Why do big dogs live shorter lives?

Whether you have a large or small , depending on your taste, there is one thing you should know: in the animal kingdom, the larger you are, the longer you live.

No one expects a house mouse to live more than two years, but an African elephant can live 65 years or more, not to mention the Antarctic blue whale, which can live to 90.

There is, however, one exception to this rule: the domestic dog. In this case, the rule is that the smaller you are, the longer you live. For example, the tiny Yorkshire Terrier, which weighs an average of three kilos, has an average life expectancy of 15 years. Compare that to an English Mastiff, which has an average lifespan of only eight years and can weigh 104 kilos. Why is that?

As reported by Psychology TodayBefore asking why different breeds of dogs age at different rates, it is worth looking at the process itself. The general consensus on how we age is based on the concept of wear and tear. This explains how we age, of course, but not why we age, which could be posed as an evolutionary question: “Since some species evolved to live longer than others, why didn't all species evolve to have a longer lifespan?

Since some species have evolved to live longer than others, why haven't all species evolved to have longer life spans?

The answer given by evolutionary theorists is that there are two different paths that any type of animal can “choose”, based on the idea that each species has only a fixed amount of biological resources. This seems a bit cruel, and perhaps that is why there is another alternative theory, that of self-preservation: here, the genetic programming provides for a slower growth rate, fewer offspring, and the maintenance of sufficient resources to build defenses against mutations and harmful conditions that will adversely affect the animal later in life.

Live fast, get fat, leave a pretty corpse

A new study collected data on 164 dog breeds. It included information on adult body mass, birth weight, age at which breeds reach 50% of their adult mass, litter size, average age at death, and, of course, typical causes of death.

The first examination of these data confirmed what others had shown, that large dogs tend to die younger than small dogs. It also showed that larger breeds of dogs tend to have more offspring. For example, the small Yorkshire Terrier has litters of three puppies on average. In comparison, the gigantic English Mastiff tends to have litters of nine puppies on average.

Large breeds are more likely to suffer from physical ailments once they reach their adult size, which is usually at age two.

The authors suggest that the development of dog breeds larger or smaller than the ancestral gray wolves actually occurred through genetic manipulation of the initial growth rate. The pressure to breed larger dogs resulted in a higher early growth rate. Unfortunately, this means that instead of devoting resources to general body maintenance and , genetic selection for larger dogs focuses on using body resources for rapid, early growth, instead of using them to strengthen the immune system, repair damaged DNA, and improve endurance. Data show that this results in larger breeds being more susceptible to physical ailments after reaching adult size, usually at age two.

In addition, the same mutations that increase the size of dogs also lead to an increased risk of . In summary, the data show that the larger the dog, the more likely it is to suffer from the disease. The researchers divided the causes of death into four categories: trauma or accident, infection, toxic exposure and cancer. Their analysis revealed that death from trauma, infection or toxin was not related to the size of the dog. Instead, the likelihood of dying from cancer steadily increased with the dog's weight.

The data show that the larger the dog, the more likely it is to develop cancer.

Does this mean that if you have a big dog, you should start thinking about an epitaph? Not quite. In fact, natural selection should allow larger dogs to develop better cancer defenses and live longer, at the expense of smaller litters.

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