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Sequel to this question.

Basically, I've been designing an intelligent avian race based on bats. After reading through some of the answers for the linked question, I've decided that these bats will be a cross between our modern bats (such as the Flying Fox) and Anurognathid pterosaurs. I'm still going to call them 'Bats' just for convenience though.

However, I've been pondering one key aspect to my design: I've been wanting to make these bats with another set of arms.

This is mostly because, as an intelligent race, being capable of fine manipulation is an asset. My bats have thumbs like the Flying Fox and 2-3 non-webbed fingers. However, since many pterosaurs have long metacarpals in proportion to their fingers and thumbs I'm worried that these hands wont be as flexible. Additionally, bat feet don't seem to be as capable of fine manipulation like some bird feet are.

So here's what I've been thinking: What if these bats had a small set of arms that could fold close to the body when flying?

These arms don't just have to be for fine manipulation. Bats can catch small prey with their feet, but appear to be limited because of how their legs move up and down with their wings when they fly. If they had a set of arms that could move independently of the wings, they could probably more easily catch prey without throwing off the flight.

Or, they might be able to. My main questions regarding this setup are:

  • What anatomical changes need to be made for this setup to work?

and

  • Could a bat like this even fly?

Edit: If it helps for answering, these bats would be among many species that evolved from a hexapodal ancestor. The world they come from is separate from our own world, so don't let the fact that we don't have hexapodal mammals on Earth limit you!

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    $\begingroup$ The main problem would be fitting in another set of muscles - in the chest and new arms - which necessarily takes space away from the muscles used for flying. You'd be dealing with a different power-to-weight ratio, and you'd need to question whether the new arms could be strong enough to grab and hold prey. $\endgroup$
    – Tim
    Commented Oct 27, 2017 at 2:31
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    $\begingroup$ Why not? Insects do just fine. The only thing is that you would need a completely separate evolutionary tree going back to around the last common ancestor of vertebrates, since the 4 limb body plan is buried pretty deep in the DNA. Not a problem if they're aliens, of course. Maybe you could have another manipulatory appendage, like an elephant trunk? $\endgroup$
    – jamesqf
    Commented Oct 27, 2017 at 3:47
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    $\begingroup$ Essentially Pegasus bats? $\endgroup$
    – nzaman
    Commented Oct 27, 2017 at 5:47
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    $\begingroup$ @nzaman if that helps you imagine it better, then sure, why not. Although by that logic, angels are Pegasus humans ;) $\endgroup$
    – Lot-Of-Malarkey
    Commented Oct 27, 2017 at 6:52
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    $\begingroup$ @Lot-Of-Malarkey : or possibly Pegasi(sus?) are angel horses... $\endgroup$
    – Joe Bloggs
    Commented Oct 27, 2017 at 9:45

3 Answers 3

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However, I've been pondering one key aspect to my design: I've been wanting to make these bats with another set of arms.

This is mostly because, as an intelligent race, being capable of fine manipulation is an asset.

I honestly don't understand why this means they have to have 6 appendages, adding a functional hand to the wrist of the wing would be more efficient but if that's your goal....

The key aspect is what Tim mentioned:

The main problem would be fitting in another set of muscles - in the chest and new arms - which necessarily takes space away from the muscles used for flying. You'd be dealing with a different power-to-weight ratio,

Your bat would need larger wing spans and improved musculature. If basing this on terrestrial examples its going to be a meatier bat with wider wings. This doesn't necessarily dictate your end appearance. On your world organisms could have discovered a more efficient and powerful muscle equivalent which would be less weight but more power. Also its skeleton could be made from a lighter stronger substance thus saving weight.

This is the inherent problem with trying to ask if a creature is realistic. The possibilities of evolution are only limited by physics, chemistry and imagination.

I just wanted to clarify another point:

Why not? Insects do just fine

Many of the mechanics used by insects do not scale well to larger organisms. A great deal of their abilities are only achievable because of their size with those constructs.

If you scaled an ant to the size of a human, it would no longer be able to lift 10x its body weight.

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    $\begingroup$ Thanks for clarifing "Insects do not scale" in the question. It's important for this type of question. Along with brains need to be a certain size. $\endgroup$
    – P Chapman
    Commented Oct 27, 2017 at 16:02
  • $\begingroup$ Your right it might need a bigger brain to accommodate the new dexterous appendages. But that potentially depends more on its intelligence, for us humans more of our brain is dedicated to processing difference senses and interpreting that information. But neural development can be way variable the muscle development. $\endgroup$
    – anon
    Commented Oct 27, 2017 at 16:10
  • $\begingroup$ Still on scale: You got me thinking a bit about. could we design a brain that gets the same power in a much smaller form that could that goes into a bunch of specialties I do not have. $\endgroup$
    – P Chapman
    Commented Oct 27, 2017 at 17:35
  • $\begingroup$ It's not unrealistic, the problem is we really don't know enough about the brain to even begin trying to optimize it. But we do know that intelligence isn't directly proportional to brain size, though there is a correlation. $\endgroup$
    – anon
    Commented Oct 27, 2017 at 17:47
  • $\begingroup$ Not only don't insect features not scale, insects have exoskeletons, so you can stick more wings almost anywhere. A third pair of arms necessitates some place to put them, which means another set of shoulders. $\endgroup$
    – RonJohn
    Commented Jul 23, 2018 at 22:19
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If you are proposing an entirely alien world then there is nothing to stop this arrangement from occurring. In our world the Cambrian explosion generated a large number of different body plan types each of which would constitute distinct phyla today. The vagaries of specific conditions at the time and luck have given us the body plans and phyla we see today. So animals such as Hallucigenia are sadly absent.

Two arms would not be very aerodynamic on a flying animal, so they should be capable of folding flat against the body as you suggest. You might want to think of some good reasons why arms and hands were indispensable to these creatures to ensure that they would be selected for.

Perhaps the main issue that you face is ensuring that this creature is not too heavy to fly. There is a physical limitation on what wings are capable of. As well as all the normal animal bodily functions you also need to equip it with two legs, two wings, two arms and a head that is capable of supporting a brain that can develop intelligence. That’s quite a load, but given the ingenuity of nature I suspect it’s doable.

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This creature would likely have evolved from a creature with a large heavy tail, in order to make the central pair of limbs into the wings. It is likely that such a creature would naturally stand hexapedally, and only raising the hands to do things.

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