By Dr. Becker
Tens of thousands of years ago, gray wolves were domesticated by humans through artificial selection (picking which animals to breed) and inbreeding.
Many experts in the field have long believed that not all the genetic changes that occurred as a result of domestication were desirable, however, little research has been done to evaluate the effects of such human interference on dog genomes.
Domestication Has Had a Negative Influence on the Fertility and Health of Dogs
For the study, the UCLA researchers collected complete genome sequencing data on 90 canines, including:
- 19 wolves
- 25 semi-feral "village dogs" from 10 different countries
- 46 domesticated dogs across 34 different breeds
The research team analyzed genetic variations among the different groups, looking specifically for markers that were deemed detrimental and would be extinguished in the natural world so as to not affect future generations.
The researchers learned that the dogs had approximately 115 more such markers than the wolves. They speculated the difference was probably due to "population bottlenecks" associated with domestication, coupled with aggressive breeding programs.
Population bottlenecks, which are temporary reductions in populations, lead to less diversity, which in turn can lead to harmful gene mutations that persist from one generation to the next.
The researchers also discovered that lack of diversity seems to have resulted in dangerous DNA changes in isolated groups of wolves as well, including Isle Royale wolves and Tibetan wolves.
The bottom line for domesticated canines? Dogs with less ability to reproduce than wolves, and a higher risk for certain disorders including asthma, arthritis, eye diseases, and certain cancers.
An Increase in Diversity in Dog Populations Can Reverse the Trend
The researchers suggest that increasing diversity in dog populations would allow the negative genetic changes to disappear naturally over time. Conversely, maintaining the current practice of selective breeding of dogs from small populations will lead to even more problems in the future.
Kirk Lohmueller, Ph.D., senior study author and assistant professor of Ecology and Evolutionary Biology at UCLA, observes that:
"… The use of small populations artificially bred for desired traits, such as smaller body size or coat color, may have led to an accumulation of harmful genetic variations in dogs."2
These types of variations, according to Lohmueller, may at some point lead to a number of different developmental disorders and other health risks.
He suggests that selective breeding programs - especially those designed to conserve rare and endangered species - may need to involve large populations to weed out detrimental genetic changes.
Selective Breeding and Canine Cancer
In order to be registered with the American Kennel Club (AKC) and other kennel clubs, a dog must be the product of other registered dogs, thereby insuring no new genes are introduced into the breed. That means every purebred dog is a relative, however distant, of other dogs of that breed.
Since the majority of owners of purebreds don't breed their dogs, the gene pool stays small. Professional breeders decide which dogs to breed based not on several generations of lifelong robust health, but on a specific set of physical traits they want to reproduce or "enhance."
The result is that in some breeds, the genes that increase the risk of cancer are reproduced in generation after generation of dogs. For example, the Mastiff group includes several breeds that commonly get cancer.
When researchers compare the DNA of Golden Retrievers with hemangiosarcoma and other breeds with the disease, the genetic abnormalities are different. Interestingly, Golden Retrievers in the U.K., where the breed originated, rarely develop cancer.
Their genes are significantly different from those of U.S. Goldens, which indicates the risk of hemangiosarcoma in American Goldens is the result, in part, of a fairly recent gene mutation.
Golden Retrievers are also at significant risk for lymphoma, and researchers studying cancer in Goldens have identified genetic alterations common to dogs with either type of cancer.
Selective Breeding and Brachycephalic Respiratory Syndrome
In the U.K., Pugs and Shih Tzus, both brachycephalic breeds, are increasing in popularity, which is exposing the tragic fact that many of these dogs have health problems so serious their owners aren't equipped to deal with them.
Due to their altered facial construction, all brachys have what is called brachycephalic respiratory syndrome to varying degrees. These pets often have very small, tightly scrolled nostrils that are so narrow it can be hard to move air in and out.
They also have an elongated soft palate, which is a flap of skin at the back of the throat that causes the characteristic snorting and other respiratory sounds often heard in brachy breeds.
Often the windpipe in these animals is very narrow in places, which leads to a condition called tracheal stenosis. This problem can predispose the animal to tracheal collapse, as well as problems with anesthesia.
Thanks to all these upper airway challenges, brachycephalic dogs often don't pant efficiently, which makes them prime candidates for heat stroke. Brachycephalic respiratory syndrome can be a progressive condition, so these animals can develop problems with the trachea or larynx over time.
At the Battersea Dogs & Cats Home in south London, the number of Pugs entering the shelter has almost tripled in just five years, and the number of abandoned Shih Tzus has also increased significantly. This has prompted the shelter to issue a warning that "poor breeding practices are compounding problems associated with the breeds' trademark squashed, short nosed faces, a feature popular with owners."3
Detrimental Selective Breeding is Unethical and Inhumane
Today's version of long-ago domestication efforts is the professional dog breeder who exaggerates certain physical characteristics over several generations of animals. It's become a shameful display of form over function, and the dogs pay the price.
That's why we now have legions of English Bulldogs who don't make it to age 7, German Shepherd Dogs who can barely stand up or walk, and several other breeds enduring debilitating deformities they were intentionally saddled with. According to the UCLA study authors:
"An associated cost of selection for specific traits in breed dogs is an enhanced likelihood of (inherited) disease. Considering that many modern breeds have been selected for unusual appearance and size, which reflects fashion more than function, our results raise ethical concerns about the creation of fancy breeds."4
As a veterinarian, I've seen first-hand the problems created when dogs are bred exclusively to achieve a certain look, without concern for their health, mobility, or quality of life. It is deeply disturbing to me, with all we know about the suffering these animals endure, that breeders persist in exaggerating their dogs' physical characteristics, even if it means sacrificing their health.