Why Do Small Dogs Live Longer than Big Dogs?

If you’re a dog lover, and you probably are if you’re watching this video, you have no doubt noticed that smaller dogs live longer lives than larger breeds. This is a rather well-known fact. But why?

Let’s try to dig up some answers.

When compared to the lifespan of other mammals, “smaller size, longer life” would at first glance appear to be illogical, especially if your family has had short-lived pets such as rats, hamsters, or gerbils. Compare their lifespans of about 2-4 years to an elephant that can live up to 70 years or the 65-ton Bowhead Whale that can live an estimated 200 years; it seems larger mammals have longer lives.

Scientists think that this happens because of the way differently-sized animals use energy. Big animals’ cells are slower and more efficient, so their parts wear out slower and last longer.

The mouse and the elephant have approximately the same amount of heartbeats in their lifetimes, albeit the mouse’s heart beats at a much higher rate.

Why Do Small Dogs Live Longer?

Great Dane vs. ChihuahuaWe should expect a Great Dane to live longer than a Chihuahua, by this logic, but, as you know, that’s not the case. The Great Dane only lives, on average, 6 or 8 years, while the Chihuahua can live up to 18 years.

Forget about all the other mammals and focus on just one species, though, and you see this trend reverses across many species of mammals. Scientists have seen this is in mice, horses, and even humans, and the effect is quite pronounced in dogs, which have a pretty extreme range of sizes from Mastiffs weighing 250 pounds to a Chihuahua named BooBoo that breaks the scale at a staggering 1.5 pounds. Personally, I would have named him something like Crusher or Bruiser. Anyway…

A study from a few years ago, led by Dr. Cornelia Kraus, a research scientist and evolutionary biologist at the University of Göttingen in Germany, researchers analyzed mortality data in over 56,000 dogs from 74 different breeds. Kraus states that they found that for each increase of 4.4 pounds of body weight, a dog’s lifespan decreased by 1 month.

So why, then, do small dogs have a longer average lifespan than the larger breeds? Well, they’re still unsure, at least at the time of this recording.

But, at it’s simplest, Dr. Kraus suggests that there are several possibilities, including that larger dogs may succumb to age-related illnesses sooner. Also, larger breeds grow from puppies to adults at an accelerated rate than their tinier kin, which may lead to a higher likelihood of abnormal cell growth and cancer.

And, scientists think that the reason big breeds die young has to do with the way humans have bred them and the way they grow.

Larger dogs grow very big, very fast. Take a one-year-old Great Dane, for example. From birth to his first birthday, he increases 100-fold in weight, increasing from 1-2 pounds at birth to 95-140 pounds at 1 year. In that same time frame, wolves increase 60-fold, poodles 20-fold, and humans only about threefold.

Why Do Small Dogs Live Longer? Research in the last decade has suggested that larger individual animals die younger because this sort of accelerated growth may come with an increased free-radical activity that can damage tissue and DNA.

We know that at least 3 genes determine large body size in dogs, IRS4 and IGSF1, involved in thyroid hormone pathways that affect growth, and ACSL4, involved in muscle growth and backfat thickness. But, more studies are needed on how this accelerates aging.

These findings are just the tip of the iceberg in our understanding of canine lifespans (and even human lifespans) and what determines them. Scientists plan future studies to explain the link between growth and mortality better.

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