Our furry friends come in all shapes and sizes, from tiny, handbag-sized Chihuahuas, to the house-trained horses that are Great Danes.
Unfortunately, it is somewhat known that the life expectancy of larger breeds tends to be much shorter than that of smaller breeds.
Many different explanations have been put forward for this, but scientists from the University of Adelaide believe they have finally found the real answer.
After analyzing the common causes of death in 164 dog breeds of varying sizes, they found that large animals are more likely to develop cancer.
They say this is because canine cancer’s natural defenses have not been able to keep up with the selective breeding of giant dogs.
Scientists from the University of Adelaide believe that larger dog breeds have shorter life expectancies because they are more likely to develop cancer at a younger age.
“When we analyzed these datasets, we discovered that larger dogs were more likely to die from cancer at a younger age compared to smaller dogs,” said study author Dr. Jacques Da Silva.
Larger dogs didn’t necessarily age faster than smaller breeds, but the research showed that as the breed’s average body weight increased, so did cancer rates.
We believe that the relationship between a dog’s body size and age may be due to an evolutionary delay in the body’s defenses against cancer, which cannot keep up with the rapid, recent selective breeding of larger dogs.
The Yorkshire Terrier, which can grow to a maximum of seven pounds (3.17 kg), can live up to 16 years.
But the English Mastiff, a gentle giant that can weigh 250 pounds (113 kg), tends to only live six to ten years.
for their study published in American naturalistthe researchers wanted to find out why there is such a huge and consistent difference in the lifespan of large and small dog breeds.
They analyzed published data on life spans and causes of death for different breeds from around the world.
Previous studies have attributed the life expectancy of larger dogs to a higher metabolic rate and increased risk of certain health conditions.
Some have also said that their body size causes them to age more quickly than small dogs.
However, the team found that this was not the case, and instead revealed that the larger the dog, the more likely it was that he would die of cancer at an early age.
The researchers wanted to find out why there is such a huge and consistent difference in the lifespan of large and small dog breeds
They say it’s because over time, humans have selectively bred dogs to be as big as possible so they can guard livestock or hunt big game.
Moreover, many of these breeds are now popular as pets because they look so cool, majestic, and intimidating.
But as a result, dog breeds have evolved to be of extreme sizes much faster than normal evolution would allow.
This means that the body’s defenses against cancer have not had the opportunity to develop in proportion to their size, so giant breeds are more susceptible to the disease.
The finding is consistent with a theory of aging known as “life history improvement” or “disposable soma.”
“This theory is based on the idea that if you invest most of your resources and energy in growth and reproduction, you can’t also invest them in cell repair and defenses against cancer,” said Dr. da Silva.
“In all living things, the emphasis is on early reproduction, even if it comes at the expense of maintaining and repairing the body and living longer.”
While this is sad news for large dog lovers, Dr. da Silva is optimistic that large breeds will eventually develop cancer defenses on par with their smaller counterparts.
“This may happen naturally or through selective breeding, where people focus on breeding large dogs with lower rates of cancer and thus increased longevity,” he said.
Most of the 400 or so dog breeds we know today only arose in the last 200 years.
Large dogs have not had time to develop better defense mechanisms against cancer to match their size.
“This can still happen, but it could have a reproductive cost.”
This is due to the life-history improvement theory, as a greater investment of energy in a crab’s defenses will mean it is less available to produce large litters.
The researchers hope that their findings could eventually benefit human health as well as dogs.
Dr da Silva concluded: “Dogs are a good model for studying aging in humans.
“Dogs, like humans in the industrialized world, live in an environment that tends to protect them from accidental and infectious causes of death, and so they are more likely to die from age-related diseases, such as cancer.”
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