How come smaller dogs generally havw longer life spans than larger dogs, but larger animals, like horses, have an even longer lifespan?
In: Biology
We only vaguely understand what determines lifespan. We’ve got some theories but none of them are sophisticated enough yet that we could, say, analyze an unknown species and accurately guess its lifespan.
But what we do know is it has to do with the genetic code that defines a species. Some animals have tougher organs that can go longer without aging. Other animals have DNA that’s more resilient to accidental errors during copying. There are a handful of factors like that that contribute to lifespan.
In dogs’ cases, while there are dozens of breeds they all descended from the same common ancestor and they share a lot of genetic similarities. So in general, most breeds have a heart that is just as “tough” as any other dog breed. That’s why larger dogs tend to die quicker: a bigger body works the heart more so they “wear out” faster. It’s also notable here that dogs are a HIGHLY domesticated animal, for the past few thousand years we have purposefully accelerated their evolution towards dozens of different “working breeds” and sometimes “long lifespan” wasn’t something the breeders cared about. Some of the smaller breeds have intentional birth defects that can significantly shorten their lifespans!
But while horses and dogs do have some common mammalian ancestor somewhere, it’s much more distant so their genetics are very different. Horses have a much “tougher” heart and other organs. So they handle being a larger creature with a longer life. We haven’t tinkered with horses through breeding quite as extensively as we have with dogs.
Put another way: If the “ancestral dog” is like a Honda Civic, all of the breeds are like people’s aftermarket tinkering. Some people make sporty models, but those parts tend to have a short lifespan. Other people focus on reliability and make a car that lasts even longer than the original.
But horses are like a Kenworth truck cab. They are similar to cars and have similar parts, but everything was built to haul much more weight and be very reliable for long trips. There’s not quite so much diversity among different kinds of trucks like this.
Other animals are like space shuttles, and wagons, and trains, etc. They’re all built out of such different parts it creates completely different lifespans. Nature isn’t always optimizing for longer life.
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