In the animal kingdom, the general rule is the larger you are the longer you live. Elephants and whales are some of the longest living groups of animals on the planet, and they are respectively the largest organisms on land and in the ocean. Conversely, mice, and other rodents tend to experience much shorter lifespans.

However, one strange counterexample is the contrast in lifespan among different sized dog breeds. It is well-known that large dogs don’t live as long as their smaller relatives. For example, mastiffs, weighing up to 230 pounds, live for 6 to 10 years, while chihuahuas, which never weigh more than 6 pounds, live for 14 to 16 years (data from the American Kennel Club).

There is also a deeper paradox to the concept of aging, says Jack da Silva from the Department of Molecular and Biomedical Science at the University of Adelaide. “Surely it is much easier for you to maintain yourself in peak physical condition over your lifetime than it was to develop from a fertilized egg into a complex, multicellular adult,” says da Silva.

So why has aging evolved, he asks? He thinks the lifespan differences observed across dog breeds could help researchers answer why.

The Different Theories of Aging

Our intuitive answers for why we age usually point to the impact living has on our bodies. Over time, our bodies degrade due to wear and tear. But this explains how we age, not why, and there is an important difference. Some species have evolved to live much longer than others, so why don’t all species evolve to live longer if aging is avoidable?

Natural Selection

According to da Silva, the answer has to do with the fact that natural selection weakens with age. As we are more likely to be alive a year from now than two years from now, due to the increased chances of accidental death or catching infectious disease, genetic mutations will have more of an impact on an individual while they are young and likely to still be alive than when they are old. This leads to two major theories for the evolution of aging, says da Silva.

Mutation Accumulation Theory

“Mutations that are disadvantageous are less likely to be removed from a population by selection if they act only at older ages. Thus, the mutation accumulation theory explains aging as the accumulation of deleterious mutations that act at older ages,” he says.

Life History Optimization Theory

According to da Silva, “any investment made in growth and reproduction cannot also be made in maintaining the body’s condition. Therefore, because selection weakens with age, you maximize the number of offspring produced by growing quickly and reproducing at a young age rather than maintaining your body in peak physical condition. This is the life history optimization theory of aging.”

Read More: The Origins of Dogs

Why Do Small Dogs Live Longer?

Recently, da Silva collected data on 164 dog breeds in a bid to better understand the causes of these lifespan differences. The data included information on adult body mass, newborn mass, at what age breeds reach 50 percent of their adult mass, litter size, average age of death, cause of death and the distribution of dogs at different ages for each breed.

Selective Breeding

The authors found that the declining lifespan for breeds with increasing body size is most likely due to recent selective breeding for larger sizes acting through increased early growth rates. But what does this mean?

Rather than devoting resources to general body maintenance, selection for greater body size prioritizes resources for early growth, which is reflected in the fact that larger breeds suffer from more physical ailments from two years old onwards (once they reach adult size).

Mutations

Additionally, mutations that increase body size in dogs and laboratory mice also increase the risk of cancer. This association makes sense when we consider that most cancers result from mutations that emerge during cell division, with larger individuals having more cell divisions for such mutations to occur.

In da Silva’s view, the evidence which points to larger breeds having shorter lifespans due to the investment of resources into early growth rather than body maintenance (which includes defenses against cancer such as repairing DNA copying errors and fighting cancerous cells), and the fact that larger dogs tend to have larger litters, supports the life history optimization theory of the evolution of aging.

This theory also suggests that, over time, larger dogs should evolve better cancer defenses, longer lives and smaller litters. A tradeoff that we think owners of large dog breeds will be happy with.

Read More: How a Contagious Dog Cancer Spread Around the World