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So, one of the common arguments made by vegans (and generally people who oppose factory farming) is that the average lifespan of farmed animals has decreased, and that that's a proof that lives of animals are getting worse rather than better. For instance, that cows used to live for decades, and now they only live for years. And that chickens used to live for years, but now they only live for months. Is there any truth to that?

I doubt it, because, well, a cow that lives a shorter life will give us less milk and a chicken that lives a shorter life gives us fewer eggs. There is incentive to make animals live longer, rather than shorter. Furthermore, chickens start laying eggs only when they are a few months old, so if they live less than a few months, they won't give us eggs whatsoever.

Oddthinking
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FlatAssembler
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    Most farm animals are killed when they are past their productive age. In the wild they might live longer - if no other animal ate them beforehand. Remember that carnivorous animals are not vegan. Also, chickens and cows do not exist in the wild... – hdhondt Jul 07 '20 at 10:11
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    You are talking about broiler chickens (raised for their meat). They don't lay eggs. Commercially, layers are kept for 1 to 3 years, the exact time is a financial decision: to maximise revenue. Farmers supply what the market wants, that is why broilers do not live long. As for their welfare: how they are kept is governed by law. – Weather Vane Jul 07 '20 at 10:43
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    @hdhondt I think the claim is regarding the lifespan of farmed animals now, compared to farmed animals some time in the past. (Not compared to their wild counter parts, which as you say do not really exist.) – TimRias Jul 07 '20 at 11:19
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    @FlatAssembler Do you happen to have a source for the claim? E.G: website, news or else? It would be nice to know the exact wording as to avoid misinterpretation. – Entropy Jul 07 '20 at 11:57
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    You need to cite a source. – Daniel R Hicks Jul 07 '20 at 12:05
  • Perhaps animals raised for food have been intentionally selectively bred to decrease the time required to reach market weight. – GEdgar Jul 07 '20 at 13:25
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    @GEdgar Not "perhaps". Definitely. –  Jul 07 '20 at 16:52
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    We want to focus our attention on doubtful claims that are widely held or are made by notable people. Please [provide some references](http://meta.skeptics.stackexchange.com/questions/882/what-are-the-attributes-of-a-good-question/883#883) to places where this claim is being made. – Oddthinking Jul 08 '20 at 12:29
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    It isn't clear what the scope of this claim is. Compared to wild, or earlier farms? Just chickens? (Bred for eggs or meat?) Just cows? (Bred for milk or meat?) Pigs? Goats? Ducks? Turkeys? Does it include whether an animal is culled before it dies naturally, or not? (We've already seen the [lifespan of a male chick at an egg farm](https://skeptics.stackexchange.com/questions/47707/do-male-chicks-get-ground-up-on-most-farms-in-the-industralized-world) is very short.) – Oddthinking Jul 08 '20 at 12:33
  • I found this source - particularly the image - which is a much clearer statement but there are two many animals/claims here. Let's whittle it down to one that you are doubtful about: https://www.meatthetruthforyourkids.com/english/ethics-and-morals/ – Oddthinking Jul 08 '20 at 12:48
  • One might expect that the theoretical "natural" lifespan of a farm animal would have increased over its wild counterpart due to human care of its food supply, health, and protection from predation or other natural hazards. The actual life span of an individual animal is determined by its commercial value to humans; the longer an animal is kept, the more food it consumes, so the more it costs. Animals raised for slaughter would thus have relatively short lives; those raised for milk, eggs, or genetic material would live longer - until there production drops below an economic threshold. – Anthony X Jul 12 '20 at 18:41

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Tl;dr, artificially, yes. Farm animal lifespans have decreased. Naturally? I doubt it.

The average lifespan of a cow is usually 15-20 years (Cow Longevity Conference, 2013), though a website that claims to be a farming sanctuary claims that cows can live for over 20 years. From what I've read, an herbivore/carnivore lifestyle doesn't really affect lifespan. Tortoises and whales are some of the longest-lived herbivores on the planet, likewise for birds like parrots or ravens. Humans live from 65-80 years on average depending on environmental conditions and genetics--however, gorillas (herbivores) and chimps (mainly herbivores but they do eat meat/insects) live 35-40 years in captivity. So diet doesn't really measure longevity. This doesn't mean not avoiding salt, sugar, and trans fats won't do any good. It would but body weight and BMI are better measurements of overall health and eventually lifespan.

Most cows that are raised for meat are usually killed at around 1000-1200 pounds (453-544 kilograms), or about 18 months of age with the conventional methods of feeding (corn, vitamins, prophylaxis antibiotic regimens). Diary cows are slaughtered after 5 years, once they are decided to be "spent."

So, to answer your question. Lifespan of farm animals have decreased artificially because humans decide when they live and when they die. Humans also determine which traits to continue breeding, so a trait to gain weight might be detrimental for long-term health but evolutionary rewarding for short-term/fast growth.

References: http://www.milkproduction.com/Library/Scientific-articles/Management/Cow-longevity-economics-The-cost-benefit-of-keeping-the-cow-in-the-herd/

https://www.farmsanctuary.org/learn/factory-farming/dairy/

https://chimpsnw.org/2013/03/how-long-do-chimpanzees-live/

Michael Tran
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    The whole bit about herbivores vs carnivores since utterly irrelevant to the question (and the answer). – TimRias Jul 08 '20 at 08:25
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    [Welcome to Skeptics!](http://meta.skeptics.stackexchange.com/questions/1505/welcome-to-new-users) I am not sure why this has been selected as an answer. It only addresses cattle (pedantry: cows are female) and not other farm animals. It goes into a side issue about herbivore versus carnivore, which seems irrelevant. It only considers longevity shorter because the cattle are killed when they ready to eat or don't produce milk. If that is included as shortening the lifespan, it isn't clear what the point of the original claim is. – Oddthinking Jul 08 '20 at 12:41