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Bat Evolution
Joseph Turcotte
Historical Geology
Spring 2008

                                                
The Evolution of Bats

 

            Bats are some of the least understood mammals in regards to their evolutionary past, yet are one of the most diverse groups of mammals alive today, accounting for ¼ of all mammalian diversity.  Up until recently, the oldest fossil was dated at 50 million years old.  This find placed bats’ evolutionary past within the Eocene.  There are very few fossilized bones of these mammals because they have small, light bones that don’t preserve well.  As such, it is difficult to classify these mammals in regards to the animals with which they are related.  The best classification of order that can be attributed to the two families of bats is Chiroptera, meaning hand-wing, which is a completely different group from any other type of mammal.  It was not always this way, as the father of modern taxonomy, Carl Linnaeus, first classified bats as primates.  Much of the bats’ evolutionary past is still a mystery, but new research and discovery is beginning to shed some light on the evolution of some of their traits.  Their anatomy seems to provide very little evidence of their evolutionary past.  Their forelimbs are different from any other flying creature and are hardly comparable to any mammal, despite the existence of the basic elements.

 

Megachiroptera and Microchiroptera

 

            The two sub-orders of bats are Megachiroptera (commonly known as megabats) and Microchiroptera (commonly known as microbats).  Megabats most commonly eat fruit; have no echolocation, larger eyes than the microbats, and a longer snout.  The name megabat is misleading, because several species of Megachiroptera are in fact smaller than some Microchiroptera.  However, the most common type of megabat, the Pteropus (flying-fox), is about one foot long.  The microbats possess echolocation (except for Rousettes and relatives), eat insects, blood, small mammals, and fish, lack the claw at the second forelimb, have poor eyesight, and possess a broader snout than the megabats.  One of the most popular bats in fiction, Desmodus rotundus (vampire bat), belongs to the Microchiroptera sub-order.  Did one of these sub-orders of bats evolve from another?  Did they simply have a convergent evolution?  Why are these the only winged mammals?  These are questions that science has been trying to figure out for quite some time and which are only made more complex by the ever-increasing species of bats.
 

Microbat                                                 Megabat

Megabat                                                 Microbat

Classification of Chiroptera

Megachiroptera--the megabats

Family (scientific name)

Family (English name)

Approximate no. of genera

Approximate no. of species

Pteropodidae

Old World fruit bats

42

170

Microchiroptera--the microbats

Family (scientific name)

Family (English name)

Approximate no. of genera

Approximate no. of species

Rhinopomatidae

Mouse-tailed bats

1

3

Emballonuridae

Sheath-tailed, sac-winged, pouched and ghost bats

12

50

Craseonycteridae

Hog-nosed bats

1

1

Nycteridae

Slit-faced bats

1

14

Megadermatidae

False vampire and yellow-winged bats

4

5

Rhinolophidae

Horseshoe bats

1

64

Hipposideridae

Old World leaf-nosed bats

9

66

Noctilionidae

Bulldog or fishermen bats

1

2

Mormoopidae

Naked-backed, moustached and ghost-faced bats

2

8

Phyllostomidae

New World leaf-nosed bats

51

152

Natalidae

Funnel-eared bats

1

5

Furipteridae

Smoky bats

2

2

Thyropteridae

Disc-winged bats

1

3

Myzopodidae

Sucker-footed bats

1

1

Vespertilionidae

Vesper bats

44

350

Mystacinidae

New Zealand short-tailed bats

1

2 (1 extinct)

Molossidae

Free-tailed or mastiff bats

13

89

 

Suggested Timelines and Recent Research

 

            One of the most likely animals for bats to have evolved from is a shrew-like creature that climbed trees.  This isn’t saying much, because all mammals are believed to have evolved from shrew-like creatures.  A sugar glider is a good example of what the ancestors to bats are thought to have looked like.  Bats are believed to have evolved around 70 million years ago or later, during the early Eocene.  Flowering plants had started to diversify and there were an increasing number of insects during this time, which made the environment perfectly suited for bats to exploit.  Around 65 million years ago, there was a mass extinction event that killed off 75% of all life on Earth.  The evolutionary ancestors to modern bats are believed to have survived this extinction event.
 

Sugar Glider                                        Theoretical ancestor to bats

A comparison of four different models of bat evolution with nearly equal genetic support.

 

            A shows: Megabats and colugos are sister taxa to the primates, with microbats well-separated and more basal on the tree.  B shows: Megabats and microbats do not share a common flying ancestor (as in A), but megabats are not the closest sister taxon of primates.  C shows: Microbats are not a monophyletic group but are split by other taxa, especially by megabats. 

D shows: Megabats and microbats share a single common (flying) ancestor.

 

            Monophyly: comprised of members derived from a common ancestor, which includes all descendants.  Polyphyly: constituents are derived from different ancestors.  Diphyly: do not share a common ancestor

 

            The recent discovery of a bat that dates from 52.5 million years ago named Onychonycteris finneyi seems to point to the fact that bats could fly before they acquired radar.  This is the most primitive bat fossil ever found and possesses claws on all five “fingers” rather than just the one or two that many other bats possess.  It appears to have a snout similar to that of a modern day microbat.  This information may be taken way out of context when we consider that many bats today don’t possess echolocation.  However, it is the most likely evolutionary ancestor to both sub-orders of bats found today, as it seems to possess some of the traits common to both types.  It had previously been thought by some that megabats had evolved from microbats due to the current genetic research.  However, this genetic research is based on the genetics of current bat species and could be misleading.
 

The oldest bat fossil to date

 

            The fact that many modern Megachiroptera share traits with primates, as was noticed by Carl Linnaeus, has caused many to believe that they are from a different order altogether.  This would mean that the adaptation of flight had occurred twice during mammalian evolution.  This has been called the “flying primates theory”.  It is based mostly on the fact that flying foxes have brain adaptations that are not found in microbats and would seem unlikely to have developed through parallel evolution.  This theory is no longer the main theory in practice due to the findings of the 52.5 million year old fossilized skeleton.

 

            More research is going into bat genetics as more techniques become available.  We may know how to create a bat before we know exactly how they evolved.  Recently, a team of researchers at MD Anderson Cancer Center was able to replace the gene regulatory element responsible for forelimb growth in mice with the gene regulatory element responsible for forelimb growth in bats.  The mice grew unusually long forelimbs.  While the article doesn’t state it, this would seem to show that it is likely that the ancestor of both bats and mice had this same gene expression.  This theory would not go well with those who have been trying to separate themselves from the idea that bats are mice with wings.

 

            Another piece of research shows that the FOXP2 gene in humans and bats is responsible for language evolution and bat echolocation.  There is very little difference in the expression of this gene in any other vertebrates, but when it comes to bats the differences are astounding.  This is mostly due to their echolocation, which is different between species of bats.  So, not only are bats some of the most unusually variant mammals in regards to physical aspects, but also in regards to language.

 

Conclusion

 

               Most of the research currently being done on bat genetics seems to lead in different directions, as do their fossil remains, and behaviors.  It would be impossible to say that any one evolutionary path is more likely than another for this complex and little understood animal.  The differences and similarities in genetics make it nearly impossible to place bats in any particular grouping and their evolutionary lineage is still up in the air.  I hope that the complexity of this subject does not deter scientists from continuing to research this animal, but rather attracts them to do more research.  As we understand more of the world around us, we also understand ourselves better.  The understanding of bats would be a major step forward.

 

Information

 

http://en.wikipedia.org/wiki/Bat
http://tolweb.org/Chiroptera
http://www.fathom.com/course/21701775/session1.html
http://www.sci-tech-today.com/fullpage/fullpage.xhtml?dest=%2Fnews%2FBats-Could-Fly-Before-They-Had--Radar-%2Fstory.xhtml%3Fstory_id%3D120000C0U8NC
http://www.sciencedaily.com/releases/2008/01/080114173923.htm
http://www.uq.edu.au/nuq/jack/consensus.htm
 

Pictures

 

http://www.fathom.com/course/21701775/session1.html
http://home.planet.nl/~twisk109/pagecolour.html
http://lamington.nrsm.uq.edu.au/images/fauna/Sugar.jpg
http://scienceandevolution.blogspot.com/2007/09/bats-and-humans-share-common-gene-for.html
http://wiki.cotch.net/index.php/Image:ForelimbHomology_unlabelled.png
http://www.uq.edu.au/nuq/jack/consensus.htm