If your dog has lived a lively and energetic life for 10 years, it is generally considered to be old, equivalent to a human's 70th birthday. This conversion factor, that is, one year of a dog's lifespan is equivalent to seven human years, is derived by dividing the average human lifespan (around 77 years) by the average dog lifespan (around 11 years).
The basic assumption of this algorithm is that at any stage of a dog's life, each calendar year is equivalent to seven human years. However, new research suggests that things are not so simple. The reason becomes clear if we look at some fundamental key stages of canine growth and development.
For example, most dog breeds reach sexual maturity between 6 and 12 months of age. According to the traditional conversion mentioned above, the upper limit of this age for dogs is only equivalent to a human's 7-year childhood. At the very end of the canine lifespan, although less common, we know that some dogs can live for over 20 years. Based on the rule that "1 dog year equals 7 human years," a dog in its 20s is equivalent to an unimaginable human lifespan of 140 years.
To complicate matters further, a dog's lifespan largely depends on its breed. Smaller dogs tend to live significantly longer, suggesting that smaller breeds mature and age more slowly than larger ones.
New research on dog lifespan suggests that pet dogs reach canine middle age much faster than most dog owners realize.
All of the above phenomena lead to the question: what exactly does age mean? The most obvious way to describe age is by counting the years from birth to the present. This is what is known as age defined in chronological order.
However, there are other ways to define age. For example, biological age is a definition that focuses more on an individual's physiological condition, relying primarily on assessments of indicative characteristics of a person's physiological development. These indicators include the "aging index," which takes into account an individual's disease status, cognitive impairment, and activity level.
In addition, there are more objective biomarkers of aging, such as gene presentation levels (genes produce proteins at different rates at different stages of life) or the number of immune cells. The rate of biological aging depends on individual genetic factors, mental health, and lifestyle.
For example, if you spend a lot of time eating junk food and smoking, and exercise little and eat unhealthy foods, your biological age is likely to exceed your actual age. Or, if you take good care of your health, you might be a 60-year-old with the body of a 40-year-old.
To measure a dog's lifespan, observing the level of DNA methylation is a more accurate method than calculating the number of years the dog has lived.
A dog's life journey
When comparing the ages of animals from different species, the biological definition of age is far more effective than the definition of the length of time. Even if you know that a hamster's lifespan is only 3 years, you still won't be able to tell which stage of life a 6-week-old hamster is in. But if you know that the hamster has reached breeding age, you'll understand that it has matured.
A group of researchers studying aging believe that a reasonable method for measuring biological age is to observe the species' "DNA clocks" (epigenetic clocks). DNA clocks are a method of measuring lifespan based on the level of DNA methylation in an organism. The DNA of all mammals undergoes genetic changes over time, exhibiting varying degrees of methylation.
In the first year of a dog's life, they develop very rapidly, and by the time they are one year old, they are equivalent to a 31-year-old human.
DNA methylation is particularly valuable as an indicator of biological lifespan. Methylation refers to the biochemical phenomenon of a methyl group (a group of atoms combining one carbon atom with three hydrogen atoms) being added to DNA. Despite differences in species, many significant physiological developmental markers, such as tooth development, appear to occur at similar levels of methylation. Therefore, by comparing the DNA methylation levels of Labrador Retrievers and humans, researchers derived a scientific formula that correlates dog ages with human ages.
The formula is: Human age = 16 × ln(the actual number of years the dog has lived) + 31.
In this formula, ln represents a mathematical function called the natural logarithm. Logarithmic functions are well-known for calculating non-linear numerical patterns such as the energy released by an earthquake (Richter magnitude) or measuring sound (decibels). The natural logarithm is useful for measuring numbers with vastly different magnitudes. It can even be used to calculate our perception of the passage of time, thus explaining why we feel time is passing faster as we get older.
In the chart below, you can see how the natural logarithm converts a dog's lifespan (a dog's age) into human age, as shown by the red dashed curve. This curve indicates that dogs mature very quickly at the beginning, but their aging process then slows down, meaning that most of a canine's lifespan is a long middle age.
A simpler way to calculate the logarithm is to remember that a dog's first year is equivalent to 31 human years.
A simpler way to calculate logarithms is to remember that a dog's first year of life is equivalent to 31 human years. After that, every time a dog's actual age doubles, the equivalent human years increase by 11. Therefore, 8 calendar years represent three "doublings" (from 1 to 2, from 2 to 4, and then from 4 to 8), which is 31 + 3 x 11. Thus, an 8-year-old dog is equivalent to a 64-year-old human.
This useful and simple approximation is plotted as the black curve below the red logarithmic transformation curve. The green line represents the unreliable traditional algorithm that "one year of dog age equals seven years of human age," and its trajectory shows that this algorithm is completely distorted during the dog's oldest years.
In fact, most dog lovers have begun to doubt that the age relationship between humans and dogs is non-linear, because they have noticed that their pets initially mature much faster than the "1 year equals 7 years" algorithm suggests.
More refined observations based on life experience than the "1 year equals 7 years" rule reveal that a dog's first two years are equivalent to 12 human years, while each subsequent year is equivalent to 4 human years. The blue curve in the graph above represents this particular rule of thumb, which aligns with the new logarithmic law.
Recent DNA research methods for human-dog age transition, as shown by logarithmic tables, indicate that dogs enter middle age even faster than most dog owners imagine. Dog owners need to remember that when they find their beloved dog Rex no longer jumping around and playing with a ball as energetically as before, their dog may have already lived longer than they expected.
