Why Do Large Dogs Have Shorter Life Spans Than Small Dogs?

Source: Painting by Edwin Henry Landseer (1839) — Public Domain

I found myself trying to comfort a grieving woman. "He was so young," she sobbed. "He hadn't even reached his seventh birthday. Why did he have to die?" Her distress was not over the loss of a child but rather over the passing of her beautiful harlequin Great Dane, Frederick, who had died of a cardiac problem. Although I could share in her sorrow, the rational part of my mind was telling me that I should remind her that if you get a large dog, like a Great Dane, you should brace yourself for the fact that there is a high probability that your dog will die at a young age. It is just a fact that big dogs live much shorter lives than small dogs. Of course, being a psychologist I knew that rational analysis was not what she needed at this moment, nor did she need advice which might suggest that if her next dog was a Miniature Poodle she might expect it to live twice as long as Frederick had. So I comforted her as best I could by reminding her that Frederick's life, though short, had been a happy one.

At first glance, at least to those of us who study animals, the notion that smaller sized creatures might be expected to have a longer life than larger ones is counterintuitive. When we look at the longevity of all mammals we find that it is generally the bigger animals who live longer. For example, elephants have a lifespan of around 70 years; the lifespan of a mouse is only 2 years. To go to an extreme limit, we could also look at the bowhead whale, which weighs in at around 65 tons and is 60 feet long. Scientists estimate that these whales have a lifespan as long as 200 years.

The strange quirk is that while bigger species of mammals live longer than smaller ones, large size is not an advantage if we confine our analysis to one species at a time. Within any single species we find that the trend is reversed, and it is the smaller animals that have the longer lives. This is certainly the case in dogs. Data suggests that this is even true in humans, since larger people tend to have shorter life spans. (In this case, it seems to be that body mass is more important than height alone.)

Great Danes like Frederick are good example of what the situation is for large dogs. Let's start off by noting that the most recent research suggests that the average life expectancy of a medium-sized dog is 13.6 years. Great Danes are generally classified as "giant" dogs, which are all of the dog breeds that are expected to weigh 88 pounds (or 40 kilograms) or more as adults. Great Danes clearly fit into this group since they weigh anywhere between 120 to 200 pounds. They also have very short lifespans, averaging around 6 to 8 years. Only 17% of the dogs of this breed will ever make it to 10 years of age.

Irish Wolfhounds are perhaps the tallest of all of the dogs, with males having an average height of 34 to 35 inches at the shoulder. They typically weigh around 140 to 180 pounds. Their average lifespan is around 6 to 7 years and only about 9% of these dogs will make it to 10 years of age.

The English Mastiff is one of the heaviest of the dogs and a typical male can weigh 150 to 250 pounds. Their lifespan is around 7 years of age and less than one quarter of these dogs will make it to 10 years.

A research team headed by Cornelia Kraus, an evolutionary biologist at the University of Göttingen in Germany, decided to see if they could determine why large dogs had shorter lives. To do this they collected data from veterinary hospitals. Eventually they had information about more than 56,000 dogs of 74 different breeds. Their findings were published in American Naturalist.

Although the research report contained some complex statistical analyses, the investigators were able to summarize their conclusions quite simply.

"Large dogs age at an accelerated pace, as though their adult life is running at a faster pace than small dogs. Hence, a first answer to the question of why large dogs die young is that they age quickly."

They then added an interesting set of numbers to show how powerful the effect of body mass was. Specifically they concluded that for every increase of 4.4 pounds (2 kilograms) in body mass we can expect a corresponding loss of approximately 1 month of life expectancy.

It seems as though these large dogs have lives that are unwinding in fast motion. To see how that might work let's return to the English Mastiff that we mentioned earlier. To get to a final weight of around 200 pounds that dog must do a lot of growing. Certainly his growth rate must be many times greater than what is required for a Yorkshire Terrier, who will ultimately be only 8 pounds in weight. So the English Mastiff grows quickly and in some weeks these dogs may gain over 5 pounds of weight. That requires an awful lot of cell division and cell growth — in other words a much faster pace of living with the body working harder simply to reach its normal adult size.

Nonetheless, the findings of the German team of investigators doesn't explain the actual mechanism that causes the more rapid aging in the larger dogs. We still need to know why a rapid rate of growth should result in a shorter lifespan. A hint as to what may be going on comes from a research team headed by Thor Harald Ringsby of the Department of Biology at the Norwegian University of Science and Technology in Trondheim, Norway. Their report was published in Proceedings of the Royal Society.

The search for a mechanism that can result in aging and earlier death in animals led these Norwegian scientists down to the genetic level (although not to the genes themselves). They ended up focusing their attention on something called telomeres. Our genetic material, specifically the DNA, is stored in bodies called chromosomes. Telomeres are the protective caps on the ends of these chromosomes. In young humans, for instance, telomeres are about 8,000 to 10,000 nucleotides long. However, over time they grow shorter. This comes about because the telomeres get reduced in size with each cell division. This is important, because when the telomeres reach a critically short length the cell stops dividing and might even die. The erosion of telomeres over time has been linked to aging and risk of disease (including cancer).

So what seems to be happening is that large dogs have to run their metabolism and their growth mechanisms at a high rate of speed. The cells divide quickly to allow the dogs to grow to their final size (which is based on the characteristics of their breed). Unfortunately, each cell division is going to clip off a bit of the length of their telomeres, bringing them closer to a state of affairs where their body will begin to fail, starting at the cellular level.

Whether something can be done to slow the aging process in larger dogs is not certain, however there are some promising research findings that provide some hope. (If you want to know more about the life expectancy of your particular breed of dog click here.)

Stanley Coren is the author of many books including: Gods, Ghosts and Black Dogs; The Wisdom of Dogs; Do Dogs Dream? Born to Bark; The Modern Dog; Why Do Dogs Have Wet Noses? The Pawprints of History; How Dogs Think; How To Speak Dog; Why We Love the Dogs We Do; What Do Dogs Know? The Intelligence of Dogs; Why Does My Dog Act That Way? Understanding Dogs for Dummies; Sleep Thieves; The Left-hander Syndrome

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