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Author Topic:   How do you evolve a BAT?
RAZD
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Joined: 03-14-2004
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Message 1 of 9 (484903)
10-03-2008 12:11 AM


This is meant to be a little light-hearted, not strictly scientific (so "Is it science" is probably the best forum):

How do you evolve a BAT?

First a brief history:

The (currently) oldest known bat is this guy ... the 54 million year old "new" bat fossil:

Which we can compare to this modern bat skeleton:

And we can note certain differences (longer tail, claws on all hand fingers) and certain similarities (long finger bones, shorter hind legs that forelegs).

The current thinking on bat evolution can be summarized by these two articles:

(1) http://www.si.edu/Encyclopedia_SI/nmnh/batfacts.htm

quote:
Evidence for bat-like flying mammals appears as far back as the Eocene Epoch, some 50 million years ago; however, the fossil record tracing bat evolution is scanty. Based on similarities of bones and teeth, most authorities agree the bat's ancestors were probably insect eating placental mammals, possibly living in trees, and likely the same group that gave rise to shrews and moles.

(2) Insectivore-like mammals, Tiny teeth and their enigmatic owners

quote:
However, there are only small differences between the teeth of bats and those of certain Paleocene insectivores, including members of the Nyctitheriidae and an unnamed genus known from Germany and Rumania (38, 39, 40). Some of these animals may turn out to be ancient bats when they become better known, especially when skeletons are found. This would not be surprising from the point of view of molecular studies which place the orders Chiroptera and Lipotyphla close to each other on the family tree of mammals (7, 8).

Thus we will set a starting point for our game of evolving a bat with a shrew-like animal living when the Yucatan Meteor kindly removed large predatory dinosaurs from the paths of evolution. But what is shrew-like?

This is a modern tree shrew:


It just finished eating an insect that it caught and held between its front paws (similar behavior to squirrels and racoons).

This is a modern diving water shrew:
FIGURE 3. A diving American water shrew (Sorex palustris)
(From: "Olfaction: Underwater 'sniffing' by semi-aquatic mammals" by Kenneth C. Catania, Nature 444, 1024-1025(21 December 2006), doi:10.1038/4441024a)

quote:

This semi-aquatic species has water-repellent fur and is the smallest mammalian diver.

Flight evolution in birds and the "WAIR" theory: Wing-Assisted Incline Running and the Evolution of Flight - curiously the wing beating pattern is different in juvenile birds than in flying adults:

quote:
This hypothesis is based upon the observation of ground-dwelling birds that use wing-assisted incline running (WAIR). This type of locomotion is not often seen, usually occurring in very short bursts when a bird attempts to escape a predator. By rapid flapping of the wings, the bird can create enough traction to run up a vertical surface.

Kenneth Dial has been studying WAIR for several years. In the current study he tested chukar partridges (and them’s good eatin’!) from first-day hatchlings to adult birds. He found that even newly hatched birds will climb ramps using their wings to paddle along and will leap off a drop flapping the wings to (poorly) control descent. Older birds become more adept at climbing in this manner and controlling descent on the other side of the ramp, until adult birds are able to climb a vertical surface and take off into powered flight.

Dial found the the orientation of the wingbeat remains constant from day 8 to adulthood and is different from that used in flight, and proposes that this wingbeat used in WAIR is a fundamental wing-stroke that predates and is ancestral to flight.


That would be a transitional behavior between running and flying, one with a clear survival advantage, as well as one that is observed in existing birds as they develop from down covered arms to fully feathered wings. Curiously this behavior does not include trying to glide on the developing wings.

Such flapping while falling to control descent is seen in many young birds, Wood Ducks would be an example many could be familiar with.

Now the fun part begins:

Stipulations:

(1) Why does the bat hang up-side-down: how did this evolve and why would it be beneficial? Did this behavior evolve before flight?

(2) How does the webbing between the fingers evolve? We do NOT see this formation in flying squirrels and sugargliders, probably because it interferes with other necessary use of front paws for climbing.

(3) If you start with an aquatic shrew to get webbing, how do you get back in the tree and hang up-side-down, and why do you lose the rear feet webbing?

(4) How does wing flapping evolve if you start with gliding? Conversly how are you going to have WAIR behavior in tree shrew?

Let the games begin.

Enjoy.

Edited by RAZD, : moved pictures


we are limited in our ability to understand
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Replies to this message:
 Message 3 by Blue Jay, posted 10-04-2008 10:50 AM RAZD has responded

  
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Message 2 of 9 (484942)
10-03-2008 2:06 PM


Thread moved here from the Proposed New Topics forum.
    
Blue Jay
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Posts: 2843
From: You couldn't pronounce it with your mouthparts
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Message 3 of 9 (485020)
10-04-2008 10:50 AM
Reply to: Message 1 by RAZD
10-03-2008 12:11 AM


Hi, RAZD.

I'm not a bat man ( :D ), but the evolution of flight is always an interesting topic, so I'll give it a little whirl.

RAZD writes:

(1) Why does the bat hang up-side-down: how did this evolve and why would it be beneficial? Did this behavior evolve before flight?

I'm going to go out on a limb and say upside-down hanging evolved after flight. Since bats don't have the bipedal posture to stand upright like birds, they adapted to hang. Spiders routinely hang upside from their webs, which (at least in the spiders) doesn’t require muscle tension (because they hang by their hook-like claws, I think). With the bat, it obviously requires some sort of tension in the digits, but I don’t know much about that.

RAZD writes:

(2) How does the webbing between the fingers evolve? We do NOT see this formation in flying squirrels and sugargliders, probably because it interferes with other necessary use of front paws for climbing.

I don’t think I have a good answer for this one. Bats do still have an unwebbed thumb, but I think that’s mostly used for climbing (probably testifying to bat ecology before flight, I would think).

RAZD writes:

(3) If you start with an aquatic shrew to get webbing, how do you get back in the tree and hang up-side-down, and why do you lose the rear feet webbing?

I can personally only envision the process happening in a "trees-down" direction, simply because it's ecologically the more parsimonious. Further, I think webbing is just a natural artifact of vertebrate skin stretched over bone, so I would guess that webbing could evolve in practically any vertebrate lineage that has retained long digits.

Have you heard of a colugo? They are related to primates, and have the most extensive patagium of all gliding mammals (here is a really cool picture of one in flight). Their fingers are also webbed, like bats’.

I think bats could have started out like a colugo, and gradually extended all or some of their fingers to improve their flight control, and eventually settled on keeping only the thumb for climbing and using the rest for flying, especially after they learned how to catch prey on the wing.

RAZD writes:

(4) How does wing flapping evolve if you start with gliding? Conversly how are you going to have WAIR behavior in tree shrew?

I'm going to reject WAIR in the bat’s case, simply because the bipedally cursorial adaptations of theropod dinosaurs and birds don't seem to be present in either shrews or bats.

The flapping mechanism in bats isn’t all that spectacular or complex: it’s just an enlargement of the muscles and bone structures that all mammals have, except that the shoulder blades are more free to move (or something like that; it’s been awhile since I took Comparative Physiology). I could see that adaptation coming from a gliding mammal just flailing to reach a couple inches further.

Edited by Bluejay, : Repeated line.

Edited by Bluejay, : Fix dBCode error from a frickin' long time ago.


-Bluejay/Mantis/Thylacosmilus

Darwin loves you.


This message is a reply to:
 Message 1 by RAZD, posted 10-03-2008 12:11 AM RAZD has responded

Replies to this message:
 Message 4 by RAZD, posted 10-04-2008 12:52 PM Blue Jay has not yet responded
 Message 6 by RAZD, posted 01-28-2009 9:44 PM Blue Jay has responded

  
RAZD
Member
Posts: 19756
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 5.5


Message 4 of 9 (485027)
10-04-2008 12:52 PM
Reply to: Message 3 by Blue Jay
10-04-2008 10:50 AM


The Colugo hypothesis?
Hey Bluejay,

I'm not a bat man ...

Well, here we're just taming the shrew ...

Have you heard of a colugo? They are related to primates, and have the most extensive patagium of all gliding mammals (here is a really cool picture of one in flight). Their fingers are also webbed, like bats’.

Nice picture.

http://en.wikipedia.org/wiki/Colugo#Classification

quote:
Recent molecular phylogenetic studies have demonstrated that colugos belong to the clade Euarchonta along with the treeshrews (order Scandentia) and the primates. In this taxonomy, the Euarchonta are sister to the Glires (lagomorphs and rodents), and the two groups are combined into the clade Euarchontoglires.[5]

So they share a common ancestor with tree shrews (and bats) ... but are herbivores instead of insectivores. Interesting.

Spiders routinely hang upside from their webs, which (at least in the spiders) doesn’t require muscle tension (because they hang by their hook-like claws, I think). With the bat, it obviously requires some sort of tension in the digits, but I don’t know much about that.

Actually the claws at rest allow them to hang. I would go with the hanging first hypothesis, as being able to hang on to the tree would allow the shrew greater reach to grab insects from the air, as would the development of longer fingers, and webbing would add to the net effect.

I'm going to reject WAIR in the bat’s case, simply because the bipedally cursorial adaptations of theropod dinosaurs and birds don't seem to be present in either shrews or bats.

Agreed that the running bit is out, however I think there may be something to the young birds furious waving of the arms to partially control descent that could apply to an animal with a webbed hand.

Curiously, Wallace's Frogs don't seem to flap when they glide from tree to tree, so that doesn't necessarily develop.

The flapping mechanism in bats isn’t all that spectacular or complex: it’s just an enlargement of the muscles and bone structures that all mammals have, except that the shoulder blades are more free to move (or something like that; it’s been awhile since I took Comparative Physiology). I could see that adaptation coming from a gliding mammal just flailing to reach a couple inches further.

Or perhaps trying to grab that insect out of the air while anchored to the branches, possibly up-side-down.

I think bats could have started out like a colugo, and gradually extended all or some of their fingers to improve their flight control, and eventually settled on keeping only the thumb for climbing and using the rest for flying, especially after they learned how to catch prey on the wing.

I think that extensive wing aft would make flapping unproductive. One distinctive difference is that gliding seems to be limited to escape behavior - getting away from predators on one tree by gliding to another.

Flight, on the other hand, could be an adaptation for predatory behavior.

Enjoy,


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.


• • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •

This message is a reply to:
 Message 3 by Blue Jay, posted 10-04-2008 10:50 AM Blue Jay has not yet responded

  
RAZD
Member
Posts: 19756
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 5.5


Message 5 of 9 (486914)
10-25-2008 2:18 PM


Onychonycteria flight
This is from the Nature article:

Abstract: Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation
Nature 451, 818-821 (14 February 2008)
doi:10.1038/nature06549;

quote:
Phylogenetically informed comparisons of the new taxon with other bats and non-flying mammals reveal that critical morphological and functional changes evolved incrementally. Forelimb anatomy indicates that the new bat was capable of powered flight like other Eocene bats, but ear morphology suggests that it lacked their echolocation abilities, supporting a 'flight first' hypothesis for chiropteran evolution. The shape of the wings suggests that an undulating gliding–fluttering flight style may be primitive for bats, and the presence of a long calcar indicates that a broad tail membrane evolved early in Chiroptera, probably functioning as an additional airfoil rather than as a prey-capture device. Limb proportions and retention of claws on all digits indicate that the new bat may have been an agile climber that employed quadrupedal locomotion and under-branch hanging behaviour.
bold color for emPHAsis.

So we have a bat ancestor that is capable of quadrapedal locomotion as well as tree climbing ability, and the flight pattern is not fully developed.

Here is a picture from the actual article:

Notice that the fossil is intermediate in characteristics between non-flying mammals (in general) and bats (in general), ie it is transitional by Darwin's definition.

Curiously genetics is now showing bats are not necessarily descended from Nyctitheriidae
http://en.wikipedia.org/wiki/Bat

quote:
Bats were formerly grouped in the superorder Archonta along with the treeshrews (Scandentia), colugos (Dermoptera), and the primates, because of the similarities between Megachiroptera and these mammals. However, genetic studies have now placed bats in the superorder Laurasiatheria along with carnivorans, pangolins, odd-toed ungulates, even-toed ungulates, and cetaceans.

Looks like another round of scientific revision is in order due to new evidence ...

Modern bats have evolved since the first fossils known of bat ancestors, and when older bats are found we will know more about bat evolution - until then we can make some educated hypothesis, but that is all they are. Was there a common ancestry between sloths and bats? Or is branch hanging a just a similar behavior? Time will tell.

Enjoy.

Edited by RAZD, : picture


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
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RAZD
Member
Posts: 19756
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 5.5


Message 6 of 9 (496534)
01-28-2009 9:44 PM
Reply to: Message 3 by Blue Jay
10-04-2008 10:50 AM


Interesting picture
Hey bluejay,

They are related to primates, and have the most extensive patagium of all gliding mammals (here is a really cool picture of one in flight).

Found another picture of a colugo


Click to enlarge

http://www.squidoo.com/wildgliders (cool information on lots of gliders: lizards, snakes, geckos, frogs ...)

Now why would a gliding animal hang upside-down from a branch?

It's enough to drive you batty ... :D

Enjoy.

Edited by RAZD, : photo


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.


• • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •

This message is a reply to:
 Message 3 by Blue Jay, posted 10-04-2008 10:50 AM Blue Jay has responded

Replies to this message:
 Message 7 by Blue Jay, posted 01-28-2009 10:25 PM RAZD has responded

  
Blue Jay
Member (Idle past 775 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 7 of 9 (496536)
01-28-2009 10:25 PM
Reply to: Message 6 by RAZD
01-28-2009 9:44 PM


Re: Interesting picture
Hi, RAZD.

RAZD writes:

Now why would a gliding animal hang upside-down from a branch?

Awesome: I didn't even think about that.

ToL puts bats and colugos as sister groups, so you could be right about the "hanging-first" model.

But, much of the genetic data is returning the four-way Eutherian split (Xenarthra, Laurasiatheria, Afrotheria, Euarchontoglires), which puts bats in Laurasiatheira as the relatives of Perissodactyla and the Carnivora/Pholidota, and colugos and sister to Primates within the Euarchontoglires. If this split is correct, your hanging observation may not be relevant.

Edited by Bluejay, : Very bad wording.


-Bluejay/Mantis/Thylacosmilus

Darwin loves you.


This message is a reply to:
 Message 6 by RAZD, posted 01-28-2009 9:44 PM RAZD has responded

Replies to this message:
 Message 8 by RAZD, posted 01-28-2009 10:43 PM Blue Jay has responded

  
RAZD
Member
Posts: 19756
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 5.5


Message 8 of 9 (496539)
01-28-2009 10:43 PM
Reply to: Message 7 by Blue Jay
01-28-2009 10:25 PM


Re: Interesting picture
If this split is correct, your hanging observation may not be relevant.

Yet the behavior could evolve repeatedly, it could help keep the folds of skin out of the way for climbing around, and as seen above, it forms a nice cozy pouch for young.

From wiki

quote:
Although they are placental mammals, colugos are marsupial-like in their breeding habits. The young are born after just 60 days of gestation in a tiny and undeveloped form, and spend their first six months or so of life clinging to the mother's belly. To protect them and transport them she curls her tail up to fold the gliding membrane into a warm, secure quasi-pouch. Breeding is fairly slow as the young do not reach full size until they are two or three years old

Enjoy


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.


• • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •

This message is a reply to:
 Message 7 by Blue Jay, posted 01-28-2009 10:25 PM Blue Jay has responded

Replies to this message:
 Message 9 by Blue Jay, posted 01-29-2009 9:51 AM RAZD has acknowledged this reply

  
Blue Jay
Member (Idle past 775 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 9 of 9 (496608)
01-29-2009 9:51 AM
Reply to: Message 8 by RAZD
01-28-2009 10:43 PM


Re: Interesting picture
Hi, RAZD.

RAZD writes:

Yet the behavior could evolve repeatedly...

Okay, so you weren't arguing common ancestry: you were just arguing for proof-of-concept. I gotcha.


-Bluejay/Mantis/Thylacosmilus

Darwin loves you.


This message is a reply to:
 Message 8 by RAZD, posted 01-28-2009 10:43 PM RAZD has acknowledged this reply

  
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