Dark Banquet (7 page)

As far as what
really
happened, that's still open to debate. But given the opportunistic nature of modern vampire bats, it wouldn't be a stretch to learn that ancient protovampires actually exploited some combination of wounds, ectoparasites,
and
larger forms of arboreal fauna on their evolutionary road to becoming modern vampire bats. Perhaps, though, blood-feeding bats came about through a completely different scenario, as of yet unknown to scientists and leaving the question of vampire bat origins open to further debate and future research.

Some of you may have wondered why I chose to describe the various scenarios for the origin of blood feeding as, for example, the arboreal-omnivore
hypothesis
and not the arboreal-omnivore
theory.
Although you wouldn't know it from seemingly countless examples in the literature, there is a
major
difference between a hypothesis and a theory. A hypothesis is really a “best guess,” based on an accumulation of evidence (generally observations or experimental data). Hypotheses are starting points as researchers attempt to answer questions that arise in science, such as “How did vampire bats evolve?” Often short lived, they're commonly modified as new evidence accumulates. Theories, on the other hand, can start out as hypotheses, but they are
far stronger
—having withstood the test of time (and vigorous scientific challenge) and garnering support from numerous and varied fields of study. For example, a
theory
exists that life on this planet evolves. There were several
hypotheses
as to just how this scenario came about—with natural selection being the mechanism best supported by the evidence.

However vampire bats evolved, there is fossil evidence that at least three additional blood-feeding phyllostomids lived somewhere between two million years ago and six thousand years ago. Interestingly, this includes at least two North American species (
Desmodus archaeodaptes
and
Desmodus stocki
). In all likelihood, the extinction of these ancient vampires (which had ranges extending from California to Florida) was linked to climate changes, as a cycle of cooler summers and warmer winters transitioned into our current climate of hotter summers and cooler winters. Unable to find enough food during the winter or migrate long distances, their highly specialized blood-feeding diet would have sealed their fate, preventing them from packing on the fat necessary to survive a winter's hibernation.

The presence in the fossil record of a giant vampire bat,
Desmodus draculae,
suggests that this creature was feeding on megamammals like the giant ground sloths and heavily armored glyptodonts.
Desmodus draculae
was significantly larger than modern vampires and there is some evidence that they lived as far north as northern West Virginia.

Presumably, all of the ancient vampire bat species died out in North America following the great megafaunal extinctions of the late Pleistocene. One vampire bat expert, however, was convinced that at least some of them had not gone extinct.

“I think
Desmodus draculae
might still be alive,” Arthur Greenhall told me during lunch one afternoon in Boston.

“How do you figure that?” I said, after nearly choking on my sandwich.

Greenhall explained that there were still regions in South America where very few people visited—the deep Amazon, and parts of Brazil's Planalto Central, for example.

“Besides,” he added,
“draculae
bones have been uncovered right alongside the remains of living species.”

These facts, combined with the relatively recent discoveries of “living fossils” such as the coelacanth (a lobe-finned fish thought to be extinct for sixty million years) apparently gave the old vampire meister at least some hope that
Desmodus draculae
might still haunt the South American wilderness.
^*31

“If only,” I replied. If only.

I have not seen anything pulled down so quick since I was on the Pampas and had a mare that I was fond of go to grass all in a night. One of those big bats they call vampires had got at her in the night, and, what with his gorge and the vein left open, there wasn't enough blood in her to let her stand up.

—Bram Stoker

3.

SNAPPLE, ANYONE?

I
t was still dark when we arrived at the slaughterhouse, but there were already several cars parked outside the nondescript, single-floor structure. We stood in the parking lot, finishing hot drinks—and not saying much—until the loud clank of a metal gate caused me to jump slightly.

“You need
more
caffeine,” my companion said, sipping her decidedly low-test tea-water thing.

I responded by pouring the remainder of my cup onto the gravel.

As we approached the door, the aroma of my coffee gave way to the acrid tang of disinfectant and something else, something metallic, coppery.

There were new sounds coming from within the building, shouts and a deep organic vibrato.

At the abattoir, the workday was about to begin and we entered without knocking.

My companion that morning was Cornell undergrad Kim Brockmann, whom I'd met several months earlier when she began showing up at our weekly Zoology Journal Club. After one such meeting I had inquired if anyone might be interested in volunteering to help me maintain some vampire bats that I hoped to bring back from Trinidad. Kim's hand shot up without hesitation. Now, bundled up against the predawn chill, she was clutching six large plastic bottles and a spaghetti strainer, and I wondered if this is what she'd had in mind.

In Trinidad, obtaining blood on a daily basis hadn't been much of a problem, basically because Farouk Muradali, and more recently, his right-hand man, Keith Joseph, had been maintaining colonies of the common and white-winged vampires on and off for twenty-five years. During my first visit to Farouk's lab at the National Animal Disease Center, I was actually rather bowled over at their success in keeping the white-winged vampire,
Diaemus youngi,
alive in captivity. Several references I'd previously read (including one coauthored by my friend Arthur Greenhall) stated that these bats could not be kept in captivity for any length of time.

“Yes, we know all about those references,” Farouk said with a dismissive wave of his hand. “They're one of the reasons why so little is known about
Diaemus.”

I bent over to examine a cluster of shapes that were gathered in the upper far corner of a spacious rectangular cage. All of the bats were asleep, except one, and he was watching me—mouth slightly open, sharp triangular teeth—strikingly white.
Desmodus rotundus
had black, beady eyes and an unmistakable air of intelligence. It was a look that Kim and I would become extremely familiar with over the next three years, one that always gave me the impression that the bats were waiting for me to make a mistake—the kind that would result in either their escape from captivity or the infliction of a savagely deep bite.

Farouk nodded toward the cage and continued.
“Desmodus
is not a picky eater. If you capture one tonight and put out cow blood for it tomorrow night—it will drink until it's full.”

I nodded, recalling that these vampires had been given the species name
rotundus
because of their round-bellied appearance. Unfortunately, the naturalist Geoffroy never realized when he named them that their rotund abdomens were due to a gastrointestinal tract bloated with blood. Had he dissected a specimen he would have certainly noticed (as did Darwin's friend and supporter Thomas H. Huxley) that the common vampire's esophagus didn't empty directly into the stomach, a feature that typifies all mammals. Instead, the lower end of the esophagus ended in an inverted T, one serif leading to the stomach, the other leading to the intestine. Furthermore, the stomach wasn't J-shaped (as is seen in most mammals). It was tubular, a blind-ended U that was nearly two-thirds as long as the ropelike intestine that it closely resembled.

Not surprisingly, researchers who sought to determine the route of ingested blood in vampire bats found that the going was weird indeed. In an experiment using barium-laced cow blood, an X-ray machine, and five (presumably grumpy) common vampire bats, G. Clay Mitchell and James Tigner determined that freshly ingested blood moved from the vampire's mouth to the esophagus and then into the intestines
before
passing into the stomach.

We know now that these variations in digestive anatomy and physiology (like other anatomical and behavioral adaptations) are related to the vampire bat's unique lifestyle. In all other mammals, a primary function of the stomach is bulk storage of food—with some breakdown of that food (digestion). There is little transfer of nutrients or other material from the lumen of the stomach to the circulatory system (which then distributes it to the body).
^*32
This last, and generally overlooked digestive system function (absorption), is usually carried out by the small and large intestines and the network of blood vessels that supply and drain them. In vampire bats a key role of the stomach appears to be the rapid absorption of water (which makes up approximately 80 percent of the ingested blood volume). This excess water is carried to the kidneys (via the circulatory system once again) where much of it is converted into urine (i.e., water plus dissolved nitrogenous waste products). As mentioned earlier, since blood contains a negligible amount of fat, a substance in which energy is typically stored for later use, vampire bats are required to drink about half their body weight in blood each night.
^†33
This sudden weight gain is an extremely dangerous proposition for an animal that might be required to take flight at a moment's notice. Because of this, an important adaptation for blood-feeding bats is their ability to shed weight as quickly as possible, and their digestive and excretory systems reflect this emphasis. Researchers who have observed common vampire bat feeding sessions know that the bats begin urinating well before they've finished feeding. Along with modifications to the typical mammalian stomach and intestines, evolution has cranked up the vampire bats' excretory system, enhancing its ability to deal with its owners' rather unique dietary requirements.

Vampire bats get most of their nutrition from protein (in this case, hemoglobin and blood plasma proteins like albumin, fibrinogen, and globulin). These proteins are composed of smaller subunits called amino acids. In mammals, a problem arises when these nitrogen-bearing amino acids are broken down during digestion, releasing the toxic compound ammonia. Most mammalian excretory systems deal with ammonia by having the liver quickly convert it into a less toxic substance—urea. Urea is not only safer to have circulating around the body than ammonia but can also be more easily excreted by the body as urine (which is basically urea diluted with water extracted by the kidneys from circulating blood plasma).

As a vampire bat begins to feed, the kidneys immediately begin producing an extremely dilute form of urine in order to shed excess weight (approximately 25 percent of the blood volume consumed is excreted as urine within the first hour after feeding). Soon, though, the excretory system shifts into a very different gear. The urine produced becomes more and more concentrated as the kidneys work frantically to eliminate the rapidly accumulating urea without causing dehydration in the bat.

Because of the vampire bat's need to excrete massive amounts of urea, water loss is a real and constant problem. Dehydration may have been one of the key factors that limited vampire bat ranges to regions that had a high relative humidity and may be one more reason why prehistoric vampire bats disappeared in North America. In a similar vein (sorry, I had to get that out of my system), vampire bats are restricted to relatively short-duration flights each night.
^*34
Presumably, this limitation is due to the evaporative water loss that typifies bat flight. So, although many bat species are known to migrate or undertake lengthy nocturnal flights, a sanguivorous diet appears to have selected against these behaviors in vampire bats.

In 1969, Cornell University vampire bat expert William Wimsatt and his coauthor William McFarland put things in perspective: “In a very real sense the vampire bat can be considered to inhabit a desert in the midst of the tropics. But the desert is delimited not by environmental aridity, but rather by the nutrition and behavior of the vampire.”

In both the vampire bats' digestive and excretory systems, there have been evolutionary trade-offs related to blood feeding, as speed of food processing has been increased at the expense of nutrient storage. This type of trade-off is a hallmark of biology. Some organisms adapt to the changing environments in which they exist (e.g., vampire bats are able to feed on blood in a humid environment but can't migrate or fly for extended periods). More often, though, organisms are unable to adapt—like many of the browsing mammals that died off as North American forests transitioned into grasslands. Although it's certainly more fun to think about the big, sexy mass extinction events (like the one that occurred around sixty-five million years ago), the vast majority of species that have ever lived on this planet apparently disappeared rather gradually. Most extinctions, it seems, were accompanied by a whimper, not a roar.

Farouk and I moved past the
Desmodus
cage to a smaller unit and peered in. Like their “common” cousins, all of the white-winged vampires were asleep except for one—but that was where the similarities ended. Where
Desmodus
has sharp, hard features (I've always maintained that they
looked
the part of vampires),
Diaemus
reminds me of a stuffed-animal version of a vampire bat. Their face is much softer—the sharp angles seem smoothed out and the eyes are huge and gentle looking. Their demeanor reflects these anatomical differences. In three years of handling
Diaemus
in captivity, they never attempted to bite me. Not once.

Farouk shook his finger at me and then continued his lesson. “Now, if you leave out the same tray of cow blood for newly captured
Diaemus,
you'll have dead bats in two nights.”

I soon learned that Farouk's “secret” for success was that each night he hand fed his newly captured white-winged vampires using a five-cubic-centimeter syringe full of cow blood. If the bats refused to eat, he didn't force the issue but instead provided them with the opportunity to feed on a live chicken. Over the course of several weeks, even the most finicky of his “babies” got the message and soon enough he had them feeding on cow blood that he'd poured into an ice-cube tray. He supplemented his captive bats' diet once a week (and probably on holidays) with feeding sessions on live chickens. These usually consisted of four hungry vampire bats to one soon-to-be-anemic chicken.