March 29, 2008

Bats And Flying Foxes

Stupid Question ™
Aug. 23, 2004
By John Ruch
© 2004

Q: What’s the difference between a bat and a flying fox?
—Bill D., Columbus, Ohio

A: It’s not so much what the difference is as how much of a difference there is. Because you have unwittingly stumbled into a 30-year scientific war about whether the two suborders of bats share a common ancestor or evolved separately. Some scientists claim the flying foxes are more closely related to us than to fellow bats.

As usual for one of our forays into the nightmarish world of scientific classification, I’m going to have to start by correcting your terms. All flying mammals are bats. The flying fox is a type of bat that looks pretty much like the name suggests—a miniature fox with bat wings.

More specifically, “flying fox” is the common name for the genera Pteopodidae, which is a subcategory of the fox-like South Asian/Australian bats widely known as fruit bats. (Most of them eat fruit and/or flower nectar.)

In scientific terminology, all bats fall under the order Chiroptera (that’s literally “hand-wing”). But the differences between the fruit bats and the “regular” bats we see in North America and the rest of the world are so distinct that no bat is simply called a Chiropteran.

Instead, science has instantly broken Chiroptera down into two suborders. There’s Megachiroptera (or “megabats”), which covers the fruit bats (“mega” because they include the largest bats, with wingspans tipping 6 feet). And there’s Microchiroptera (or “microbats”), which includes the classic bats with which Americans, Europeans and Africans are familiar.

All this scientific gibberish aside, I think the German language makes the obvious distinction much more beautifully. To them, a microbat is a “fledermaus”—a “flying mouse.” And a megabat is a “flughund”—a “flying dog.” This really hits the broad anatomical differences on the head.

Of course, neither term is biologically accurate. Microbats are more like flying shrews; they are mostly insectivorous, like shrews, and there are arguments for a common ancestor. (In this vein, there are also arguments that some shrews echolocate, like bats do.)

And megabats are not related to canines in any direct fashion. But what, exactly, are they related to? That is our burning question.

What unites the Mega- and Microchiropterans is their wings and flight mechanisms (and the fact they’re all mammals). All bats have forelimbs and paws that have morphed into membranous, scallop-edged wings which extend down to include the hind limbs. They all fly in the same way, by alternately flexing back and chest muscles. (This is different from birds, incidentally). They also have similar hind legs that can swivel the claws forward, essentially reversing their direction.

And that’s about it for similarities. Now, the differences.

Microbats use echolocation; megabats don’t (except for two species, which do so crudely, in an entirely different physiological manner). Microbats have snub snouts and fancy ears often highly evolved for echolocation; megabats have a prominent, toothy, dog-like snout and typical mammalian animal ears (they look more like an average mammal).

Microbats generally have tails; megabats generally don’t. Microbats mostly eat insects; megabats mostly eat fruit/nectar. Microbats have one mobile, clawed finger on each wing; megabats have two.

In short, they are really, really different, except for their manner and physiology of flight.

In the 1970s, a controversy finally erupted over this obvious difference, with several scientists proposing megabats and microbats had entirely different evolutionary origins. Rival studies flew back and forth, and while the controversy has largely died down in favor of a single-origin theory, studies still pop up occasionally on both sides.

Like most scientific controversies, the matter has been temporarily settled more out of boredom with the whole thing than any sort of case-closing smoking gun turning up.

Also like most such controversies, the debate has been tinged with all sorts of ulterior concerns. The main one is the complexity a separate-origin theory introduces. The main argument for a single origin remains: “Look, they both have wings that look the same, and they fly the same way!”

If separate origins were accepted, the separate evolution of flight in two different mammalian orders would also have to be accepted, and we know flight is biologically rare indeed. Furthermore, scientists know there are creationists waiting in the wings, so to speak, to seize such a rhetorical opportunity and wax on about how utterly fantastical it is to think that flight would evolve in an identical way twice.

I’m not daunted. It would be far more fantastical for flight to evolve in two totally different ways in the same family of animals; biological morphology has built-in restraints. For that matter, I’m not sure we can accept the implicit presumption that flight in birds and insects all comes from a single origin, either. But then, I’m not a bat scientist.

There are other arguments in favor of a single origin, including studies of mitochondrial DNA indicating a common origin. However, mitochondrial DNA has its own limitations; interestingly, it hasn’t settled the question about whether humanity itself has a common or multiple origins, though it points toward the single-origin theory with us as well.

Another single-origin argument sometimes made is that all known extinct bats in the fossil record are essentially microbats, with the implication being that megabats branched off later. However, the fossil record for bats is so scanty as to render this line of argument virtually meaningless.

As for the separate-origin theory, it has not so much presented evidence of differences from microbats as it has presented evidence of similarities to primates. Specifically, the argument is they are descended from lemurs, or have a common ancestor.

These arguments are almost exclusively anatomically based. It is argued that megabat nervous system and related arterial system are much closer to primates than microbats in arrangement. There’s also a naughty-bits argument that the megabat penis and female breasts are much more like those of primates than of microbats.

Most scientists are more willing to accept separate evolution of similar neurological and sexual apparatus than they are of winged flight.
There is one study that claims megabats have amino-acid characteristics closer to primates than to fellow bats. But that is far less compelling than mitochondrial DNA.

Thus, the single-origin theory stays dominant. And, to finally answer your question, the difference is…well, it’s either an actual difference or just a distinction. It’s an aesthetic matter more than a scientific one; it’s less Megachiroptera and Microchiroptera than it is fledermaus and flughund.

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