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In my book series, some of the people of the planet Aurea use giant vultures called Argentavis for transport and aerial combat. These domesticated, selectively bred birds are far larger than their wild couterparts, at around 10 ft tall at the shoulder, 13 ft long, and with a 40-foot wingspan. These birds, being as large as they are, prefer soaring at extremely high altitudes to actual flight, sometimes reaching altitudes of 30k ft above sea level or more. The people of Aurea only have an early Renaissance tech level, so how could they survive riding these creatures without either freezing to death at the high altitude or suffocating from lack of oxygen?

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    $\begingroup$ Are people of Aurea physically similar to humans? Do they have similar biological needs? Do we design a planet for this scenario or change the anatomy of the creature to fit the planet? $\endgroup$
    – Sachin
    Apr 5, 2021 at 10:21
  • $\begingroup$ It's important to note that Hypoxia is known to set in for humans as low as 10,000 feet, 20,000 feet is the altitude at which no human has surpassed without having hypoxia set in, and at 30,000 feet, the pressure of the altitude is only 1/3 what it is at sea level.. Essentially you're asking how to have a living creature operate effectively @ 11.1% of their normal oxygen consumption... Difficult, to say the least. $\endgroup$
    – tuskiomi
    Apr 5, 2021 at 17:54
  • $\begingroup$ The people of Aurea could have developed pump-action lungs or air sacs, both of which would make them more efficient breathers. They could also have developed adaptations similar to seals or whales due to being aquatic in their early evolutionary history. $\endgroup$
    – Alendyias
    Apr 5, 2021 at 19:40

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Simple solution: Aurea's atmosphere, when compared to Earth's one, is more dense, thus at the quote of 30k feet on Aurea the air density and oxygen content are equivalent to the Earth's 18k feet: one feels it, but can still get acclimated, which is something they do.

For getting protection from the cold, don't forget that many populations lived in harshly cold conditions way before somebody brought them so called modern means of staying warm: think of Inuit, Sami, Ainus and so on: furs can do their job in protecting someone from cold.

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    $\begingroup$ Lower gravity worlds will have a higher scale height. Low gravity doesn't just make it easier to fly (useful, if you want to have passenger-carrying birds) but you can also fly higher without necessarily having a dangerously low PPO2. $\endgroup$ Apr 5, 2021 at 11:18
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    $\begingroup$ If you just make the pressure higher, then wouldn't the natives become acclimated to the pressure at the altitude they live at, anyway, and still have problems with changes in pressure? $\endgroup$ Apr 5, 2021 at 13:42
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an early Renaissance tech level

Though it would be slightly anachronistic by real-world standards, you could actually make use of a chemical oxygen generator or rebreather.

According to wikipedia's entry on the history of rebreathers:

Around 1620, in England, Cornelius Drebbel made an early oar-powered submarine. To re-oxygenate the air inside it, he likely generated oxygen by heating saltpetre (potassium nitrate) in a metal pan to emit oxygen. Heating turns the saltpetre into potassium oxide or hydroxide, which absorbs carbon dioxide from the air. That may explain why Drebbel's men were not affected by carbon dioxide build-up as much as would be expected. If so, he accidentally made a crude rebreather more than two centuries before Saint Simon Sicard's patent

Though this is clearly late in a fairly expansive definition of the renaissance, it isn't implausible that the trick could have been devised by a clever chemist (or alchemist) before then. Certainly, gunpowder technology was widespread in the real world by even quite generous definitions of "early renaissance" so access to saltpetre would not be anachronistic.

Engineering even a fairly simple rebreather isn't an entirely trivial exercise, but not an implausible one. They're also safer to build and use if you're not diving and in need of high-pressure air.

As for keeping warm... furs might be good, but also consider the benefits of simple things like hot waterbottles or suitably insulated containers with smouldering coals in. If the passengers are mostly sitting still, these make good additional choices, depending on the loadbearing capabilities of the bird in question.

Note that whilst down makes excellent modern insulation, making the best use of down requires very light fabrics with a very close weave, something that's considerably more awkward and expensive to source as you go back in history. This may or be important, depending on the economic status of the bird-riders. Fur is obviously much easier to make use of, but it is also heavier which may be problematic depending on how strong the birds are.

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It seems unrealistic to me that the birds themselves could survive at that altitude, but perhaps they have specialised metabolisms.

In any case, there is a simple solution: weigh them down when you ride them, if your own weight is not enough.

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  • $\begingroup$ Real world Ruppel's griffon vultures have been seen flying at ~37k feet so it's at least possible for the birds. $\endgroup$ Apr 5, 2021 at 13:58
  • $\begingroup$ @TheWeaselSagas That's likely due to thermals, which, yes, is flight, but the energy expenditure is much lower.. Given, I'm not certain this is the case, but I can say that at least up to the cumulus barrier, it's very likely $\endgroup$
    – tuskiomi
    Apr 5, 2021 at 17:58
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The people of Aurea could be fossorial psychrophiles. This would give them adaptations for cold and lack of oxygen. While they would have no protection against barotrauma, this could be averted by taking care to avoid quick altitude changes

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30000 feet is not a problem for survival, in the short term.

It is just a bit higher than the peak of Mount Everest.(29030ft). 30000ft is only 6% less pressure.
People have not only survived, but actively climbed the mountain to get to the peak of Everest without any oxygen equipment. The human body is quite capable of surviving this environment with minimal exertion, but it does require extensive training to do so, especially if any level of physical exertion is required.

Your riders will be physically completely inert, just strapped in to their mounts.

They will, of course, have to be acclimatized to mountain air.
They will need to be wearing what amounts to arctic wear. Parkas or equivalent.
They will need a period of recovery after a flight at that altitude.

It would not be a good idea to spend a cumulative total of too many hours at altitude. Any time spent above about 26000ft does do permanent damage to the body, regardless of acclimatization one cannot indefinitely survive above that altitude.

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