Dark Banquet: Blood and the Curious Lives of Blood-Feeding Creatures Read online

  Vampire bats, feeding at a bite they’ve inflicted, use their tongues to draw out the blood. Contrary to popular belief, they do not suck blood from their victims. In fact, the physics involved is very similar to what happens when a phlebotomist draws up a patient’s blood into a capillary tube. Basically, these thin glass tubes work because their inner diameter is so small that the force of attraction between the blood and the glass is greater than the downward pull of gravity. Thus, the blood pulls itself up the inside of the tube, filling it to a considerable degree.

  In the case of vampire bats, the pistonlike motion of the bat’s tongue causes blood to flow (via capillary action) along a pair of grooves located on the bottom of the tongue and directly into the bat’s mouth. There’s even a cleft on the bat’s lower lip and a space between its lower incisors to facilitate the blood flow. While feeding in this manner, saliva is constantly applied to the wound.

  Vampire bat saliva contains several ingredients that inhibit the body’s normal clotting mechanisms. One such anticoagulant compound works by preventing blood platelets from clumping together—an important step in the formation of a plug that will eventually become a blood clot. Meanwhile, another salivary ingredient inhibits the torn blood vessels from constricting—a process that normally reduces blood flow to the wound site and thus from the wound itself. Finally, an enzyme that medical researchers would christen desmokinase (and, later, desmoteplase or DSPA for Desmodus rotundus salivary plasminogen activator) breaks down the protein framework upon which the remainder of the blood clot forms.

  Primarily because of its antihemostatic properties, vampire bat saliva has drawn considerable attention from the medical community as a potential treatment for certain strokes, namely, those in which a blood clot inhibits blood flow within the brain’s blood vessels. In these instances, cells in the region of the brain on the downstream side of the clot are denied oxygen and nutrients. If this blockade continues for long enough, the cells die and the function they were responsible for is impaired. Traditionally, stroke victims have been treated with a compound called tissue plasminogen activator (t-PA). Unfortunately, t-PA must be administered within three hours of the stroke to be effective. After that, the risk of bleeding into the brain increases, and as a result so does brain cell death. Since the average stroke patient waits more than twelve hours before going to the emergency room, t-PA is rarely administered and cannot be considered an effective treatment for the nation’s third-largest killer (after heart disease and cancer). Unlike t-PA, though, studies have shown that the vampire bat–derived DSPA (an extremely potent clot buster) can be administered up to nine hours after a stroke has occurred and has no detrimental effects on brain cells.

  Since vampire bats often lick the site before inflicting their bite, there has also been some speculation that their saliva might contain either a pain-killing agent that prevents their prey from feeling the bite, or an enzyme that could function to soften up the potential bite site. Even without a painkiller or a skin-softening enzyme, the vampire bat’s razor-sharp teeth are likely capable of producing a wound that causes little or no pain to the prey.

  In what amounts to a strange sort of payback, one modern technique used for vampire bat eradication involves the use of the anticoagulant warfarin. Isolated from a clover mold, warfarin has been marketed for humans since the 1950s as Coumadin. It remains (along with the lung-and intestine-derived compound heparin) a popular blood thinner for millions of patients prone to strokes or blood clots.

  Vampire bats treated with warfarin are subject to no such medical benefits. After capturing the bats in mist nets, the animals are painted with a mixture of warfarin and Vaseline and then released so that they can fly back to their roosts. Since vampire bats spend a considerable amount of time grooming one another, the toxic paste is soon spread throughout the members of the roost—with deadly results. Bats soon perish as the ingested warfarin induces massive internal hemorrhaging, causing them to bleed to death.

  Although some might consider vampire bats dying from a clot-busting anticoagulant to be poetic justice, others might consider it to be a cruel waste of neat bats. In any event, warfarin paste is certainly several steps up from the days when cries for vampire bat heads sent folks scurrying for dynamite, poison gas, and flamethrowers. As long as vampire bat control personnel are applying the paste to the correct bats, this eradication method is relatively species specific. The drawback is that it’s successful only in places (like Trinidad) where individuals are trained to capture the right bat species, untangle them from mist nets (no easy task), and then apply the poisonous paste. All of this must be accomplished without injuring the bat, getting hammered by its powerful jaws, or painting a nonvampire bat by mistake.

  A related, but less-cost-efficient, method of vampire control involves the inoculation of livestock with low doses of anticoagulants. Vampires feeding on the inoculated bovine blood suffer the same hemorrhagic fates as those grooming warfarin paste off their roost mates. While this systemic method eliminates the need to capture and correctly identify vampire bats, it does require the treatment of the entire herd in order to be effective.

  In the end, both of these methods are successful because of the low reproductive rate in vampire bats. Like the vast majority of bats, vampires give birth to only one pup per year—a far cry from other mammal pests like rodents, who can crank out babies faster than a baseball player can split sunflower seeds.

  Back in the slaughterhouse, Kim and I used the same colander to strain off the strange, woven clots that materialized in the barrel—squeezing out the blood they held, before discarding the sponge-like clumps in a waste bucket. After about fifteen minutes of this fun, the defibrinated blood that remained in the barrel (i.e., the blood minus the clotting factors and proteins making up the clots) was poured, still warm, into the two-gallon plastic containers we’d brought with us. By stimulating clots to form, and then removing them, we were assured that our defibrinated blood would remain liquefied and clot free during storage, and when we placed it out to feed our bats.

  Although we didn’t realize it at the time, a similar method was employed between the 1820s and 1920s to defibrinate a human donor’s blood prior to transfusion. In the days before the medicinal use of anticoagulants, donated blood was collected in a bowl, whisked, and filtered before being transfused into a recipient.

  Some researchers use an alternative method to facilitate the storage of blood (for vampire bat meals and other purposes). The technique involves “citrating” the blood by adding the compound trisodium citrate to it. This also prevents the formation of clots, and although we never employed this method, in hindsight it could have provided our captive vampires with a slightly more nutritious meal. This is because, unlike our whisk and filter method, the clotting proteins aren’t actually removed from the citrated blood.

  After returning to our lab at Cornell’s School of Veterinary Medicine, Kim and I transferred the blood into several dozen Snapple bottles we’d collected earlier from the cafeteria (yes, we cleaned them first). We froze the blood-filled bottles, thawing one out each morning so that the liquid would reach room temperature by nightfall. That was when we fed our vampire bat colonies—pouring the blood into an ice-cube tray and elevating it with a wooden block so that the roosting bats wouldn’t have to strain themselves while they ate. As Farouk had done in Trinidad, we supplemented the diet of our white-winged vampires with a live chicken (once per week and on holidays). This turned out to be a vital step in maintaining our vampire colony, as I found out three years later.

  Shortly after passing the bats off to another Cornell grad student (who had proposed a study on their digestive physiology), I received a rather frantic call from Kim. I discovered that the new researcher had not only relieved my friend of her bat-keeping duties but had decided to suspend the colony’s live chicken supplement (basically to save a few bucks each week while eliminating the far from insubstantial hassle of dealing with live chickens). Within ten day
s, vampire bats began dying at an alarming rate—a trend that stopped immediately after a “talk” with the grad student led to a resumption of weekly chicken dinners for the colony.

  During the three years that we maintained our colonies of common and white-winged vampire bats, it’s safe to say that we saw some strange stuff, much of it relating to feeding behavior or social interactions between roost-mates. We found out later that Farouk and his Trinidadian bat crew had already noted much of what we were observing at Cornell. Their reluctance to publish, however, made it news to us, and we were grateful that these bat experts had (for some reason) decided to take us on as collaborators and coauthors. There were numerous occasions when something very much like the following exchange took place over a crackling long-distance phone line.

  “Yes?” Farouk’s Trinidadian accent made it sound more like yes-ah.

  “Farouk?”

  “Yes.”

  “You’re not going to believe what we just saw.”

  Silence.

  “I think Diaemus is mimicking chicks. They’re snuggling right up to these hens—then biting them on the chest. It’s unfriggin-believable!”

  Silence.

  “Farouk?”

  “Yes.”

  “Have you ever seen that before?”

  “Yes. The bites are on the brood patch.”

  “Oh…Cool. Okay, I’ll talk to you soon.”

  “Yes.” Click.

  I’ve always considered my friend Farouk Muradali to be one of the most generous and nurturing people I’ve ever met. But to say that he is a man of few words…well, you get the picture.

  My collaborators and I also learned from the start that Arthur Greenhall had been right about the significant differences that existed between vampire bat species (in our case, between Desmodus rotundus and Diaemus youngi)—and we would discover that most of this variation was related to the bat’s preference for either mammalian or avian blood, respectively.

  “Diaemus doesn’t jump,” Farouk had said (in what would become his equivalent of the Gettysburg Address). And after a hundred-plus trials on our miniature force platform, we had to agree. But why was this so?

  Initially, we tested our system out with the common vampire bat, Desmodus, and as in previous studies, we confirmed that these bats could make spectacular, acrobatic jumps, in any direction. Pushing off the ground with their powerful pectoral muscles, Desmodus used its elongated thumbs (the last things to leave the ground) to impart precise direction to jumps that could reach three feet in height.

  These amazing jumps, along with their ability to run at speeds of up to two meters per second, were adaptations for terrestrial blood feeding. They enabled the common vampire bat to escape predators, avoid being crushed by their relatively enormous prey, and initiate flight after a blood meal. The ability to feed efficiently on large quadrupeds is the primary reason why Desmodus rotundus has been so successful in terms of numbers and range, but in all likelihood this success was a rather recent development.

  Until about five hundred years ago, Desmodus rotundus may have been anything but “common.” In fact, populations would have been severely restricted not only by climate but by the finite number of large mammals that were present in any given area. Quite possibly the vampires would have been compelled (as they sometimes are today) to feed on smaller mammals as well as birds and other vertebrates like snakes and lizards.

  Starting in the early 1500s, however, the influx of Europeans and their domestic animals into the Neotropics would have spelled big changes for Desmodus, as well as the other two vampire genera. Suddenly, enormous four-footed feeding stations would have sprung up in places where the pickings might have previously been sparse for thousands of years. Additionally, not only would there have been plenty of new animals to prey upon, many of these quadrupedal blood bags would have been penned in, making them super easy to find and ultimately making meal time a whole lot more predictable than it had ever been before. Populations of the opportunistic Desmodus would have exploded as more and more land was cleared for cattle farming. The more cows, pigs, and horses, the larger the vampire bat populations that could be sustained by their blood. Human victims weren’t necessarily preferred, but they did give the vampires additional opportunities to feed, long before windows, screens, and protective netting would keep them at bay.

  From the standpoint of the newly arrived humans, it must have seemed like yet another plague had descended upon them, for with the mysterious nocturnal attacks and the gruesome postbite cleanup came diseases, rabies being the most feared. Soon, stories of vampire bats, their gory attacks, and the horrible diseases that they inflicted, were making the rounds throughout Europe, and from there they spread to the rest of the world. What little scientific knowledge there was on the topic became blurred by misconception and misidentification, turning tales of these creatures into an unreliable blend of vampire fact merged with vampyre fiction.

  Unlike Desmodus, Diaemus youngi (which resembles a winged teddy bear) has contributed little if anything to vampyre folklore. Perhaps they were once fast and aggressive, and maybe they even initiated flight similarly to their spring-loaded cousins. But now their movements are more deliberately paced and show little sense of urgency. When placed on the surface of our force platform, white-winged vampires would give a little hop or two, then scuttle off to find a dark corner in which to hide.

  Watching Diaemus feed arboreally, we saw why they didn’t need to catapult themselves into the air. Approaching a roosting bird from below the branch, white-winged vampires moved slowly and stealthily—advancing one limb at a time—and always keeping the branch between itself and the underside of its intended prey. Once situated beneath the feathered lunch wagon, Diaemus picked a potential bite site, usually on the bird’s backward-pointing big toe (i.e., the hallux). This made perfect sense, since feeding from this particular digit kept the bat better hidden from above than if it had chosen to feed on one of the three forward-facing toes. After licking the chosen site for several minutes, an apparently painless bite was inflicted using the razor-sharp teeth that characterize all three vampire bat species. The bite was never violent and very often occurred as the bird shifted position slightly on its perch, as if reacting to some slightly uncomfortable irritant. Still hanging below its completely oblivious prey, Diaemus began feeding, and within five minutes it began peeing. It did so by extending one hind limb sideways and downward, deftly avoiding the embarrassment of soiling itself while eating. After feeding for between fifteen and twenty minutes, the bat would release its thumbs from a branch, hang briefly by its hind limbs, then drop into flight. Initiating flight in this manner, there was absolutely no need for Diaemus to jump, and so it didn’t, at least not into flight.

  On numerous occasions, we did observe Diaemus feeding on birds from the ground. Supporting its body in a low crouch (as compared with the extreme upright stance of Desmodus), the white-winged vampire was quite adept at hopping around (rather comically) in pursuit of a feathered blood meal. This behavior had not been reported in the wild and we used it to propose that the white-winged vampire bat had made a relatively recent return to the trees, thus avoiding competition with its ground-feeding cousin, Desmodus.

  During these terrestrial feeding bouts we occasionally recorded behavior that approached chick mimicry on the “weird-o-meter.” This occurred after the bat leaped or climbed onto the chicken’s back, then scuttled forward, intent on biting the back of the bird’s head or its fleshy comb. Male chickens mounted in this fashion quickly grew agitated and dislodged the bat with a shake and a peck. Hens, however, had an entirely different response. Rather than showing annoyance, female chickens quickly assumed a crouching posture that they maintained until after the vampire bat had finished feeding and hopped off. With a little research into poultry behavior, we learned that this was the identical posture taken by a hen while being mounted by a male bird—for a completely different reason.

  Another way that Diaemus diffe
rs from Desmodus and Diphylla is by the presence of a pair of cup-shaped oral glands located at the rear of the mouth. When Diaemus gets upset (or, as we observed, during dominance hierarchy behavior), the glands are projected forward and they can be seen quite easily when the bat opens its mouth. As it does, Diaemus produces a strange hissing vocalization that is accompanied by the emission of a fine spray of musky-smelling liquid from the oral glands. Although a detailed study remains to be performed, the oral glands of Diaemus appear to function in self-defense (like the scent glands of skunks) and as a means of communicating information like status, mood, and territorial boundaries to others of its kind.

  Besides their actual ability to feed on blood, perhaps the most fascinating of all vampire bat adaptations is one that we observed only once in our colony of Diaemus.

  In 1984, zoologist Gerry Wilkinson reported that vampire bats in the wild commonly share food by regurgitating blood. Wilkinson, who made his initial observations on Desmodus rotundus, determined that about 75 percent of the time blood sharing occurred between a mother and her dependent offspring (until about the age of one). In other instances, sharing took place between related or unrelated bats.

  Gerry’s results indicated that there were several reasons why this behavior occurred. Blood sharing between mothers and newborn pups presumably transfers nutrients and bacteria to the infant’s digestive tract. In humans there are normally over two hundred species of bacteria living somewhere on or in our bodies (it’s rumored that in some college dorms this number can hit five million species). In any event, these essential microbes (termed bacterial flora) are vital components of several physiological processes, most notably digestion.