In spite of belonging to one of the most diverse orders of mammals, with over a thousand separate species classified, bats remain something of a mystery - often unobserved by the casual naturalist. Perhaps I tend towards a Chinese interpretation of things because of my childhood travels, but I must admit that I have long been rather fond of the nocturnal wonders. While I can not be certain, I suspect that I saw my first real bat long ago at the Denver Zoo, but my earliest memorable encounters with the creatures in their natural environment were never through their direct presence, but always of the carnage they left in their wake.
While living in Singapore, we had an enormous mango tree growing in the front yard of our home on the hill at 8A Rochalie Drive, but we never did get the chance to taste the mangoes that grew from that tree. As soon as they would begin to ripen, an unseen horde of flying foxes would descend out of the Malaysian sunset and devour the fruit, scattering their thick guano - now rich in partially digested mango - all over the driveway. This relationship presented a classic example of inadvertent predatory mutualism. The mango tree expends energy producing fruiting bodies to support its offspring, but by day the rich ethylene put off by the ripening fruit attracts birds and primates - and by night it draws bats who also consume the fruit. Mango seeds which can not survive passage through the intestinal column of a frugivore are gradually winnowed from the population - or at the very least, face a competitive disadvantage when compared to siblings or neighbors' seeds that can survive the process. Trees whose seeds do survive the transition may be carried some distance from their parent before being dispersed, thereby helping to expand the mango's range, and hopefully prevent competition between proximate family members. This process has been so successful that there are some mangoes for whom selection has encouraged germination only upon contact with fruit bat stomach acids after ingestion. Evolutionary relationships aside, as the season wore on, this rotting fruit would begin to ferment in the humid and warm air, and the atmosphere could become positively narcotic. Our low-slung cairn-terrier would avoid the shadow of that tree, but our evil little cat would at times bask in the alcoholic vapors, after which he would become positively silly.
My next memory of bat-contact would also be indirect, and it would take me some time to realize that I was again observing their handiwork. For some time, I had seen banana leaves and palm fronds with small cuts chewed into their undersides, perpendicular to the lobes of the leaves. These tiny cuts weakened the cross-sectional strength of the lobate processes on a leaf, and they folded along these lines, forming a small tent-like structure. One morning while waiting for the fourth-grade school bus at the top of the hill on Rochalie Drive, I happened to look up and discover the culprits responsible for this unusual gardening: a knot of tiny white-furred bats roosting in the folds of a banana leaf some thirty feet above my head.
Years later, I would watch in wonder as thousands of fruit bats left their chattering camp to pillage some hapless fruit plantation somewhere in Northern Australia. Their coterie began with one or two flyers leading the way, and eventually grew into an endless stream nearly a dozen individuals thick. The whole flock took nearly half an hour to pass, a river of airborne terriers, steadily and silently wending their way into the gradually darkening sky. It is easy to see why many farmers consider them an agricultural catastrophe, as flocks of this size are allegedly capable of denuding an entire orchard in a night.
Of course, only a third of bat species are frugivorous. The major division in bats is between suborder megachiroptera and suborder microchiroptera, and while the larger-bodied megabats tend to rely on a phytovorous diet high in sugars, the microbats tend towards carnivory. In particular, the microbats lean towards insectivory, taking up the challenge and the ecological niche of "small volant predator" when the swallows go home to roost for the night. These are the bats whose famous echolocation drives their dazzling acrobatics as they dive and flitter before bright stadium lights for moths and the precious fat they carry. I've been watching small bats of an indeterminate species flit before the arc-sodium light over my apartment complex's parking lot for the last five years, and I recently played host to a small colony living inside the bedroom wall of my apartment for a few weeks late in December of 2005. They woke up and began their flight at seven to eight in the evening, and returned between one and two in the morning. While their chittering was often comforting to one who has lived alone for far too long, my apartment complex elected to have them removed. While the bats themselves are mostly inoffensive, their droppings could provide a nutrient rich home for many noxious fungi and bacteria that could grow and spread throughout the walls of the complex.
Just as the habits of living bats remain somewhat mysterious, their evolutionary origins are also considered controversial. The earliest known bat fossils are found from 50 to 52 MYA in the Eocene, but their crania are already in possession of the traits key to echolocation in extant microbats, and their fossils are also already broadly distributed across the globe. Creationists seize upon this "full-formed and sudden appearance" within the fossil record without a solid lineage of transitional fossils as evidence refuting evolution. It is unfortunate that bats, like birds, are small-bodied animals with fine bones that rarely survive the process of fossilization. Worse still, the majority of extant species of bat live in tropical forests whose environments are even less conducive to the creation and preservation of fossils, and it is presumed that their extinct ancestors may have lived in similar environments. In spite of these difficulties, the dentine in teeth preserves remarkably well, and incisors showing intermediate properties of Order Insectivora and the insectivorous microbats have been discovered in Cretaceous and Paleocene rocks. Of course, there is no telling what the rest of the organism might look like...
Modern "creation science" also likes to speak of "irreducible complexity", and some would claim that the evolution of flight in bats is just such an impossible trait, because there could be no successful transitional organism between full flight wings and webbed fingers. These individuals lack imagination, and I believe that I need point no farther than the relative diversity and abundance of the so-called 'flying frogs' - also of southeast asia. Remember that there are no 'partial traits' on any living organism: only fully functional traits that can be exploited with varying degrees of success! More importantly, the initial function of a trait may create spandrels ready for exaption to another function. Organisms may have webbed feet because their ancestors used them to swim, or because they act as a fly-swatting net to capture prey, or to glide from tree to tree - but their descendants may accidentally adapt them to a new alternately useful purpose.
Having personally witnessed some of these oddities glide from tree to tree, or using their webbed forelimbs as a fly-swatter to capture or stun prey while terrestrially bound, I can assure you: while the transition between partial gliding flight via webbed feet, elongated ribs, or full-body patagia may be a difficult path, it unquestionably provides a prior selective advantage to the individual in possession of these 'partial traits'. Conveniently enough, a recent paper demonstrates a mechanism by which bats' long fingers might have evolved. It appears that mutations leading to an up-regulation of bone morphogenic protein-2 (BMP2) (or the down-regulation of BMP2 antagonists such as Noggin) in the distal phalanges would result in the elongation of bat forelimb digits, providing a mechanism by which early proto-bats might literally have flown over some adaptive constraints on the path to flight.
All of this still fails to explore the actual controversy in real science: the confusing relationship between the mega- and microbats - and fails to answer where their branch or branches anchors to the tree of life. Much as tarsiers have proven difficult to classify, so have the suborders of bats - and for similar reasons. Without strong fossil evidence pointing its stony fingers at a particular time and event, it is near impossible to tell whether the megabats or the microbats evolved first, and if so - is one line derived from the other? At first glance, the relationship seems obvious - the megabats and microbats share many presumed ancestral character traits in common, but the microbats possess the derived traits contributing to echolocation. A closer analysis of skull morphology reveals some interesting differences. Morphologically, microbats share many traits in common with Order Insectivora - and surprisingly enough, the megabats share many significant traits with Order Primates. This has led some researchers to declare the megabats flying primates, relatives of the Dermoptera. Of course, filing colugoes under a particular branch has proven equally difficult. Even molecular studies remain somewhat inconclusive.
Only time and more research in fields of both paleontology and molecular science will let us know for certain. Until then, I will just have to watch and enjoy the bats at the University of Florida's bat house.