Why Do Larger Dogs Have Shorter Lifespans?

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In the animal kingdom there is a general rule that the bigger you are, the longer you live. For example, a pet or laboratory reared house mouse, that weighs around 3 ½ ounces (19 g), is only expected to live about two years. An African elephant, who typically weighs more than 13,000 pounds (6000 kg), is expected to live 65 years or more, while the Antarctic blue whale, who weighs in at over 300,000 pounds (140,000 kg), typically lives to around 90 years of age.

There is an exception to this rule, however, and that is the domestic dog. Here the rule is that the smaller you are the longer you live. For example, the tiny Yorkshire Terrier, which has an average weight of seven pounds (3 kg), has a median life span of 15 years. Contrast this to an English Mastiff which weighs around 230 pounds (104 kg) and has a median lifespan of only eight years. This reversal of the usual size and longevity principle in dogs caught the attention of Jack da Silva and Bethany Cross of the School of Biological Sciences at the University of Adelaide in Australia and stimulated them to look for an answer to this conundrum.

Theories of Aging

Before we can address the question of why different dog breeds age at different rates we have to look at the process of aging. The general consensus for how we age is based on the concept of wear and tear. The longer an individual lives, the more repetition of routine activities causes organs to degrade, and the weaker our immune system becomes. Note that this theory explains how we age, not why we age, which is an important distinction. We can pose this as an evolutionary question, "Since some species have evolved to live longer than others, why haven't all species evolved for greater longevity?"

The answer that evolutionary theorists give is that there are two different routes that any type of animal might "choose", based on the idea that each species has only a fixed amount of biological resources. You might remember that Darwin referred to natural selection as being based on the survival of the fittest. For Darwin "the fittest" are species that produce the largest number of healthy offspring. This leads to a cruel picture of evolution since a "fit" species puts all of its resources into having lots of offspring, and caring for them until they can survive and reproduce on their own. After that, the older animal is not needed and can simply be left to deteriorate. Its resources of all been expended by its early rapid growth and reproduction and there are few reserves left to maintain its body. We can think of this as the Fast and Furious Theory (technically called the Disposable Soma Theory).

The second evolutionary course can be called, for want of a better name, the Self Conservation Theory. Here the genetic programming is for a slower growth rate, fewer offspring, and the maintenance of enough resources to build in defences against harmful mutations and conditions that will negatively impact the animal later in life.

Big Dogs Have "Fast and Furious" Lives

This new study is not based on new laboratory testing results, but rather collates data from a number of other sources. The information that they collected was data on 164 dog breeds. It included information on adult body mass, birth weight, at what age breeds reach 50 percent of their adult mass, litter size, average age of death and of course typical causes of death.

The first run through of this data confirmed what others have shown, which is that larger dogs tend to die younger than smaller dogs. It also showed that larger breeds of dogs tend to have more offspring. For example, the tiny Yorkshire Terrier has litters with a median number of three puppies. In comparison the giant English mastiff typically has litters with a median size of nine puppies. It is interesting to note that there was a recent report of an English mastiff (Lily) who gave birth to a litter of 21 pups. While the largest litter of Yorkshire terriers ever reported was 12 puppies (and the mother was described as "an oversized, 16 pound Yorkie").

Faster Growth and the Cancer Link

The authors suggest that the development of dog breeds larger or smaller than the ancestral gray wolves has really occurred through genetic manipulation of early growth rate. We are talking about selective breeding here. The pressure for breeding dogs of larger sizes has resulted in increased speed of early growth. Unfortunately this means that instead of devoting resources to general body maintenance and health, the genetic selection for bigger dogs emphasizes using bodily resources for speedy early growth, rather than using them to strengthen the immune system, repair damaged DNA and improve stress resistance. The data shows that this is reflected by the fact that the larger breeds are more susceptible to physical ailments after they reach their adult size which is usually at two years of age.

These researchers suggest that the same mutations that increase body size in dogs also cause an increased risk of cancer. Basically the data shows that the bigger the dog, the more likely it is to suffer from cancer. In retrospect, this association makes sense since most cancers result from mutations that occur during cell division. Since larger dogs have more cell divisions and each division has a certain probability of a harmful mutation, then the likelihood that a large dog will end up with a cancerous mutation is going to be higher.

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The investigators divided causes of death into four categories: trauma or accident, infection, toxic exposure, and cancer. Their analysis revealed that deaths by trauma, infection or toxin were not related to the dog's size. However, the likelihood of dying from cancer steadily increased with a dog's weight. In fact a 65-pound (30 kg) dog was 50 percent more likely to die of cancer than was a 7-pound (3 kg) dog.

There is hope for larger dogs, however it is over a longer term since natural selection should eventually allow larger dogs to evolve better cancer defenses and longer lives at the expense of smaller litters. This process could be sped up considerably if breeders of big dogs began to selectively concentrate on breeding those dogs with the longest lives and best immune systems.

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