Most animals exhibit a peculiar trend that correlates their size and life expectancy. The delay in the onset of senescence seems to be positively correlated with an animal’s size. Humans live longer than cats, cats live longer than rats, and rats live longer than bees.
This is conspicuously true for mammals. Life expectancy rises with an increase in size, even though all mammals on average relish the same amount of heartbeats throughout their lifetime. This average has been estimated to be a billion and a half. However, this trend is turned on its head when it comes to our “best friends”, our beloved canine companions – dogs.
“Giant” dogs such as Great Danes are known to live for only 6-8 years, while retrievers live slightly longer, up to 10 -12 years (however this difference of two years is not merely “slight” in dog years). Compare that to petite dogs, such as Chihuahuas, who boast an average lifespan of 12-20 years.
What is the cause of this dissonance? Does Nature particularly detest larger dogs?
The answer seems to be deeply rooted in the intricate mathematical principles governing the physics of molecular mechanisms cavorting in their cells, coupled with the tragic repercussions of artificial or selective breeding. It comes down to the way they grow and the way we bred them.
Large dogs age ‘quicker’ than smaller dogs
Consider again the astonishing fact that mammals share roughly the same number of heartbeats throughout their life, yet exhibit a discrepancy in their life spans. The heart of an elephant and the heart of a shrew beat for the same number of times, but the reason an elephant lives longer than a shrew is that an elephant’s heart beats slower.
Mammal heartbeats seem to conform to a precise mathematical relationship between size and age. This is formally known as Quarterly Power Scaling. One theory is that Nature illustrates this biased leniency to ensure that larger animals do not wear out too quickly. Their cells are evolved to utilize metabolic energy and deteriorate more efficiently. A more detailed explanation of why we (and other animals) age can be found here.
The problem with dogs, specifically larger dogs, is that they age quickly. Their accelerated growth causes their adult life to, as Cornelia Kraus claims, “unwind in fast motion.”
Cornelia Kraus is the lead researcher of a study issued in the scientific journal “The American Naturalist” exploring the relationship between a dog’s size and its life expectancy. The study analyzed 56,000 dogs across 74 breeds and found that large dogs grow at an accelerated rate.
Consider the trend of growth in size from birth until the first birthday of different species. Humans grow 3 times as large in weight, poodles grow 20-fold, wolves 60-fold and Great Danes a staggering 100 times their initial weight! They grow very big, very fast.
Unfortunately, it is this rapid growth that leads to an exponential increase in free radical activity or molecular disruption — the root cause of senescence.
The price of selective breeding
One study reveals that mice, dogs and rats that were selectively bred for larger sizes exhibit reduced longevity. In the case of these animals, their large size is lucrative only in the short term. Sure, they attain maturity and virility overnight, but their rapid growth makes the diminishment of their ability to maintain this huge body more imminent, consequently reducing their forthcoming mental and physical life.
The trade-offs between these traits seem to be inevitable. Kraus’ study revealed one shocking fact that every 4.4 pounds of body mass reduces a dog’s life expectancy by about a month! And since they age at an earlier age, with each successive year, they grow more senile.
The increased random molecular disruption renders them more susceptible to predicaments like arrested development, musculoskeletal and gastrointestinal diseases, and particularly cancer. Because cancer is basically an outgrowth of cells, a large dog’s rapid cell growth significantly increases the chances of acquiring cancer.
This is why breeding is frowned upon by many dog lovers and purists. Large dogs might be appealing to some, but their suffering clearly outweighs our fastidious aesthetic demands. This is even more disturbing when we realize that dogs cannot vocalize their angst; they’re essentially mute.
Or maybe we simply aren’t listening.