Why do small dogs live longer than large dogs?

The issue of body size and lifespan is a fascinating topic in biology. It’s strange that across species, at least in mammals, large bodied animals live longer than small sized animals. For example, elephants live a lot longer than mice. The theory is that bigger animals have slower metabolisms than small animals, and that faster metabolisms result in more accumulation of free radicals that damage tissue and DNA. But this doesn't always hold for all animals and the “rate of living” theory is not widely accepted. What we cannot clearly understand remains fascinating.

But now if we look at within a given species, lifespan and body size are inversely correlated. This is definitively the case for dogs and mice, and it’s been proposed that this is the case for humans too. Why would this be? A possible explanation is that larger dogs (or mice, or people) grow faster than their smaller counterparts because they reach a larger size in more or less the same time, and that faster growth could be correlated with higher cancer rates.

We do not have a clear understanding of why growing faster leads to accelerated aging. But it seems that it is an accelerated rate of aging, or senescence, that causes larger dogs to have a shorter lifespan than little dogs.

The figure is from: Ageing: It’s a Dog’s Life (possibly behind a paywall). The data is from 32 breeds. Note that the inverse correlation is pretty good, however some large dog breeds, at around 40 to 50 kg, live 12 or 13 years in average while some other dog breeds of equal body size live only 8 or 9 years in average. This is due to dogs being a special case as they were artificially bred by humans to select for looks or behavior and not necessarily health, and that considerable inbreeding was necessary to produce “purebred” dogs. For example, boxers are big dogs, but their higher cancer rates may result in a shorter lifespan. However, the really giant breeds all consistently live 8–9 years on average. So there is something going on besides simple breeding quirks that led to bad genetics and ill health. Something more general.

A few years ago, a large study was published using mortality data from thousands of dogs across 74 breeds, testing 3 hypotheses: Large dogs may die younger than small dogs because of (1) an earlier onset of senescence, (2) a higher minimum mortality hazard or (3) an increased rate of aging. The conclusion from their study is that aging starts more or less at the same age in small and large breeds, but large breeds age faster. We do not have a clear understanding of the underlying mechanism for faster aging in dogs. It seems that when we selected for large body size, we selected for faster aging as well. But we do not know all the genetic components of this. We know that there are at least 3 genes that determine large body size in dogs, IRS4 and IGSF1, involved in thyroid hormone pathways which affect growth, and ACSL4, involved in muscle growth, and back fat thickness. But how this accelerates aging is still speculation. More studies are needed, but dogs seem to be a great model to study the evolution of body size and its relationship to aging.

This information was taken from Quora. Click here to view the original post.

Have you learned something new about your pet? Was this information useful? Please tell us in the comments below!

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What are your thoughts on this subject?
Darlene Davidson
It's a sad day when your dog dies. To suggest that this sad day will be postponed if your dog is a small breed seems to miss the point of dogs and the emotional attachmemt people have to them, big or small.
Oct 25, 2018 6:20PM
My Golden Lab, Peaches, lived well into her teens. She was rescued from a horrible family & kept running to me for help. I legally adopted her & don't know her exact age. But 14 or 15 I'd say. It's been a long time, but I'll always miss her❤❤
Mar 7, 2021 12:51AM
Cathy Cummins Keating
Steve Avery, you are not alone in the way you feel about your beloved pet , pets are like fur babies to us , I to have lost pets in death and still do miss and think of them always, and people like us do care about others who have lost their sweet pets .
Nov 6, 2019 4:44PM
Steve Avery
I know no one cares, but my Abby, a Golden Lab was truly a golden jewel. Two years have gone bye since she passed and i still expect her to come running to meet me when i come home.. She always wanted to give me a kiss, after which she would run around in a circle two or three times, guess she was so glad to have me home.. I doubt I will ever get over her being gone.
Oct 8, 2019 5:38PM
Myrna Cooksey-Watson
I had a lab cross that was about 55 lbs., and she lived 17 years.
Jul 12, 2019 1:04PM
Wendy Stamatovich
Darlene Davidson, oh absolutely it is a horrible day when our pets die. I no longer have dogs, but have cats and always have. Pets are one of the best things in life!!!
Mar 15, 2019 4:50PM
Wendy Stamatovich
My chihuahua/ rat terrier mix was tiny and she lived to be almost 20. My big German Shepherd girl lived to be about 14. I understand that is pretty old for a G Shep. Neither had many health problems at all.
Mar 15, 2019 4:48PM
Paul Bison
I have a Staffy Jack Russell cross who will be turning 19 years old in three days time.
Feb 11, 2019 12:26AM
Kate Gula
My harlequin Great Dane lived to be 9. He had blue eyes so had vision problems however very healthy. Always monitored his weight closely, had gastropexy to prevent torsion. One morning couldn’t get up. Passed away two days later total paralysis from Wobblers. Was active and symptom free his whole life until those last 3 days. He was magnificent.
Feb 5, 2019 6:41PM
Richard D. McDowell
I have an 11yo Jack Russell, I have controlled his diet to keep him under 17lbs and keeping his conformation from the top with a nipped in waist. I mix french cut green beans into his food, canned,kibble every other day. Other day just kibble.
Jan 8, 2019 10:06PM
There is no complex reason. For example; A Great Dane has a much larger circulatory system to carry all of that blood through and much more body mass to contend with... While a Chihuahua has just the opposite...
Dec 18, 2018 4:41PM
vasily tchaikovsky
My big sheep dog lived 15 years but quality of life last 2 years was not good .Interesting information.Thanks.
Dec 8, 2018 3:51PM
Don Racette
Jeri Lee Lazar You're DOGgone right it is.
Nov 17, 2018 5:48PM
Kathy Black Hutton
Oct 15, 2018 12:13PM
Pamela Spallin
Oct 10, 2018 12:58PM

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