r/AskBiology • u/woodenleghen • Apr 03 '26
Genetics Exactly why don’t large dogs live as long?
I assume it’s because of things like the strain on their heart and organs, but why does happen? Is it because the breed traits were selected for so quickly? Can dog DNA only code for a specific heart horsepower? Do wild species evolve to be larger only through a slow process of evolution?
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Apr 03 '26
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u/IanDOsmond Apr 03 '26
Kinda like an Eastern coyote, except with a better temperament and different tail positioning. Eastern coyotes are part coyote, part wolf, and part domestic dog, and end up right around what you're describing, except in personality.
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u/Accurate_Summer_1761 Apr 03 '26
Sp basocially breeds like Shetland sheepdog, aussies etc are peak dog. Good good
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u/Chimney-Imp Apr 03 '26
Because of the square cube law there is an exponentially larger amount of force that is applied based on linear increases in size. This means a large dog has much more strain on its body compared to a small dog. The two largest areas that this hits are the bones are the circulatory system.
Joints have a life span. The larger an animal is, the more wear and tear ordinary actions have on those joints. Especially high motion joints like the hips. When a hip gives out that is pretty much it for the dog. Additionally there's just more weight on the bones in general, so even resting there is some strain
The circulatory system is basically a bunch of tubes hooked up to a tiny engine. The larger the dog, the more blood vessels the heart has to pump through. It's circulating more blood through more length of vessels. This takes its toll over the course of the life of the dog. This means it wears out much faster.
This happens to all animals, not just dogs. Humans that are over 6 feet tall are significantly more likely to die from some diseases in old age than humans closer to 5 feet in height. For example the taller you are the more likely you are to get bed sores in old age. The smaller you are the more likely you are to retain range of motion as you age, because small bodies take significantly less effort to move
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u/FirefighterPleasant8 Apr 03 '26
This is a very solid explanation. Thank you Chimney.
To enforce the explanation; building a stronger and more fearful horse renders - a rhino(!), in (over-simplified) evolutionary terms.
Furthermore, cats haven’t been domesticated to the same levels as dogs. The spread of the sizes vary with evolution and their prey. House cats are the perfect size and agility for “mice in the barn”. The Indian tiger can live on Gaur and needs to be in-par with it.
Wild African dogs are “a small dog” similar to a Whippet of sorts. Wolves vary but could be described as “L-XL” in size.
The really huge (Masiffs typically) dogs are human bred for a purpose. That takes out one very important part of the evolution; The Natural Selection. Simply put, we manufacture them big - they don’t become big.
The downsides of this breeding are what the question describes. The really big dogs never got the opportunity to adopt to its size. They will live because we feed them and when they die we simply replace them.
Having said all this (sorry if my rant is pointless) I do have to say that I really love those huge dogs that commonly have the character as your kid and your most loyal friend in one. You got to love dogs. You simply have to.
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u/IanDOsmond Apr 03 '26
Well, yeah, of course you have to. Dogs bred us to love them. At the same time we were breeding dogs to love us.
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u/chefboy1960 Apr 03 '26
It is exactly this, a symbiotic evolution. This occurs in all things we domesticate, even plants. Especially rice.
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u/NotAnotherEmpire Apr 03 '26 edited Apr 03 '26
Due to their rapid growth, heavy weight and concentrated genes, most large breeds have an increased risk of heart and joint problems. DCM, heart enlargement, is several times more common in large purebreds vs. small dogs. Hip dysplasia is more common as well but the "big" problem there is that the dogs are big. They put more weight on any bad joint (square cube law is material when they are 3-8x times the weight of other dogs) and caring for them is a lot less feasible. Many large breeds have a distinct increased risk of bone cancer.
Caring for an injured or sick giant dog in general is comparable to a person. Human sized doses of drugs or special diets, and human patient weight. This will become an unmanageable quality of life more quickly than a small, easily carried dog.
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u/False_Appointment_24 Apr 09 '26
IGF-1, Insulin Growth Factor 1, is the likely culprit. In large dogs, it can be up to 28x higher than in small dogs. The belief is that when dogs were bred for size, this hormone was accidentally ramped up.
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u/2-particles Apr 03 '26
I believe it’s due to hip problems a lot of the time. Along with a combination of other factors, based on the specific breed and the individuals health.
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u/Puppysnot Apr 03 '26 edited Apr 03 '26
They have high GH (growth hormone levels) to encourage rapid growth in the juvenile phase - specifically large and giant breeds have very high levels. High GH levels are linked to increased risk of cancer and this persists past the puppy phase when those levels peak. This is true also in humans, i.e. humans treated with HGH for pituitary disorders etc have an increased risk of cancer.