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 emotionally, my rational mind wanted to remind her that if you get a large dog, you should brace yourself for the fact that there is a high probability that your dog will die at a young age. It is simply a fact that big dogs live much shorter lives than small dogs. Being a psychologist, I knew that rational analysis was not what she needed at this moment—nor did she need me to suggest that if her next dog was a Miniature Poodle, she might expect it to live twice as long as Frederick. So I comforted her as best I could by reminding her that Frederick’s life, although 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 a larger one is counterintuitive. When we look at the longevity of all mammals, we find that, generally, the bigger animals live longer. For example, elephants have a lifespan of around 70 years, compared to the lifespan of a mouse, which is only two years. To go to an extreme limit, we could look at the bowhead whale who weighs in at around 65 tons and is 60 feet long. Scientists estimate that these whales have a lifespan of up to 200 years.
“The strange quirk is that while bigger species of mammals live longer than smaller ones, large size is not advantageous if we confine our analysis to one species at a time.”
The strange quirk is that while bigger species of mammals live longer than smaller ones, large size is not advantageous 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 longer lives. This is certainly the case in dogs. Data suggests that this is even true in humans since larger people have shorter life spans. When we are talking about size, body mass is more important than height alone
Great Danes, like Frederick, are a 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, defined as all dog breeds expected to weigh 88 pounds (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 six to eight years. Only 17 percent of dogs of this breed will ever make it to 10 years of age.
The English Mastiff is one of the heaviest of dogs. A typical male can weigh 150 to 250 pounds. Their lifespan is around seven 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, accumulating information about more than 56,000 dogs of 74 different breeds.
Although the research report contained 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 demonstrated how powerful the effect of body mass was, concluding that for every increase of 4.4 pounds (two kilograms) in body mass, we can expect a corresponding loss of approximately one month of life expectancy.
“Within any single species, we find that the trend is reversed: it is the smaller animals that have the longer lives.”
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. To get to a final weight of around 200 pounds, a dog must do a lot of growing. Certainly, the growth rate must be many times greater than what is required for a Yorkshire Terrier, who will ultimately weigh only eight pounds. So, the English Mastiff grows quickly—the growth rate of English Mastiff puppies may be over five pounds per week. 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 don’t explain why a rapid rate of growth should result in a shorter lifespan. A hint 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.
The search for a mechanism that can result in aging and earlier death led these Norwegian scientists all the way back to a genetic level (although not to specific genes). They ended up focusing their attention on something called telomeres. Our genetic material, 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 it might even die. The erosion of telomeres over time has been linked to aging and disease risks, including things like 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 (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 the point where their body will begin to fail, starting at the cellular level.
Of course, there are outliers in any group—certain dogs that will outlive their breed’s average life expectancy. How, exactly, to make sure your dog lives as long as possible remains a subject of much study. Initiatives like The Dog Aging Project are bringing together dogs, owners, veterinarians, researchers, and volunteers to understand how genes, lifestyle, and environment influence aging. The goal is to help pets and people increase “healthspan,” the period of life spent free from disease. If you’d like to become a citizen scientist, you can nominate your dog to participate at dogagingproject.org.
This article originally appeared in the award-winning Modern Dog magazine. Subscribe today!
