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Dog owners have probably heard the method of counting their pet’s age that says one human year is the equivalent of seven dog years. But in reality, that math isn’t entirely accurate – or that easy to calculate.
How quickly your dog ages compared to the human aging cycle depends on a number of factors, including their size and breed. And since dogs develop and mature faster earlier in their lives than humans do, it’s more likely that the first year of their life is more like the equivalent to 15 years, and 9 months in dog years being closer to 9 to 11 human years.
Here, we’ll look at some more accurate ways of calculating a dog’s age in human years, physical signs of age in dogs, and why smaller dogs mature at a slower rate than larger breeds.
Is One Dog Year Seven Human Years?
The idea that one dog year was equivalent to seven human years started in the 1950s and is likely to have been based on the fact that, at that time, the average human lifespan was around 70 years and the average dog’s lifespan was around 10 years.
It’s also been considered that the 7:1 rule was a ‘ploy’ simply designed to stress the idea that dogs age at a faster rate than humans, and therefore will need to be treated as older than their ‘age’ in human years from a health perspective.
But despite the math being far more complex than this, the myth of the 7:1 rule is still rife, even among those who do not own dogs.
Not Sure How Old Your Dog Is? Look For These Clues
Despite not being entirely accurate, the 7:1 rule might be a good way to calculate an average age for your dog – as long as you know when they were born. But for owners of rescued or adopted dogs, there’s no obvious calculation that can help them define the age – and therefore health and other care needs – of their pup.
In these cases, there are still ways you can figure out your pet’s rough age without knowing their date of birth. Vets can give you a prediction of their age following a full body examination, and senior dogs can be identified by signs of age like gray fur, clouded eyes, and stuff joints.
At home, simply looking at your dog’s teeth you will be able to tell:
- If they’re over 8 weeks, all baby teeth will have grown in
- By 7 months, all permanent teeth are grown in and are fresh white and clean
- At 1-2 years, their teeth will have grown duller with some yellowing at the back
- At 3-5 years, their teeth will be showing signs of wear at tartar buildup
- At 5-10 years, you might spot signs of disease which is more common by this age, and their teeth will be more worn down
- At 10-15 years, a dog might be missing some teeth, most will be worn down, and there will be heavy tartar buildup
How Old Is My Dog In Human Years?
Some key numbers to keep in mind as a guidelines when calculating your dogs age are:
The first year of a medium sized dog’s life is roughly equal to 15 human years
- The second year of a medium sized dog’s life is about nine human years
- From the third year onwards, a medium sized dog ages about five years for every human year
- Small dog breeds are considered ‘senior’ from age seven despite their longer life expectancy
- Larger dog breeds are considered senior from 5-6 years of age
- These numbers are generally used in relation to a dog’s maturity and development, and when age-related problems begin to present themselves
Epigenetic Clock Study, 2019
In 2019, researchers from the University of California San Diego presented a new way to calculate a dog’s age based on DNA. Known as the Epigenetic Clock Study, it uses DNA methylation to compare the activity of DNA molecules in both dogs and humans and compare how they change over time.
In the Epigenetic Clock Study, the researchers only tested on Labrador Retrievers, so, since we know that different breeds age at different rates, the findings of this experiment can’t be said to apply to all dogs. But it does show more accurate results than the 7:1 method.
The study performed targeted DNA sequencing on 104 Labradors aged up to 16. Researchers added methyl groups to DNA molecules throughout the aging process and compared this to the results of humans in an attempt to compare ‘dog years’ and ‘human years’.
The results helped the researchers come up with a formula that multiplied the natural logarithm of the dog’s age by 16 and adding 31 could more accurately adjust a dog’s age into ‘human’ years.
Though complex and inconclusive for all other breeds, this research has introduced a new, science-backed method for calculating a dogs’ age that could be more useful for medical purposes than the debunked 7:1 rule.
Why Do Smaller Dogs Live Longer?
Another factor that is apparent in dog’s aging processes is that smaller dogs mature and age slower than larger dogs. This sets them apart from other mammals, where the general rule is that larger species have far longer lifespans than smaller animals.
Though no research has yet been able to explain why dogs with larger body mass have a shorter lifespan, studies have shown that large dog breeds age at a faster rate than their smaller counterparts. In fact, it’s believed that for every 4.4 lbs of body weight, a dog’s lifespan is reduced by a month.
Though no biological reason for this is clear, it’s thought that larger dogs will suffer from age-related conditions and illnesses sooner, and accelerated growth rates could lead to the formation of certain types of cancers.