It used to be that everyone believed that 1 dog year was equal to 7 human years.
More accurate research has shown, however, that aging in dogs isn’t quite so simple.
Aging in dogs is governed by size. So the theory that all dogs age 7 years for 1 human year just doesn’t fit.
That is to say that a chihuahua and a St. Bernard who are both 7 are at different places in terms of their life stage.
Why do larger dogs live shorter lives than smaller dogs?
When it comes to mammals of different species, the general rule of thumb is that larger mammals live longer than smaller mammals.
Interestingly enough, though, when it comes to mammals within the same species, the opposite is true.
So, while human beings tend to live longer than, say, dogs, St. Bernards live shorter lives than chihuahuas.
But, why? Let’s start by looking at animals of different species.
One reason that larger mammals live longer lives than smaller mammals is the result of environmental and ecosystem constraints.
For example, a blue whale has fewer predators than a field mouse and consequently it is going to face less population loss as a result of predation.
But this doesn’t particularly answer the question, does it? So, what does?
Metabolic Rate and Heart Rate
One popular theory in the scientific community says that metabolic rate and heart rate is where we find one of the biggest causative factors for lifespan differences between species.
Every animal needs to use up energy keep the body functioning at it’s most basic level ie: breathing, staying warm, etc. within a set period of time. (This amount of energy expended is referred to as the basal metabolic rate.)
One of the most energy-hungry processes in the body is maintaining body temperature. So the harder a body has to work to stay warm, the more energy it is going to require and the higher that animals BMR is going to be.
So, does a blue whale or a field mouse have to work harder to maintain it’s body temperature?
The answer is a mouse. How can this be, though, if a blue whale is so much larger?
It all comes down to math.
For the sake of simplicity, let’s work with hypothetical numbers here.
So, let’s say that the surface area of our blue whale is 2,400 cubic feet and the volume of our blue whale is 8,000 cubic feet. This means that inside 2,400 cubic feet of skin, there are 8,000 cubic feet of tissue, cells, fat, etc. The ratio of outside to inside is then 3:10. There is more whale “inside” than there is “outside.”
Obviously, our field mouse is going to be smaller, so let’s say that the surface area of our mouse is 24 cubic feet and the volume of our mouse is 8 cubic feet. This means that inside 24 cubic feet of skin, there are 8 cubic feet of tissue, cells, fat, etc. The ratio of outside to inside is then 3:1. There is more mouse “outside” than “inside.”
Now when the body loses body heat it is lost through the skin or the “outside,” this means that the larger the skin surface, the more heat will be lost. BUT, while heat is lost from the “outside” it is maintained from the “inside,” so the higher the volume of a creature, the more heat it is able to maintain over a longer period of time.
So which creature needs more energy to stay warm? We simply look at the ratios above.
A lower ratio of surface area to volume (or more inside than outside) as in the case of our whale, means that there is more heat maintained than heat lost by the body.
A higher ratio of surface area to volume (or more outside than inside) means that there is more heat lost by the body than maintained.
So, the smaller creature – our mouse, loses more body heat at a faster pace than our elephant.
This means that our mouse must expend more energy to stay warm than our elephant so it has a higher basal metabolic rate based on mass.
Since metabolic activity leads to the production of free radicals which damage cells, this is one way that increased metabolism can contribute to a shorter lifespan.
Additionally, if our mouse has higher energy requirements based on its size, it must spend a good majority of its life eating to provide the constant need for energy.
Now, higher energy requirements and increased need to generate heat mean a more active creature which means a faster heart rate. This faster heart rate delivers increased levels of oxygen to fuel the cells of a faster-moving body.
Now this increase in heart rate has an effect on the lifespan of our fieldmouse.
Researchers estimate that the heart of various species tends to have a lifetime beat capacity of a million and a half beats. This means that the mouse – with it’s faster heart rate – is going to use up all of those beats at a much faster pace than our blue whale.
Our whale, however, has a slower metabolic rate, so its heart rate is slower, but it must also be powerful. So while the blue whale’s heart may beat slower, it beats with much more force in order to ensure that oxygen is delivered throughout its immense body. Since these stronger beats occur more slowly, though, the lifespan of the blue whale is longer.
Another influencing factor in longevity, according to more recent research on the lifespan of large and small animal species is cell structure and efficiency.
Larger creatures have more efficient cells than smaller creatures.
Why? Because it is much more difficult to power a larger body than it is a smaller one.
Think of how far nerve networks and blood vessels must stretch to maintain normal functioning in a blue whale. Now think about the field mouse.
If the cells in the body of the blue whale were equal in size and functioning ability to the cells in the body of the field mouse, the blue whale would live a much shorter life than the mouse.
So nature gave the larger creatures a more efficient internal network that requires less fuel to function.
This internal network pushes bodily systems like the heart to pump more blood with each beat so that it does not have to work fast to pump blood around that massive body…it just has to work hard. Which is great for the blue whale because if it had to power a rapidly beating heart like the mouse, it would have to eat incredible amounts of food every day!
Another theory in the scientific community links brain size to longevity among species.
Recent research suggests that animals with large brains in comparison to their body size live longer. This theory builds upon Darwin’s idea of survival of the fittest and the ability to adapt. These animals with larger brains are more able to adapt to changes in their environment and survive by avoiding predators.
The problem with this theory, however, is that currently, researchers can’t tell whether increased brain size is due to increased longevity among species or vice versa. All they can say for sure is that there is a positive correlation between animals with large brain size for their body and increased longevity.
Back to Dogs
So what does all of this have to do with dogs anyway?
If you can remember all the way back in the beginning of this article, I mentioned how the trend seen in longevity and size in different species is inverse to that of animals within the same species.
So, let’s get back to our Saint Bernard and our Chihuahua.
Why does the Chihuahua live longer than the Saint Bernard?
Why is it that the interspecies rule is reversed?
Predation certainly doesn’t come into play. Heart rate and metabolism are different between large and small breeds, but certainly nowhere near as drastic as the difference between a blue whale and a mouse. Brain size to body size ratio isn’t much of an issue…so what is causing the longevity differences between breed sizes?
Rate of Aging
As with interspecies aging studies, there are a number of theories in play, but the most promising are related to the aging process itself.
Researchers suggest that larger breed dogs experience a faster aging process than smaller breeds which results in a shorter lifespan for bigger dogs.
A research study at American Naturalist found that rather than aging sooner than smaller dogs, larger dogs age at a more accelerated rate once adulthood begins. Just how quickly does increased size affect age acceleration? The numbers come down to around 1 month lost per additional 4.4 lbs of body weight.
But why does the speed of aging increase with age in larger dog breeds?
Unlike the mouse which has a much faster metabolism and heart rate than the blue whale, the Great Dane does not have drastically different vitals to the Chihuahua so faster aging isn’t a result of increased energy use or faster beating of the heart. So what could it be?
There is no definitive answer as of yet, but one answer may lie in the accelerated growth rate in larger breeds.
Could it be that our larger dog breeds are experiencing faster aging and early death as a result of “faults” found at increased rates in faster growing bodies? There is some evidence that points in this direction, but until further research studies have been conducted we can’t know for sure.