Brennen McKenzie, MA, MSc, VMD

Dogs age physically and functionally as they get older, but these changes are not solely due to the passage of time or the body “wearing out”. 

There are two broad ways to look at the causes of aging: why should physical aging happen at all, and what specific mechanisms lead to the changes we see in aging dogs? The first question is often answered in terms of evolutionary and developmental biology and why it can lead to animals experiencing deterioration and death with age. The second question focuses more on the biological events that happen in an individual body that lead to this deterioration, and what we might do to change these events and delay aging.

Many theories have been proposed for why animals experience aging at all. The oldest notion is simply the “wear and tear” hypothesis. It seems logical that our physical bodies should wear out over time. It was once thought that each individual only had a limited number of heartbeats before the heart became too damaged to function, or that too much activity caused cumulative damage to joints, resulting in arthritis. 

However, living animals have systems for repairing and regenerating tissue. We now know that exercise actually extends life and preserves function, partly because it stimulates these systems for repairing and maintaining the body. Mechanical wear-and-tear turns out to be a relatively minor factor in aging.

It is true, of course, that some kinds of damage do accumulate over time. Injuries and imperfect healing can lead to dysfunction later in life. Damage to DNA or proteins can build up over the years and cause body systems to lose or have decreased functionality. The idea of damage accumulating over time has some truth to it, but overall, it is too simplistic and can’t explain many important aspects of how dogs age.

A more sophisticated idea for why animals age is the “rate of living” theory. Scientists have found a general relationship among animal species between body size and lifespan. Larger species tend to live longer than smaller ones. Part of the reason for this may be that smaller species have a faster metabolic rate; they use more energy and more fuel than bigger species because all their bodily functions run faster. This higher metabolic rate leads to greater production of damaging by-products, such as free radicals, and thus, faster accumulation of damage and breakdown of body functions. These smaller species literally live fast and die young compared to larger creatures!

A classic example would be the differences between a mouse and an elephant. Although the elephant is a lot bigger, and so needs a lot more food overall, the mouse actually consumes and uses a lot more calories than the elephant relative to its body size. Its heart rate and breathing rate and every other aspect of its bodily functions run much faster than those of an elephant. As a result, mice age faster and die younger than elephants.

The rate of living theory works pretty well as a general principle, but there are plenty of exceptions, and it doesn’t perfectly explain differences in aging between or within species. In fact, larger individuals within a species tend to have shorter lifespans. This difference is especially exaggerated in dogs due to intensive breeding. There is still much to discover about the root causes of aging and the specific mechanisms that lead to the physical and functional changes we see as our dogs get older.

Why do dogs age?

<h2 class="wp-block-heading" id="why-dogs-age">Why Dogs Age</h2>



Dogs age physically and functionally as they get older, but these changes are not solely due to the passage of time or the body “wearing out”.&nbsp;



There are two broad ways to look at the causes of aging: why should physical aging happen at all, and what specific mechanisms lead to the changes we see in aging dogs? The first question is often answered in terms of evolutionary and developmental biology and why it can lead to animals experiencing deterioration and death with age. The second question focuses more on the biological events that happen in an individual body that lead to this deterioration, and what we might do to change these events and delay aging.







<h3 class="has-xl-font-size wp-block-heading" id="wear-and-tear" style="font-style:normal;font-weight:700">Wear and Tear</h3>



<img loading="lazy" decoding="async" src="https://lh3.googleusercontent.com/Oi6o0fNb9RaW4ly56cSWftIlEIl-OdlAv1Lzu7iYAyKme4QW9JB50xFcFqATzbpd09tZxErQ273LbmNHukC30z2fhN4zneaMG9lLFpPcaz2auych7PN60ShQQ6YYEQQ_fkYjnBX9" width="461" height="308">



Many theories have been proposed for why animals experience aging at all. The oldest notion is simply the “wear and tear” hypothesis. It seems logical that our physical bodies should wear out over time. It was once thought that each individual only had a limited number of heartbeats before the heart became too damaged to function, or that too much activity caused cumulative damage to joints, resulting in arthritis.&nbsp;



However, living animals have systems for repairing and regenerating tissue. We now know that exercise actually extends life and preserves function, partly because it stimulates these systems for repairing and maintaining the body. Mechanical wear-and-tear turns out to be a relatively minor factor in aging.



It is true, of course, that some kinds of damage do accumulate over time. Injuries and imperfect healing can lead to dysfunction later in life. Damage to DNA or proteins can build up over the years and cause body systems to lose or have decreased functionality. The idea of damage accumulating over time has some truth to it, but overall, it is too simplistic and can’t explain many important aspects of how dogs age.







<h3 class="has-xl-font-size wp-block-heading" id="rate-of-living" style="font-style:normal;font-weight:700">Rate of Living</h3>



A more sophisticated idea for why animals age is the “rate of living” theory. Scientists have found a general relationship among animal species between body size and lifespan. Larger species tend to live longer than smaller ones. Part of the reason for this may be that smaller species have a faster metabolic rate; they use more energy and more fuel than bigger species because all their bodily functions run faster. This higher metabolic rate leads to greater production of damaging by-products, such as free radicals, and thus, faster accumulation of damage and breakdown of body functions. These smaller species literally live fast and die young compared to larger creatures!



A classic example would be the differences between a mouse and an elephant. Although the elephant is a lot bigger, and so needs a lot more food overall, the mouse actually consumes and uses a lot more calories than the elephant relative to its body size. Its heart rate and breathing rate and every other aspect of its bodily functions run much faster than those of an elephant. As a result, mice age faster and die younger than elephants.



<figure class="wp-block-image"><img decoding="async" src="https://lh3.googleusercontent.com/ueo7iLqdWg7F7AWd6w70N3UYu_aCQZzAQ-ZnjqJADyR9VoLazlvJOlcvvvtLhzjmVSIlhohMYVBF2agN95NMgXGiatKnRWA7vRwPZhvnayhPckP2ocFWa49r_64udtOFitYWlC_l" alt=""/></figure>



The rate of living theory works pretty well as a general principle, but there are plenty of exceptions, and it doesn’t perfectly explain differences in aging between or within species. In fact, larger individuals within a species tend to have shorter lifespans. This difference is especially exaggerated in dogs due to intensive breeding. There is still much to discover about the root causes of aging and the specific mechanisms that lead to the physical and functional changes we see as our dogs get older.

Age-related diseases have long been thought of as inevitable, natural consequences of getting older. The traditional medical response to age related diseases such as cancer has been to detect and treat them as early as possible, to cure if possible and reduce suffering and prolong life when an illness cannot be cured. These are important and valuable approaches but in an ideal world, the best approach is always prevention.

Preventative veterinary medicine has been incredibly effective at reducing the burden of infectious disease, parasitism, malnutrition, injuries, and many other causes of suffering and death in dogs. However, the options are limited for preventing the many diseases of aging. Maintaining a healthy body weight, regular exercise, and a few other lifestyle interventions do reduce risks to some extent, but we need to turn our growing understanding of the mechanisms of aging into therapies that address aging itself.

The direct way to accomplish this is through interventions that prevent or delay the occurrence of specific age-associated diseases. If targeting particular mechanisms of aging can reduce the chances of our dogs developing cancer, for example, this would prolong life and preserve health for many individuals because cancer is a leading cause of death in aged dogs. 

However, the immediate cause of death for most dogs is not any specific disease but rather euthanasia. We choose to let our dogs go when it is clear that they are experiencing significant suffering or loss in quality of life. 

Aging also leads to a general loss of comfort, function, and resilience, a condition known as frailty. One of the greatest potential benefits of targeting the underlying mechanisms of aging would be to slow the onset of frailty. Keeping dogs biologically younger for longer evwould extend healthspan and could also indirectly lengthen life by delaying the point at which quality of life has declined so much that euthanasia is necessary. 

Immortality, of course, seems unlikely for complex biological organisms, and fatal illness or unacceptable frailty is likely inevitable for all dogs eventually. However, there is great potential to improve overall quality of life by compressing the period of significant disease and disability to as small a fraction of the lifecycle as possible and extend those prime, healthy years. 

Is aging inevitable?

Age-related diseases have long been thought of as inevitable, natural consequences of getting older. The traditional medical response to age related diseases such as cancer has been to detect and treat them as early as possible, to cure if possible and reduce suffering and prolong life when an illness cannot be cured. These are important and valuable approaches but in an ideal world, the best approach is always prevention.



Preventative veterinary medicine has been incredibly effective at reducing the burden of infectious disease, parasitism, malnutrition, injuries, and many other causes of suffering and death in dogs. However, the options are limited for preventing the many diseases of aging. Maintaining a healthy body weight, regular exercise, and a few other lifestyle interventions do reduce risks to some extent, but we need to turn our growing understanding of the mechanisms of aging into therapies that address aging itself.



The direct way to accomplish this is through interventions that prevent or delay the occurrence of specific age-associated diseases. If targeting particular mechanisms of aging can reduce the chances of our dogs developing cancer, for example, this would prolong life and preserve health for many individuals because cancer is a leading cause of death in aged dogs.&nbsp;



However, the immediate cause of death for most dogs is not any specific disease but rather euthanasia. We choose to let our dogs go when it is clear that they are experiencing significant suffering or loss in quality of life.&nbsp;



Aging also leads to a general loss of comfort, function, and resilience, a condition known as frailty. One of the greatest potential benefits of targeting the underlying mechanisms of aging would be to slow the onset of frailty. Keeping dogs biologically younger for longer evwould extend healthspan and could also indirectly lengthen life by delaying the point at which quality of life has declined so much that euthanasia is necessary.&nbsp;



Immortality, of course, seems unlikely for complex biological organisms, and fatal illness or unacceptable frailty is likely inevitable for all dogs eventually. However, there is great potential to improve overall quality of life by compressing the period of significant disease and disability to as small a fraction of the lifecycle as possible and extend those prime, healthy years.&nbsp;

Dogs and humans are uniquely connected as species. Studies have found that 

dogs are even much better than chimpanzees

, our closest evolutionary relatives, at understanding human gestures and communication. For tens of thousands of years, dogs have lived symbiotically with humans, and for at least several hundred years we have intensively bred and shaped dogs to participate in human society. As hunting companions, protectors, and increasingly as members of our close family, dogs are bonded to us and integrated into our lives unlike any other animal.

One consequence of this relationship is that dogs are well-suited to help us understand and manage aging, a biological process with a multitude of factors, many of which are tied to lifestyle and habits. Unlike laboratory animal models, such as rats and mice, dogs share our environments, routines, and the same potential risk factors for death and disease. This makes dogs a more relevant model of natural aging than animals in controlled research settings.

Dogs also share many of the diseases and causes of death as humans. While they grow and age faster than us, they do so in similar ways. Injury and infections, for example, are more common in young dogs as they are in young people, whereas older humans and dogs are more likely to experience cancer or degenerative diseases. These similarities allow research data from dogs to be more readily applied to human health than data from lab animal species. The shorter lifespan of dogs also allows us to study aging and longevity therapies on a more practical timescale than similar studies with humans.

Comparing how dogs and humans age. From 1. Wang T, Ma J, Hogan AN, et al. Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome. 

Another advantage of studying our faithful companions is the fact that intensive breeding in dogs has produced perhaps the greatest degree of variation within any species. This includes physical differences, such as the huge variation in size from the toy breeds such as the Yorkshire Terrier to giants like the Great Dane or Irish Wolfhound, and of course, variation in how dogs age. Large breeds age faster and live shorter lives than smaller breeds. Investigating these differences can give us unique insights into the underlying mechanisms of aging that are harder to identify in other species with less intraspecific variation.

There is also a comprehensive scientific healthcare system in place for dogs as well as humans, which allows not only medical testing and treatment for individual dogs but widespread research on aging and age-associated health problems. There is also ample demand and resources for this research because so many of us love dogs and are willing to contribute time, effort, and money to improving canine health. 

While there are, of course, important biological differences between humans and dogs, our canine companions are uniquely suited to help us understand our own aging. Helping us to understand and alter our own aging is just one more of the many gifts our canine best friends bring into our lives.

Dogs are great models for aging biology

Dogs and humans are uniquely connected as species. Studies have found that <a href="https://www.sciencedaily.com/releases/2012/02/120208180251.htm">dogs are even much better than chimpanzees</a>, our closest evolutionary relatives, at understanding human gestures and communication. For tens of thousands of years, dogs have lived symbiotically with humans, and for at least several hundred years we have intensively bred and shaped dogs to participate in human society. As hunting companions, protectors, and increasingly as members of our close family, dogs are bonded to us and integrated into our lives unlike any other animal.



One consequence of this relationship is that dogs are well-suited to help us understand and manage aging, a biological process with a multitude of factors, many of which are tied to lifestyle and habits. Unlike laboratory animal models, such as rats and mice, dogs share our environments, routines, and the same potential risk factors for death and disease. This makes dogs a more relevant model of natural aging than animals in controlled research settings.



Dogs also share many of the diseases and causes of death as humans. While they grow and age faster than us, they do so in similar ways. Injury and infections, for example, are more common in young dogs as they are in young people, whereas older humans and dogs are more likely to experience cancer or degenerative diseases. These similarities allow research data from dogs to be more readily applied to human health than data from lab animal species. The shorter lifespan of dogs also allows us to study aging and longevity therapies on a more practical timescale than similar studies with humans.



<figure class="wp-block-image"><img decoding="async" src="https://lh5.googleusercontent.com/Y2YiLEfNaIWJ6U3k2-sUHGwAAvCj1izu5LISlDgDe6C5Hg7sAdqdamBhJpL62vh1Ajzgpasm6OAa-70P6gTBzf7p9W5-UraJiN8brzB67zaYBDU_cO5E45k1NzEOJp5ZIDN9i2mr" alt=""/><figcaption>Comparing how dogs and humans age. From 1.&nbsp;Wang T, Ma J, Hogan AN, et al. Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome.&nbsp;Cell Syst. 2020;11(2):176-185.</figcaption></figure>



Another advantage of studying our faithful companions is the fact that intensive breeding in dogs has produced perhaps the greatest degree of variation within any species. This includes physical differences, such as the huge variation in size from the toy breeds such as the Yorkshire Terrier to giants like the Great Dane or Irish Wolfhound, and of course, variation in how dogs age. Large breeds age faster and live shorter lives than smaller breeds. Investigating these differences can give us unique insights into the underlying mechanisms of aging that are harder to identify in other species with less intraspecific variation.



There is also a comprehensive scientific healthcare system in place for dogs as well as humans, which allows not only medical testing and treatment for individual dogs but widespread research on aging and age-associated health problems. There is also ample demand and resources for this research because so many of us love dogs and are willing to contribute time, effort, and money to improving canine health.&nbsp;



While there are, of course, important biological differences between humans and dogs, our canine companions are uniquely suited to help us understand our own aging. Helping us to understand and alter our own aging is just one more of the many gifts our canine best friends bring into our lives.