Why-are-bats-the-only-flying-mammal? (¿Por qué los murciélagos son los únicos mamíferos que vuelan?)

(If someone ask why some animals are the way they are, chances are forth they recieve a long explanation about the way evolution proceeds, in which they will be told that it evolution is random and unpredictible.

I.Don't.Agree.Period.

Some paths in evolution are quite more predictible and understandable examining the enviromental and mechanic stress forces that organisms had to face as well the difficulties of acquiring some new structures that would allow them to overcome the aforementioned factors.
I don't think that Darwinian theory of evolution is the epithome of all theories regarding this fact, in fact I find it poor in explaining evolution. Its major contribution has been telling that there is a selection among the organisms before reaching reproduction. This is not more than pure observation of the facts. But it fails to give any perspective about pure mechanical forces acting on the way organisms changes. Worse than that, the absurd scenario in the USA, with the advantages given to creationists that try to depict their beliefs as an acceptable scientific theory, has driven to a fierce defence of the randomness and unpredictibility of the proccess, forgotting that other sciences can as well as pure observation can give us important clues about how change operates.)
 ---------------


Grabado de Haeckel de los murciélagos.  http://www.taringa.net/comunidades/derribando-mitos/7204419/Los-Murcielagos-no-son-Ciegos.html



In spite that they are "the only", we shouldn't forget that bats are over 1000 different species, in fact the second largest order of mammals by the number of species, only outnumbered by rodents.
Also, we shouldn't forget that "the only" extant archosaurs are birds. And that there is only one more order of flying vertebrates, the extinct pterosauria order.
And that actual flight only evolved once in invertebrates, in insects, and three times in vertebrates, the mentioned bats, birds and pterosaurs.
So, as mammals we have been very lucky to have an order of flying animals.

But why only once?. The easiest answer is to say that evolution doesn't have intention or direction, it happens because it happens. This type of explanation never convices me.
The mechanic explanation is that flight is very difficult to evolve in tetrapods (that is vertebrates with four limbs), because the adapatations to achieve it are very demanding.
A tetrapod has to dedicate a pair of their limbs to fly, evolve an efficient way to power it, decrease the weight of its skeleton and body in general to make it possible, gain an efficient way of mantaining basic metabolism while flying (birds have very especialized adaptations in their breathing system), and in placental mammals females have to find a way of develop a successful gestation even while flying.
All in all, an actually self powered flight demands too many adaptations, so the chances of getting it are really low.