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I am building a world where most of the wildlife is capable of flight and there is an ecosystem that is almost totally isolated from the ground. Is it possible to have a mammal, roughly the size of a labrador retriever, be capable of gliding and landing softly on the ground, without any injury? I am currently thinking of a flying squirrel and bat hybrid, however with hollow bones to make it lighter. But could hollow bones support the weight of such a animal? Could it be possible to have solid bones in areas like the leg, and hollow ones in other areas?

The world would be exactly like Earth in almost every way, including air density and gravity. The mammal can be the size of a border collie, but it needs to be larger than a falcon and any bat or flying squirrel. It is also predatory.

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  • $\begingroup$ It's hard to say without more details about the planet your creatures live on. Is your planet earth-like? If not, what's the atmosphere like? Gravity? Those are the big two I'm aware of, but you might want to add those or consider them going forward because $\endgroup$ – Pleiades Oct 31 '17 at 0:30
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    $\begingroup$ Do you mean the same size as a dog, or the same weight? A Giant golden-crowned flying fox has a 5 1/2 foot wingspan, but weighs less than 3 1lbs. $\endgroup$ – Arcanist Lupus Oct 31 '17 at 0:43
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    $\begingroup$ dinosaurs could do it way bigger $\endgroup$ – Fleon_ Oct 31 '17 at 0:44
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    $\begingroup$ Welcome to Worldbuilding YZ88! I'll answer this question later, but I reccomend distinguishing if you want this to be a true flying animal or a gliding animal (as in, does it flap to gain lift or does it fall with style?). Your question says 'glides to the ground' but your title says 'Flying'. $\endgroup$ – Lot-Of-Malarkey Oct 31 '17 at 1:07
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    $\begingroup$ I would say that the creature is the same size as the dog, but not necessarily the same weight. It could be much lighter. I am aiming for a flying creature, but I didn't think it would be possible for such a large animal. $\endgroup$ – YZ88 Oct 31 '17 at 10:35
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Two things affect the possibilities:

  1. What is is the air density. Thicker air means less wing needed, or slower speeds needed for a given amount of lift. You can do this with either higher pressure, colder temperatures, or by mixing a heavier gas as one component. E.g. Sulfur hexafluoride.

  2. What is the local gravity? Lighter gravity means less muscle needed.

To get both lighter gravity and higher pressure, make your planet larger, but lower density.

Edit for earth conditions:

The largest birds I know of are condors and albatrosses. I've found one mention of 15 kg -- 33 pounds. Weight of a small border collie. These birds do not actually fly much, so much as soar. They are very good at finding and using updrafts. And for these critters we're talking wingspans of 10 feet. This is not a critter that maneuvers well through the treetops.

While lighter bones will help, especially if they are built like trusses, and not like columns, most of the weight is muscle. So things you can do to make a larger flying critter possible:

  • A type of muscle that can produce more power per pound. Faster individual fibre recovery time. Lots more mitochondria per cell. A better reaction to create ATP from ADP
  • A mechanism so that muscles can use short chain fatty acids for energy, instead of exclusively on glucose.
  • A replacement for hemoglobin that can carry more oxygen per volume of blood.
  • Viscosity agents to make blood slide through capillaries more easily.
  • Valving in the veins in flight muscles to help the heart circulate.
  • Some form of flow through lung, so lungs don't spend time exhaling. If you set it up so that air flows in through the mouth, and out through an opening further back, and set up the blood flow in the opposite direction, you have all the benefits of a counter flow exchanger. This should double to triple the effectiveness of the lungs. The animal has to have some kind of diaphram to breath when stationary, but once flying, can use flow through.
  • A mechanism to store energy that can be metabolized FAST.
  • A mechanism that can store oxygen for later use in burst mode. See Niven's "Legacy of Hereot" series on Grendel metabolism.

A 'tree top' ecology would also have room for really good jumpers. Think in terms of self contained catapult, where the muscles can ratchet back tendons to store energy, and then the critter leaps. Grasping limbs would be optimized for shock absorption to catch branches on the other side.

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    $\begingroup$ For the sake of the question, the world would be exactly like Earth in almost every way, including air density and gravity. The mammal can be the size of a border collie, but it needs to be larger than a falcon and any bat or flying squirrel. $\endgroup$ – YZ88 Oct 31 '17 at 0:50
  • $\begingroup$ Avian quality lungs is a major necessity. But the biggest requirement is. .. food, lots and lots of food that being a flyer makes the dog better at catching $\endgroup$ – Garret Gang Oct 31 '17 at 19:37
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According to Wikipedia, the Andean Condor is about 2 to 3 times the size (not weight) of a Labrador.

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    $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. $\endgroup$ – Vylix Oct 31 '17 at 1:39
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    $\begingroup$ We generally avoid one line answer. Can you elaborate more? A brief paragraph is enough, and maybe add an image to make it better! $\endgroup$ – Vylix Oct 31 '17 at 1:41
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    $\begingroup$ @Vylix - this answer seems perfectly fine to me. What is your issue with it? And one line answer are perfectly acceptable when they answer the question in one line. $\endgroup$ – Davor Oct 31 '17 at 10:09
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    $\begingroup$ @Davor How does it answer the question when it specifically asks for a mammal? We already know that birds can fly. $\endgroup$ – pipe Oct 31 '17 at 10:24
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    $\begingroup$ @pipe: The question hinges on the viability of an animal of this size flying. The manner in which it reproduces does not meaningfully affect its flight capabilities. The answer points out that animals larger than the OP's intended size already exist in real life, and therefore there is no issue with OP trying to create a large-dog-sized animal that can fly. $\endgroup$ – Flater Oct 31 '17 at 10:27
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As Fleon mentioned, the Pterosaur flew on earth. It is estimated to have weighed 250 kilograms, according to wikipedia. There certainly were some with a wingspan of 10 feet or more.
https://www.livescience.com/24071-pterodactyl-pteranodon-flying-dinosaurs.html

So lab sized seems easy to imagine, and sure, there is no reason to suppose it couldn't safely land.
As far as hollow-bones go, they'd definitely be an advantage for flight. To continue with the pterosaur example, it could likely also could run on the ground, using its wings like front legs.
It may be possible for a solid-boned creature of dog-size to fly, but easier to go with hollow bones.

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    $\begingroup$ Bats are a great example if you want to see what running on your front wings looks like. Notice how the bat's wings are clearly lifting its body more than its hind legs. It makes sense that the wings are stronger due to being used a lot more. $\endgroup$ – Flater Oct 31 '17 at 10:52
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    $\begingroup$ to add birds and pterosaurs have a mix of hollow bones and solid ones, and the largest are the size of giraffe so a dog sized animal is no problem at all. $\endgroup$ – John Oct 31 '17 at 14:33
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I am going to base my reply by incorporating some of the information supplied in other questions, and adding some input of my own.

As mentioned by Fleon and Vincent the pterodactyl (member of the pterosaur family) was quite large, and was capable of gliding and/or flying.

As mentioned by James McLellan the Andean Condor is larger in size than a Labrador, but does not definitively say that it weighs less, but it probably does. (Fact check: Male Labradors weigh between 29.5 and 36 kg, while an Andean Condor weighs in at up to 15 kg).

And then of course there are bats - mammals capable of true flight, but usually quite small. Bats do not have hollow bones, but their bodies are tiny compared to their wingspans. (Fact check: The largest bat is the giant golden-crowned flying fox with a wing-span of 1.5 to 1.7 m, though they are very light, weighing 0.7 to 1.2 kg).

So the short answer to your question is Yes it should be possible. But what would the survival imperative be to develop the ability to fly or glide?

One might be the presence of very large, prolific burrowing carnivores. That would be a great motive for getting off the ground and staying off it.

Fact checks provided aided and abetted by @Mauro, so thanks again!

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    $\begingroup$ en.wikipedia.org/wiki/Giant_golden-crowned_flying_fox fruit bats are quite large (extremely light) and an Andean Condor weighs in at up to 15kg (33lbs) en.wikipedia.org/wiki/Andean_condor $\endgroup$ – Mauro Oct 31 '17 at 8:49
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    $\begingroup$ Fat check: Labradors tend to weigh in at whatever weight their owners let them get to! :) Your numbers are correct for show dogs, generally. Working dogs are much smaller and lighter though. 20kg would be on the low side but not unknown. $\endgroup$ – Graham Oct 31 '17 at 13:20
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Your suggested size is not an issue.

I would say that this albatross is roughly the size you're after?

enter image description here

The existence of the albatross in real life proves that the size of the animal in no way precludes it from flying.

Not convinced? How about this bad boy?

enter image description here

That's the Argentavis Magnificens, and it is massive.

The Andean Condor was mentioned in another answer and is an equally relevant addition to the list of massive birds.


Hollow bones.

I don't think they are necessary. There are a few interesting things to note about hollow bones, and I suspect these differ from your current interpretation of why hollow bones are useful:

Reference link

With hollow bones a bird can fly very long distances without getting worn out from carrying its own weight.

The benefit from hollow bones isn't so much that they allow you to fly, but rather than they minimize exhaustion. Once you're flying, your torso is essentially suspended from your wings. The heavier your body is, the more stress that puts on your shoulders.

Unless your flying mammal is capable of long-distance flight, the hollow bones don't seem all that necessary. But there's nothing wrong with giving the animal hollow bones.

You might think these bones are fragile, like empty egg shells, but birds can't afford to have bones that break easily. The hollow bones are supported by internal struts – structures inside that help brace the bone so that it can withstand longitudinal pressure (pressure along its length).

As per your question:

But could hollow bones support the weight of such a animal?

Given the explanation quoted above, I'd say that the hollowness of the bones does not meaningfully affect the animal's capability of supporting its own weight.

Birds have hollow bones, and let's look at avian weight record holders:

  • The largest bird in the fossil record may be the extinct elephant birds (Aepyornis) of Madagascar, which were related to the ostrich. They exceeded 3 m (9.8 ft) in height and 500 kg (1,100 lb).
  • The largest carnivorous bird was Brontornis, an extinct flightless bird from South America which reached a weight of 350 to 400 kg (770 to 880 lb) and a height of about 2.8 m (9 ft 2 in).

Those are massive birds, which proves that it's possible.

Do note that there are no currently living flying birds that weigh more than e.g. a labrador. It seems that heavy flying creatures are at a disadvantage (unless they went extinct for an unrelated reason?).

But that doesn't mean that yours can't exist. You're talking about a completely different ecosystem where most of the wildlife flies. It's perfectly possible for a heavy flying animal to not go extinct.

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  • $\begingroup$ I think you are wrong about flying relying on agility. Being heavy is a disadvantage for a flier because they need to generate more lift, and that is hard (and costs a lot of energy). $\endgroup$ – Martin Bonner supports Monica Oct 31 '17 at 11:17
  • $\begingroup$ @MartinBonner: The hollow bone argument (which is what I was talking about) is not referring to the lift of the wing, but rather the animal's ability (= lack of exhaustion) to keep the wing steady in order to carry its body weight. Less weight to carry = less exhaustion. The lift argument is equally correct but simply not what my answer was focusing on. Notice that the OP gave no restrictions as to wingspan (thus rendering the lift argument mostly moot), and the question itself is focusing on whether the animal's wings could support its weight, not whether they can generate enough lift. $\endgroup$ – Flater Oct 31 '17 at 14:05
  • $\begingroup$ @MartinBonner: Nonetheless, I removed the mention to prevent any wrong inferences. $\endgroup$ – Flater Oct 31 '17 at 14:06
  • $\begingroup$ they went extinct for hte same reason everything large went extinct at the time, becasue a mountain fell out of the sky and screwed the global ecosystem like a roofied prom date . $\endgroup$ – John Mar 3 '18 at 4:40
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The largest flying animals weighed 500lbs and were the size of a giraffe, on an earth like world a dog sized animal (whether chihuahua or great dane) is no problem whatsoever.

https://www.researchgate.net/profile/Mark_Witton/publication/279618696/figure/fig5/AS:306094300778505@1449989976915/Figure-5-Comparative-sizes-of-Quetzalcoatlus-northropi-25-m-tall-at-shoulder-250-kg.png

Let's be clear hollow bones let you be bigger for the same mass, it is why a giraffe sized animal only weighs 500lbs. All animals with hollow bones have a mix of solid and hollow bones, not all bones in a bird are hollow so you can mix them up just fine. There are also varying degrees of hollow cormorant bones are much thicker walled while still being hollow.

The only issue you have with hollow bones is mammalian breathing does not work with air sacs (hollow bones need air sacs to be hollow), but your alien life is not earth mammals (even if they look like them) so you can give them a bird like breathing system with no issue.

Lastly hollow bones can be just as strong if not stronger than solid bones, this is quite easy to see since a goose wing strike can break a human's arm. The air sacs in hollow bones are pressurized which stiffens them additionally hollow bones are cross braced they keep the bone anywhere it is need to maintain strength. Think of it like an I-beam vs a solid steel bar, the I-beam is just as strong but a lot lighter.

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