A Dog’s Lifespan: The Effects of Size and Breeding
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If your dog is an English Mastiff, congratulations! You have one of the biggest canines on the planet. You can expect your 200-pound bundle of love to have a lifespan as long as 12 years. Conversely, if you like your pooches in smaller packages, you might get yourself a Chihuahua. Your tiny pet may top the scales at as little as six pounds, but could be part of your family for as many as 20 years!
So what explains the relationship between dog size and lifespan, with larger dogs having significantly shorter lives than their miniature brethren?
Why Size Matters
Large dogs tend to grow more rapidly than smaller dogs, which puts significant strain on their organs and physiological systems. This rapid growth leading to large size means that, in simple terms, big dogs have more cells in their bodies, and therefore more sites potentially vulnerable to cancer.
Rapid growth also produces more free radicals in the bodies of big animals, which can damage cells and contribute to aging. Larger dogs also have a higher metabolic rate, which means they burn more energy and put more stress on their systems. Because of their size, large dogs are more prone to joint problems like dysplasia, which can reduce mobility and quality of life.
Being small does not automatically mean a long life, however. Brachycephalic dogs (those with stubby noses, like Pugs and Bulldogs) tend to have reduced lifespans because of specific pathologies and respiratory problems resulting from their compact skull shape.
Consequences of Inbreeding
The breeding of dogs to select and perpetuate specific physical and behavioral traits has been taking place for several hundred years. The term “inbreeding” was coined from the Victorian practice of “breeding in” a specific feature:
A curled tail.
A coat pattern.
The ability to learn quickly how to herd sheep, for example.
Parent/offspring and sibling cross-breeding was the preferred technique to cause such traits to become fixed in a population quickly.
The challenge arises when traits not targeted (or desired) are passed from parents to offspring. Genes associated with harmful traits (like heart and hearing problems or orthopedic issues) can “hitchhike” with the genes being bred for. Consequently, the breeding process produces the unintended consequence of building harmful characteristics into the genetic profiles of purebred animals. In Chinese Shar-Peis, for example, the genetic variant that produces the breed’s distinctive wrinkled look is also implicated in Shar-Pei fever, a condition characterized by episodes of fever, inflammation and joint pain.
Outbreeding
Outcrossing breeding, also known as outbreeding, is the process of breeding two genetically different individuals to increase genetic diversity. It's a reproductive strategy used in animal and plant breeding to reduce the negative effects of inbreeding.
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Relationship Among Inbreeding, Body Size and Lifespan
A joint American-German research team investigated how inbreeding of dogs is related to body size and lifespan. In turns out that inbreeding and body size are strongly correlated. For the reasons noted above, larger dogs tend to have shorter lifespans than smaller animals. Moreover, the team discovered mixed-breed dogs live an average of 1.2 years longer than purebreds of the same size. When they looked specifically at data for Golden Retrievers, they found female dogs tend to outlive male dogs and outbred individual animals (those born of unrelated individuals) live longer than inbred dogs.
The bottom line of their research: occasional outcrosses in breeding populations (animals from separate genetic pools) can produce significant improvements in the health and the lifespans of dogs.
Post script: There is evidence that large size can also have negative lifespan effects in humans. Hall of Fame basketball player Larry Bird once commented that he didn’t expect to live to be 75. At six feet nine inches, he seemed to understand his size worked against him in the lifespan game. “I know there are a few of us who live a long time, but most of us big guys don’t seem to last too long,” he said. “I tell my wife all the time. ‘You don’t see many seven-footers walking around at the age of 75.’” Bird’s wife and fans are understandably upset by his actuarial projections.
The source for this blog (and an interesting read):
Yordy, J. et al, “Body size, inbreeding and lifespan in domestic dogs,” Conservation Genetics, 2019 Dec 5;21(1):137–148.
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