Different breeds of 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.