Dogs age very quickly compared to humans, the next reason being that they have no set bones, but breed characteristics give them a long, flexible neck. It is because of this flexibility of the neck that the dogs are able to age at such a rapid rate, being in their teens before they have even completed the first year of their life. The skeletal system of a dog is very complex, with a number of muscles and ligaments constantly working to keep the various parts of the body taut and balanced. Therefore, there is a high rate of metabolism in dogs which allows for more energy to be consumed than an equivalent human.
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There is now, however, some research into how dogs age, using DNA samples from living and previously bred dogs. Previously, dogs have had their DNA sequenced to determine their overall health, but this process has been slow because it is difficult to extract DNA from living samples. However, researchers now have the tools to sequence dogs' DNA quickly and efficiently, producing methylation data on a routine basis, which can be used to assess both the general health of the dog, as well as the physiological mechanisms involved in disease. This research has implications for both owners and veterinarians.
One study compared the methylation patterns present in two groups of dogs, one from the UK and one from Germany. The research found that although German dogs appeared to have higher methylation rates than did their UK counterparts, the general pattern was no different between the breeds. Nonetheless, when the researchers examined the differences between the German and UK dogs using a method called genomics, they found significant age effects on both sides of the spectrum. The German dogs between the ages of three and seven had significantly more methylation than did the UK dogs. Similarly, the UK dogs showed significantly more non-methylation than did the German dogs.
The study concluded that there is a genetic factor associated with the increased speed of aging at a young age, but that it does not account for the significantly younger age of the German dogs. Instead, they postulated that the increased methylation rate observed in the UK dogs may be due to the diet they receive or to environmental factors, such as higher air and food temperatures or higher home temperatures. Alternatively, the researchers opined that the observed methylation rate in dogs is likely to be a direct result of the increased levels of glucocorticoids that are present in their bodies at these young ages. Glucocorticoids are naturally produced by the liver but are excreted from the dogs' bodies in higher quantities during a rapidly growing period. As a consequence, the dogs tend to age rapidly, and the glucocorticoid surge adds to the acceleration of methylation rates seen in humans.
If correct, this new evidence could support the view that dogs age faster than humans. It will be interesting to follow the research, but so far there appears to be no definitive answer to the question. However, there is substantial controversy in some circles. Some researchers argue that the dogs can accurately age seven years old, while others think that they can age much faster. The answer may lie in how we separate the dogs' chronological years from their genetic years.
In humans, we use chronological age as a way of classifying and measuring time. By deducting the number of calendar years from their birth dates, we can roughly estimate their chronological age. However, dogs have evolved into a much different animal. Like humans, they breed over time, reproduce through a process called spermatogenesis, and undergo a metamorphosis (change of form) at a relatively early age. While this means that we can't accurately count the number of calendar years old of a dog, it also means that we can make an educated guess as to when the dogs might actually reach their "milestone" - the point where their maximum reproductive potential is at its greatest.
This is where dogs age compared to humans becomes relevant. Because dogs are not able to use the same body-weight or chronological age system that humans do, we rely on a system of numbers to make sure that our dogs are in the right age group. We also use this number to roughly calculate the "aging gap," which is the difference between how old a dog is compared to how young his potential to reproduce is. (The actual number of years that a dog could be expected to reproduce during his lifetime is unknown, of course.)
Scientists have recently begun looking at the relationship between dogs and humans through the lens of genetics and ecology. They've found that a variety of genes and sperm RNAs are responsible for creating these differences, and that these environmental factors can account for as much as 60% of the variation in reproductive potential among people. What's more environmental variables such as diet, exercise, and overall health can also act as epigenetic clocks, slowing down the clock or enabling certain individuals to procreate at a much earlier age than others. These results have been in the works for many years, but it's only now with the help of advanced DNA analysis that they're finally being applied to questions of aging and disease resistance.