Hate the early darkness because it marks the end of my garden and the beginning of muddy paws and shoes needing to be cleaned before entering the house, but otherwise winter is beautiful.

I think that's being a bit obtuse. It's obvious that "natural" in this context means not cut short by slaughter/disease/etc. The natural lifespan of the human being is what could be said to mean how long a human lives if an early death doesn't take them. Moreover, that range can include the tribal human living without processed food or electricity or the sheltered city dweller living safely alongside their McDonald's.

Define artificial assistance? If you mean human care like feed and safety, then I fail to see how that is relevant. It's similar to asking if I have data on Boston terrier lifespan without human assistance in the wild. We created these breeds.

The lifespans I gave are lifespans with care (feed, shelter) given in the case of the domesticated species. Im very interested in why you think data on domesticated species living in the wild is relevant, but if you are interested in the hardness of domesticated species, there is an interesting case of a cow escaping the slaughterhouse in Poland to live with a herd of wild bison. No idea what ended up happening, but it caused quite a stir that she survived winter just fine.

Coworker has a cow/calf pair. The cow and calf do get separated in the morning, but after milking they are turned out together on pasture for the rest of the day. Of course, most folks aren't lucky enough to have the space or means (or desire) to raise their own food the way they want. I have friends who love milk but cant stand the sight of an udder... funny. In their words, it is "better not to think about it." Wild. My dream has always been to have a homestead, so I cant imagine the thinking...

Some numbers for relevance:

American bison lifespan is 10-20 years, african buffalo is 10-25, Indian water buffalo averages 25 years, and guar are around the same.

Now let's look at some heritage breeds of domestic cow:

White park cattle are not unheard of to live around 30 years, highland cattle 15-20, Dexters around the same.

Edit: I forgot the dairy cows... holstein 15-20, jersey around the same, friesian around the same.

It is in the nature of cattle to live at least into their teens. Dairy cattle are treated terribly and bred to overproduce, but they are killed when production drops. A good bull, on the other hand, can be kept around for longer, but most dairies do ai now so they dont even have to have a dangerous bull on site.

As an aside, the entire thing is similar to production hens. A production hen's system gives out early (usually due to reproductive issues or cancer) due to being designed to lay an egg a day regardless of anything else. There is a reason family farms who want pets generally avoid the high production ladies if they value health over production.

I really enjoyed this answer. I have definitely noticed a more positive spin on having children from my non American or well traveled friends, and this reminded me of their views.