Answer by Suzanne Sadedin, Ph.D. in zoology from Monash University:
Lifespan in general is determined by trade-offs between survival and reproduction. Wolves, the ancestors of dogs, can live 15-20 years, roughly twice as long as comparable-sized dogs. They start breeding in the wild no younger than 2 years old. They need to form pairs and establish a territory before breeding. Older wolves will often have help raising their pups from older juveniles who have not managed to mate or find territories. In contrast, most dogs can breed from 6 months to 12 months of age, and they don't benefit from having territories, pair bonds, or packs. Whereas wolves breed until they die, dog breeders will usually retire older females. So the whole life history of dogs is shifted to more of a "live fast, die young" style compared to wolves.
On top of that, artificial selection and inbreeding have created huge problems for dogs. Here's what dogs look like when they're not bred to conform to human expectations.
- Long snouts for breathing, panting, and eating. Shorten the snout, and you get squished teeth, reduced heat tolerance, and breathing problems.
- Broad skulls for brains. Several modern breeds have been reduced to what Temple Grandin calls "brainless icepicks." Reducing brain size doesn't just make animals stupid, it also contributes to neurological issues that can kill them.
- Pointy ears. They don't get ear infections. Make them floppy, and you create a nice environment for pathogenic bacteria.
- Strong hips. They're for walking and running. Breed them into a cool-looking German Shepherd slouch and you'll get hip dysplasia.
- Slender, lithe forms. Carrying extra weight puts increased wear and tear on all body systems.
Striving to breed to an idealized "type" while ignoring basic physiological necessities doesn't create a robust organism. That's how we get tortured monstrosities like English bulldogs, which can barely breathe without snorting and whose pups must be cut out of the mother's womb because she can no longer deliver them. Even seemingly harmless traits often bring a higher probability of serious health problems. White fur, for example, is often accompanied by neurological deficits ranging from subtle behavioral abnormalities to deafness or even early death. Generally speaking, working dogs have sustained longer lifespans because they're required to be physically fit to do their jobs. Show dogs are mostly just required to meet peculiar aesthetic requirements and be easily managed. The lethargy resulting from chronic health problems is actually a positive for champions who dominate the gene pool, even if it shortens their lives. (It's a bit like foot-binding, whose victims were prized as wives for their passive and mild behavior, which resulted from being crippled and in constant pain.)
Loss of genetic diversity also shortens lifespans. In a healthy population, essentially all individuals have several defective genes, but each defective gene is rare in the population as a whole. Each individual holds two copies of each gene, so in a randomly mating population, it's rare for an individual to have two defective copies. Usually, as long as the individual has at least one good copy, it will be fine. Health problems only arise when an individual has two defective copies. But when the population experiences a genetic bottleneck—that is, only a few individuals get to breed—any defects they have will spread to a large proportion of the population. That means that when these individuals mate, a large proportion of their offspring will carry two copies of the genetic defect and therefore be unhealthy.
Unfortunately, for the last century or so, dog breeders have actively pursued a misguided strategy of purifying breeds by demonizing cross-breeding and allowing only "champions" to breed. Healthwise, this is terrible. You can't eliminate all the subtle genetic problems that plague breeds by selective breeding. They arise faster than you can purge them. While some of the more severe defects have been reduced by conscientious breeders who test their dogs before breeding, this selective breeding further narrows the gene pool and has thus promoted many more defective genes that cause mild reductions in health and lifespan. We can't test for these defects, and consequently they are now prevalent throughout most nonworking breeds.
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