only an average of 7-8 years. In contrast, the smallest breeds, such as the Chihuahua, Miniature Poodle, and Yorkshire Terrier, have an average lifespan longer than 12 years and commonly live well into their teens, almost twice as long as much larger breeds.
Genetics also influence health in aging dogs
In addition to overall differences in lifespan, large and small breeds tend to experience somewhat different age-related health problems, those there is much overlap. Large breeds, for example, are more likely to suffer from musculoskeletal disorders, such as arthritis. Cancer also tends to be a more common cause of death in larger breeds. Smaller breeds, on the other hand, are more likely to experience diseases of the nervous system or metabolic disorders, such as glandular diseases and kidney failure.
It is not completely clear why large dog breeds have shorter lifespan than small dog breeds. One possibility is that they may age faster, accumulating physiologic abnormalities and health problems sooner than smaller dogs. Alternatively, they may experience the same overall health impacts of aging as smaller breeds but be less able to cope with and survive these. Research and debate is ongoing to decide between these possible explanations.
Based on Figure 1. Lifespan negatively scales with adult body mass in male and female dogs in Fan et al (2016)
Loyal is studying size and its impact on aging
It is clear, however, that body size and growth rate are key factors, and this gives us hope that we could extend the lifespan and healthspan of large dogs by targeting mechanisms associated with these. We understand the genetic differences between small and large breeds, and we know a lot about the effect of these differences on growth rate and metabolism. Loyal is actively investigating therapies to target possible mechanisms underlying the shortened lifespan of large breeds, and we are planning clinical trials to test these in the near future
While owners can’t change the genetics of our dogs, there are many things we can do to give our pups the longest, healthiest life possible whatever their size. Keeping our dogs at a healthy weight, providing regular exercise and behavioral enrichment, and keeping up with regular preventative veterinary care can all help maintain good health and keep our canine companions happy and healthy.
Sources
Colin Selman, Daniel H. Nussey, Pat Monaghan, Aging: It’s a Dog’s Life, Current Biology, Volume 23, Issue 10, 2013, Pages R451-R453.
Fan R, Olbricht G, Baker X, Hou C. Birth mass is the key to understanding the negative correlation between lifespan and body size in dogs. Aging (Albany NY). 2016 Dec 8;8(12):3209-3222.
Galis, F., Van Der Sluijs, I., Van Dooren, T. J. M., Metz, J. A. J., & Nussbaumer, M. (2007). Do large dogs die young? Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 308B(2), 119–126
Kraus, C., Pavard, S., & Promislow, D. E. L. (2013). The Size–Life Span Trade-Off Decomposed: Why Large Dogs Die Young. The American Naturalist, 181(4), 492–505.
Rimbault M, Beale HC, Schoenebeck JJ, Hoopes BC, Allen JJ, Kilroy-Glynn P, Wayne RK, Sutter NB, Ostrander EA. Derived variants at six genes explain nearly half of size reduction in dog breeds. Genome Res. 2013 Dec;23(12):1985-95. doi: 10.1101/gr.157339.113. Epub 2013 Sep 11. PMID: 24026177; PMCID: PMC3847769.