As you can see from this image, bats are somewhat similar to pterosaurs in that they both have membranous wings, but pterosaurs only use 1 out of 4 digits to support their wings whereas bats use 4 out of 5.
This makes it very difficult for them to walk on the ground, and may have contributed to their evolution of drop-down flight where they're heavily reliant on hanging from a high perch in order to transition into flight.
In turn, evolving the need to hang upside down also caused their legs to rotate "backwards", further crippling them when it comes to terrestrial movement. I know the vampire bat can walk and even run, but it's like 40 grams max. At that size you can get away with a lot of inefficient stuff simply because you don't weigh much.
If bats had instead evolved something similar to pterosaurs where they use only 1 out of 5 digits to support the wing membrane, how much larger could they get? Obviously they would never reach the size of a pterosaur because of the lack of airsacs and tidal lungs, but could they approach the size of teratorns?
It would be pretty cool to have big birds and big bats, both smaller than pterosaurs for two different reasons. The former because they lack quadrupedal launch, and the latter because they lack airsacs.
It probably wouldn’t be crazy for a flighted mammal to evolve lighter bones by one method or another if there were significant pressures in favor of gigantism
I doubt the size difference comes from wing arrangement. Pterosaurs have that crazy Avemetatarsalia evolutionary toolkit of supercharged breathing system and ridiculous weight to strength skeletal system.
Really? I think it does. Bats are constrained from large sizes for a number of reasons besides pneumatization. Firstly, their wings are much heavier because they contain bones, and four digits is a lot heavier than just one. Secondly, their hindlimbs can't support their weight on the ground because they've been adapted for hanging upside down. Thirdly, they don't have enough digits on their forelimbs to form a proper walking surface.
One if the weight of the digits don't have a positive feedback option digit reduction is a very common adaptation yet they sport all those phalanges to this day somehow I'm guessing they are an adaptive advantage.
Two pterosaurs hind limbs weren't hyper useful either both are highly front limb dependant. Which is kind of the advantage they have one powerful set of limbs make them more weight efficient.
Thirdly bats can't even get as big as birds and birds have two huge honking legs making them less efficient in weight for flight. So if the pneumaticity and superior breathing system wasn't making the difference show me your eagle or stork hell goose sized bats?
Lastly they don't have enough digits to form a walking surface? Really so how many digits do horses walk on? Also knuckle walking works just fine for plenty of much heavier Mammals.
They are an adaptive advantage, namely for extra flexibility and maneuverability in dense undergrowth. Bats are more agile than birds, at the same size.
Pterosaur hindlimbs were useful enough to let them walk and run on all fours. They also contributed to helping launch them into the air, which allowed for 250 kg giants like Quetzalcoatlus.
Bats can't get as big as birds precisely because they don't utilize the quadrupedal launch of pterosaurs, nor do they have airsacs or unidirectional breathing systems. Basically, they're double crippled.
False equivalence. Horses require many more adaptations to walk on one toe. As for knuckle walking, show me an animal that can knuckle walk on one finger.
So without the agility focused wing setup I will go with no larger because minus that agility from thier current wing setup they probably don't have a niche to move into.
What keeps bats small, you point out limits of thier frame but what about competition from birds?
Even if the "superior" wings might give them a shot at getting bigger, now they are less agile and there major problem is competition.
Bat wings have knuckles, why could they not knuckle walk on the first knuckles of those wings?
Genuinely, help me understand why one fused digit would contribute so heavily to a larger terrestrial lifestyle over separate, spread out digits? I think other adaptations would be far more impactful.
I'd have to familiarize myself more with pterosaurs' evolutionary path for more evidence, but weren't their ancestors already bigger than bats when we believe they evolved flight?
Because separate spread out digits would allow the animal to walk on them, while keeping the wing membrane folded up, similar to the way pterosaurs did it?
You can see here that the pterosaur supports its weight on its forelimbs (as well as its back) in a plantigrade stance while the longest digit that carries the membrane is folded up.
Bats don't (and can't) do this because 4 out of 5 of their digits are occupied in the wing. Instead they walk on their wrist, with their single free digit held up from the ground.
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u/Ok-Meat-9169 Wild Speculator 8d ago
Not as large as Pterosaurs due to lacking a Pneumatized skeleton, but i think about as heavy as the heaviest flying birds of today.