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By Christian Yates

6th January 2020

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92彩票网平台If your dog has been alive and kicking its paws about for a decade, the widely held belief is it has aged as much as a human would have done over 70 years. This conversion factor – each year of a dog’s life accounting for seven human years – comes from dividing human life expectancy of around 77 by the canine life expectancy of around 11.

92彩票网平台The underlying assumption is that each calendar year a dog lives through is equivalent to seven human years at any stage of a dog’s life. But new research . And if we look at some basic developmental milestones, it’s clear why.

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For example, most dog breeds  between the ages of six and 12 months – the upper end of that range corresponding, by the traditional conversion, to a human age of seven. And at the other end of the spectrum, although unusual, some dogs have been known to live for over 20 years. Under the “factor-of-seven” conversion rule, this would equate to an unfathomable 140 human-equivalent years.

92彩票网平台New insights into how dogs age suggest our pets move into middle age more rapidly than most owners might suspect (Credit: Getty Images)

To make matters more complicated, dogs’ life expectancy depends significantly on the breed. Smaller dogs , suggesting that they age more slowly than bigger dogs.

92彩票网平台All of this raises the question of what exactly we mean by age. The most obvious way to describe it is simply the length of time that has passed since birth. This is known as the chronological definition of age.

When it comes to comparing animal ages across species, the biological definitions of age are far more useful than their chronological counterparts

However, there are other descriptions. Biological age, for example, is a more subjective definition, which relies on assessing physiological indicators to identify an individual’s development. These include measures like the “” – surveys that take into account an individual’s disease status, cognitive impairments and levels of activity.

Then there are the more objective , such as  (genes produce proteins at differing rates at different stages of life) or . The rate at which biological age increases depends on .

92彩票网平台Rather than celebrating chronological age, looking at the levels of methylation on a dog's DNA is a much more accurate measure of aging (Credit: Getty Images)

For example, if you’ve spent a lot of time eating junk food and smoking cigarettes instead of taking exercise and eating healthily, the chances are your biological age will exceed your chronological age. Or, you might be a 60-year-old with the body of a 40-year-old if you’ve looked after yourself well.

A dog’s life

When it comes to comparing animal ages across species, the biological definitions of age are far more useful than their chronological counterparts. Knowing a hamster is six weeks old doesn’t give you a good picture of that animal’s life stage, even if you know the life expectancy of a hamster is only three years. Learning that a hamster has reached an age where it can reproduce gives a much better picture of its level of maturity.

The authors of the new ageing study suggest that – changes to the packaging of our DNA that accumulate over time in all mammals.

92彩票网平台In their first year of life, puppies grow up so quickly that they age the equivalent of 31 human years (Credit: Getty Images)

In particular, “methylation” – the addition of methyl groups (a carbon atom bonded to three hydrogen atoms) to DNA – seems to be a good indicator of age. Many prominent physiological markers, such as the development of teeth, seem to occur at the same levels of methylation across different species. So by matching the levels of methylation in Labrador retrievers and humans, the researchers derived a formula to map dog age to its human equivalent.

That formula is: human equivalent age = 16 x ln(dog’s chronological age) + 31.

92彩票网平台Here “ln” represents a  known as the . The logarithm function is well-known in the non-linear scales for energy released during earthquakes (Richter) or for measuring sound (decibels). It comes in useful for measuring quantities whose sizes vary over many orders of magnitude. It’s even possible that a logarithmic experience of the passing of time might explain why we perceive .

A handy short cut is to remember that the first dog year counts for 31 human years

In the graph below, you can see how the natural logarithm works to convert the years a dog has lived (dog age) into the equivalent human age in the red dashed curve. The curve suggests that dogs mature extremely rapidly at first, but that their ageing then slows down, meaning that most of their lives are experienced as a form of protracted middle age.

92彩票网平台A handy short cut is to remember that the first dog year counts for 31 human years. Then, after that, every time the dog’s chronological age doubles, the number of equivalent human years increases by 11. So eight calendar years represents three “doublings” (from one to two, two to four and then four to eight) giving a dog age equivalent of 64 (that’s 31 + 3x11).

This useful approximation is plotted as the black curve on the conversion figure below. The green line represents the discredited factor-of-seven rule that suggests unrealistic ages at the higher end of the dog age spectrum.

In eight calendar years a dog will approximately age the equivalent of 64 years (Credit: Christian Yates)

92彩票网平台Most dog lovers will already have suspected that the human-to-dog age relationship is non-linear, having noticed that, initially, their pets mature much more quickly than the linear factor-of-seven rule suggests.

A  to the factor-of-seven rules has suggested that each of the dog’s first two years correspond to 12 human years while all subsequent years count for four human equivalents. The blue curve in the above figure, which represents this ad hoc rule, shows better agreement with the new logarithmic law.

92彩票网平台In practice the new molecular insights into human-to-dog age conversion encapsulated by the logarithmic law suggest that dogs move into middle age even more rapidly than most dog-owners would have suspected. It’s worth bearing in mind, when you find that Rex is reluctant to chase the ball like he once did, that he’s probably got more miles on the clock than you’ve been giving him credit for.

Christian Yates is a senior lecturer in mathematical biology at the University of Bath. He is also the author of .

This article  on The Conversation, and is republished under a Creative Commons licence. 

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