Quite simple really. How large can a bat grow before it is too big to fly. I would like a bat with a similar external and internal structure to a real bat just scaled up. I am willing to accept changes to the wing to body size ratio as I assume that bats can't get that much bigger than they currently are by simple scaling up due to the square cube law.

  • $\begingroup$ this Earth or somewhere else? if you can mess up the atmosphere a bit, then you will be able to launch a Quetzalcoatlus-size bat with little problems. $\endgroup$ – Thỏ Già Aug 23 '16 at 14:51
  • $\begingroup$ Currently the planet is Earth with different animals and no humans although this might change in the future. $\endgroup$ – Bellerophon Aug 23 '16 at 14:55
  • $\begingroup$ Don’t be so quick to accept one answer! Wait at least 24 hours, for people in different timezones to read the post. It discourages more answers and cross-fertilization of ideas. $\endgroup$ – JDługosz Aug 23 '16 at 16:11

The worlds largest bat, the Golden-Crowned Flying Fox, weighs in at a wingspan of 1.5-1.7 meters and a weight of only 1.5-2.6 lbs. But with these estimates, we can absolutely do better.

Introduce Argentavis, the largest bird ever to exist weighing in at an impressive 154-159 lbs with an estimated wingspan of 23 feet! Doing fancy math and applying the square cube law we get a bat size of 8.85-14.21 lbs for a bat with an equal height to Argentavis. But you know what they say, go big or go home.

Behold the king of evolutions air,Quetzalcoatlus, weighing up to an estimated 550 lbs with a theoretical wingspan of 10 meters (32 feet 10 inches), this was the largest animal to ever fly. Again scaling up the Golden-crowned Flying Fox using the square cube law we get a bat of equal size should weigh in at around 39.6 pounds, now while this seems odd, keep in mind 2 things.

  1. Mega bats, such as the Flying Fox, tend to fly much the same as Quetzalcoatlus is theorized to, mostly gliding with short burst of energy to increase height.
  2. Mega bats are known to weight a lot less than they appear, as I said before, the Golden-Crowned Flying Fox only weighs at most less than 3 pounds!

Now how you're going to keep a bat this size alive is another manner as it will need to almost always be eating food very high in calories and fat, but I'll let you save that for another question.

  • $\begingroup$ Looking at your numbers it seems to me that you are decreasing wing loading as you increase wing span. Is there a reason for this? Am missing something? It's completely possible I'm being a twit, but from some rough estimates I did, it looks like the aforementioned bat has a wing loading of ~2.5kg per m2. Whereas your 10 metre wingspan looks like a wing loading closer to ~1.1kg per m2. $\endgroup$ – Myrdden Wyllt Aug 24 '16 at 18:17
  • $\begingroup$ @MyrddenWyllt my math may be wrong, feel free to edit it if you feel I have erred. $\endgroup$ – TrEs-2b Aug 24 '16 at 18:22

I'm assuming by your question, you're trying to re-create something like the man-bat from the Batman series. If I'm wrong here, please correct me.

I've been researching and found that the largest current bats that exist are the Giant golden-crowned flying fox species native to the Philippines.

Bearing in mind that this bat has a wingspan longer than most humans are tall and that if this image is to be believed, then it's fair to say it's pretty big anyway.

Now, onto your question, could they get any bigger? In all honesty probably not any time soon.

Not only do we have the square cube law to take into account as you previously mentioned in your question, but you've also got the dynamics of the Planet to take into account.

During the age of the dinosaur, scientists think that the composition of the atmosphere had much more Oxygen than we do right now and that that contributed almost proportionally with the size of the dinosaurs.

Since the Industrial Revolution, we humans have been introducing carbon emissions and greenhouse gases into our atmosphere, we're slowly killing off the trees that produce oxygen and the corals in the seas (which are made of carbonaceous material) adding more carbon dioxide into the system.

TL:DR - Probably not until we either stop polluting or die off.

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    $\begingroup$ To be honest a bat with a 1.7 metre wingspan is larger than I assumed possible. $\endgroup$ – Bellerophon Aug 23 '16 at 14:24
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    $\begingroup$ I just want to point out that we haven't been 'introducing carbon emissions and greenhouse gases' since the age of the dinosaurs. I don't think you can pin that solely on humans. $\endgroup$ – DaaaahWhoosh Aug 23 '16 at 14:29
  • $\begingroup$ @DaaaahWhoosh No, you're right. Only since the industrial revolution. Sorry for the mis-interpretation, it wasn't my intention to say from the moment the dinosars died away as that wasn't the intent there. $\endgroup$ – Raisus Aug 23 '16 at 14:29

TrEs-2b gave a very good answer. I'll just add in a bit of bat physiology which might explain why modern bats don't get as big as giant pterosaurs.

Bats are warm-blooded. Their wings are made of living membrane through which blood flows. This means that bat wings are great for shedding heat when the bat gets too hot: a huge surface area to do just that. But conversely they are rubbish at retaining heat when the bat gets too cold.

All the really big bats live in the tropics. Something the size of a fruit bat would chill itself to death in the likes of Canada or Norway as heat leaked away from its wings every time it flew. Even the microbats (microchiroptera) have to take measures to avoid the cold: hibernating or migrating away.

So make it a fruit bat (megachiropteran or megabat) type of bat, and have the climate of the place it lives exactly right for getting huge yet staying not too hot and not too cold. Of course, the megabats don't have sonar - you don't need echolocation to catch fruit. So if you want a bat with sonar, those are out.

Now obviously the pterosaurs also had naked membrane wings and DID get enormous. So they cracked the staying warm problem.

  • $\begingroup$ Actually, pterosaur wings aren't as simple as you think. They were more likely solid sheets of skin strengthened by actinofibrils, fibers spaced closely together. The wing membranes also contain a thin layer of muscle, fibrous tissue, and a unique, complex circulatory system of looping blood vessels. $\endgroup$ – JohnWDailey Jan 2 '17 at 17:52

Even though I can't give an estimation of factible size for a bat. There is a problem with an important function of bats if you make them grow, the function of their radar.

Bats have a radar which works using sound, so if you increase the size of a bat, then the radar lowers its frequency. Therefore, even if the bat can fligt, probably it cannot find food, unless it changes its diet.

The second problem would be the diet, a bigger bat needs to find preys bigger as well, otherwise it will waste too much energy to find more small preys.

  • $\begingroup$ Not all bats use echolocation. So this really isn't an issue. See my answer above. That bat doesn't use it at all $\endgroup$ – Raisus Aug 23 '16 at 14:42
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    $\begingroup$ Why would the frequency lower? Couldn't the relevant organs and structures stay the same size? $\endgroup$ – rek Aug 23 '16 at 14:45
  • $\begingroup$ we don't know its diet, so there is no reason to rule out the size just because of radar malfunction. $\endgroup$ – Thỏ Già Aug 23 '16 at 14:56
  • $\begingroup$ Consider that dolphins & whales use ecolocation quite well, and in the same frequency ranges as bats. $\endgroup$ – jamesqf Jan 2 '17 at 18:20

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