A Maremma shepherd and a dachshund are indisputably very different animals despite belonging to the same species. It’s not just a matter of size: in what might seem like a paradox, the former will live a much shorter life than the latter. But why, put bluntly, do large dogs die before small ones? A study published in Science could reveal the answer, or at least part of it: hold on tight because it hides in the recesses of DNA.
The paradox of size
The relationship between size and life expectancy is not exclusive to dogs: in general, in animals the rule is that larger ones live longer (think of sharks and whales compared to mice or flies). However, the rule also applies that, within the same species, larger specimens live shorter than smaller ones: even among humans there is evidence that taller people have slightly shorter lives. The question is: why?
The Dog Aging Project has been looking for the answer since 2014, at least as far as dogs are concerned. And the fact that many of the project participants submitted multiple DNA samples of their dog over the years allowed for a comparison: the team from Arizona State University analyzed the DNA of 894 specimens participating in the project and looked for differences and changes over the years. What the researchers discovered has to do with a process called methylation, and so-called “jumping genes” or transposons.
Cellular aging
Methylation is a mechanism that leads specific chemical groups, namely methyl groups, to attach to portions of DNA, regulating the expression of certain genes. Transposons, on the other hand, are pieces of DNA that can move within the genome: when they are methylated they are harmless, but if left “free” they can damage the DNA and cause inflammation.
As we age, methylation becomes less effective, increasing the likelihood of developing pathologies (in dogs, but the mechanism is similar in humans too). Studying the DNA of dogs involved in the Dog Aging Project revealed that larger dogs age more quickly, and their chance of dying increases at the same rate – much faster than that of small dogs.
Sex also turned out to be an important element: males age faster from a cellular point of view, even those who were sterilized when young.
From here on it is still all hypotheses: the authors themselves explain that their study is not enough to say that “bigger dogs age more quickly and die earlier”, nor that the “fault” lies with transposons; furthermore, if it is true that the link between size and aging is very clear, we still do not have an explanation of why things work like this.
But the discovery is still important, and will be the basis for other similar studies: discovering the secret of the longevity of certain dogs could, one day, help us humans to age better too.
