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

  In that regard, the mammalian small and large intestines (the terms refer to diameter, not length) are home to billions of bacteria that have evolved a number of mutualistic relationships with their warm-blooded hosts. Often referred to as endosymbionts, these bacteria get food and a warm, moist environment in which to live. The mammals reap a number of benefits from the relationship, including the absorption of vitamins B12 and K, which are secreted by the bacteria as part of their day-to-day functioning.*37 Additionally, indigenous bacterial flora inhibit or kill nonindigenous forms, and they also prevent infection by stimulating the immune system to produce antibodies that can cross-react with potentially harmful nonindigenous bacteria, should they appear. In hooved mammals (i.e., ungulates), as well as wood munchers like termites, the presence of certain endosymbiotic bacteria enables their digestive tracts to break down cellulose—the structural protein that makes up the plant cell wall. These bacteria are the prime reason herbivorous creatures are able to digest plant structures like leaves, stems, and wood. Since we don’t have these specific endosymbionts, it’s also the main reason why this type of “fiber” goes through humans like the vegan version of Roto-Rooter. Young herbivores aren’t born with their bacterial flora either but instead obtain them from adults (like their mothers) through regurgitation or by consuming their feces (coprophagia). For this reason, termite “babies” denied their fecal formula are unable to digest wood and quickly starve to death.

  Other related studies, by researchers such as Long Island University geneticist Ted Brummel, have shown that symbiotic bacteria increase the life span of fruit flies, even though the bacteria are apparently not involved in the digestion of plant matter.

  Blood sharing between related and unrelated vampire bats also occurs on a reciprocal basis; that is, bats that were experimentally starved for one night before receiving blood from another nonrelated individual were more likely to donate blood to that individual when it was starved. This behavior is almost certainly related to the fact that the bats need to acquire a blood meal every night (and will starve to death in two or three days if they don’t obtain one). So, over the course of their long lives (up to twenty years), there will presumably be numerous opportunities to receive and share food. The implication here is that Desmodus can remember past donors and can also recognize cheaters—those individuals who try to beat the system by rarely sharing blood. It’s also interesting to note that although adult males share blood with females and young bats, they do not share with other adult males—which makes perfect sense. Why share food with someone you may be competing with for a mate?

  There is evidence that both Diphylla ecaudata and Diaemus youngi also share blood (as I mentioned, we saw this behavior once in two captive specimens of Diaemus).*38 Unlike Wilkinson’s in-depth study of Desmodus, however, this behavior in Diphylla and Diaemus has yet to be studied in detail.

  This brings up an important point regarding original research—and one that I found quite helpful when I was just starting out in the field. I often advise students who are looking for research projects to seek out classic studies (like Gerry Wilkinson’s) and then think about applying similar techniques to other organisms that have yet to be studied. Likewise, if the original research was done years earlier, new studies on the topic may warrant publication if the new researcher employs technology or methods that weren’t around in the past (or asks questions that wouldn’t have been asked in “the old days”).†39

  Before leaving vampire bats to their bloody business, it’s only fair that I mention the third genus, Diphylla ecaudata, the hairy-legged vampire bat. So named for the frill of hair that borders the back margin of its hind legs, Diphylla is thought to exhibit the most primitive anatomical characteristics for its group.*40 In other words, scientists believe that Diphylla has undergone the least amount of evolutionary change from ancestral vampire bats—whatever they were.

  One such primitive characteristic is that most bats (including Diphylla) have extremely thin hind limb bones (i.e., the femur, tibia, and fibula), and by thin, I mean that their diameter is quite small compared to their length. Scientists believe that this is an evolutionary trade-off related to flight. By having thinner, lightweight limb bones, bats have reduced their weight—an important factor for any flier. The downside of the trade-off becomes apparent if you watch a bat moving around on the ground (which is something you generally don’t see very often). In this regard, most of the eleven hundred species of bats can do little more than a clumsy shuffle when grounded, and even those that can walk are anything but graceful. Engineering models have shown that most bat hind limb bones did not evolve to withstand the compressive loads associated with walking. To demonstrate this for yourself, take a two-inch length of uncooked spaghetti and hold the ends between your thumb and index finger. Then bring your fingers together. You’ve just applied a compressive load to a model of a bat hind limb bone. Neat, huh? Now go pick up those pieces of spaghetti before someone steps on them.

  As you have already learned, an inability to move about terrestrially is not a problem for Desmodus and Diaemus. These blood feeders are quite adept (and, in the case of Desmodus, even spectacular) as they walk, run, and hop about on the ground.

  If you examine the hind limb bones of these two bats, it’s not surprising that compared with Diphylla, they’re thicker in Diaemus (i.e., they have greater diameter to length ratios) and much thicker in Desmodus—where they more closely resemble those of a small terrestrial mammal than they do typical bat hind limb bones. Apparently, stronger limb bones evolved in some vampire bats as they became adapted for current feeding strategies, namely, preying on large quadrupeds like pigs and cows. The evidence that Diaemus was once a terrestrial hunter lies in their robust limb bones, which seem overdesigned for their current arboreal roles. Additionally, Diaemus can scoot along quite well on the ground when it needs to, and it is quite capable of feeding while doing so.

  The fragile hind limb bones of Diphylla, on the other hand, are clues to this bat’s arboreal feeding habits; in other words, form reflects function. Unlike the requirements for walking and hopping, you don’t need thick limb bones to hang under a branch when you feed since bones like the tibia and femur would be loaded under tension rather than compression. Researchers in the 1970s cited engineering models to hypothesize that hanging behavior in bats actually evolved because of thin hind limb bones, and you can demonstrate this concept with another short piece of pasta. Using the thumb and index finger (of both hands this time), gently pull on the ends of your two-inch length of experimental noodle. Unless you’ve twisted or bent the pasta by accident, you should be holding a piece of fracture-free fettuccine.*41 There—you’ve just modeled the tensile forces encountered by the hind limb bones of a hanging bat.

  Besides Diphylla’s fragile hind limb bones, the hairy-legged vampire has another anatomical characteristic not seen in its blood-feeding cousins. In fact, this feature is completely unique to all other animals.

  Many bats have a structure called a calcar, which is a bony or cartilaginous extension of their heel bone (the calcaneus). Since bat hind limbs are rotated up to 180 degrees from the typical mammalian condition (picture your knees facing backward), the calcar generally points toward the midline of the body. Its function is to strengthen and straighten the trailing edge of the tail membrane (uropatagium) that spans the space between a bat’s hind limbs. Basically, the calcar increases aerodynamic efficiency by preventing this extra lift surface from flapping around during flight.

  As one would expect, the calcar varies in size and shape among the eleven hundred bat species. In Noctilio, the fishing bat, for example, the calcar is a huge, bladelike affair. Noctilio uses it to get its substantial tail membrane out of the way while the gafflike hind limb claws skim the water’s surface for prey.

  It’s also no surprise that the calcar is absent in bats that do not have a tail membrane. At least that’s what I thought until I started examining preserved specimens of Diphylla
at the American Museum of Natural History where I was working as a postdoctoral research fellow.

  After determining that differences in performance existed between Desmodus and Diaemus (“Diaemus doesn’t jump!”), I had started looking to see if these behavioral differences might be reflected by variations in their anatomy. While comparing the hind limbs of the three vampires, I noticed that the calcar was absent in Diaemus and reduced to a flaplike tab in Desmodus. Like I said, no big deal, when you consider that all three vampires lacked a functional tail membrane.*42

  The calcar of Diphylla was a completely different story. Not only was it present but it also stood out like a tiny finger. I immediately pulled out several additional specimens—just to make sure I wasn’t just looking at one extremely weird individual. But in each instance, I saw the same digitiform structure. Next, I hit the literature, looking for any mention of Diphylla’s calcar. “Small but well developed” ran the typical description—but nothing more. Finally, I called the vampire bat expert, Scott Altenbach, recalling that he had once maintained a colony of Diaphylla ecaudata in New Mexico. Scott had done the original work on quadrupedal locomotion in the common vampire bat in the 1970s and he’d joined us in Ithaca during our force platform project in 1993.

  I remember a conversation something like what follows taking place over another crackling long distance phone line.

  “Hey, Scott, did you ever take any photos of Diphylla climbing around the branches?”

  “Yeah, but they weren’t branches. We used wooden dowels.”

  “Well, check it out and let me know if your bats were using their calcars to grip the dowels.”

  “What?”

  “I think Diphylla is using its calcar as a sixth digit during arboreal locomotion.”

  (Long pause)

  “Scott?”

  “I’ll go get the photos.”

  (Sound of phone dropping)

  Basically, what I’d proposed was similar to the story of the panda’s thumb (popularized in an essay by Stephen J. Gould). The giant panda (Ailuropoda) feeds on bamboo leaves that it strips off branches, apparently with the aid of its opposable thumb (something not found in any other carnivores). Anatomists who examined the panda, however, found that things weren’t quite as they seemed. The panda’s thumb was actually a wrist bone (the radial sesemoid) that had become greatly enlarged. This allowed it to take on a new function—stripping off bamboo leaves.

  Gould cited the panda’s “thumb” as a beautiful example of how evolution doesn’t create; it tinkers with what’s already there (in this case, the panda’s radial sesemoid bone), modifying it for a new function rather than creating a new structure from scratch.

  Ailuropoda’s odd little digit also presented some rather daunting questions for those who support a creationist view of how we got here. Basically, if there is an intelligent designer, why did he (or she) give the panda a jury-rigged structure for stripping leaves off branches? Why not just give the Ailuropoda a real thumb?

  Back in Ithaca (and several minutes later), I heard scrambling and the sound of the receiver being picked up. “You nailed it, Bill—I’ve got some great shots.”

  “Excellent,” I replied. “Do me a favor and send them to me at once.”

  Altenbach’s black-and-white photos clearly showed a climbing Diphylla with its calcar tightly wrapped around a wooden dowel. I immediately put together a proposal to record this behavior in the field, setting my sights on a visit to central Brazil. Since Diphylla didn’t live in Trinidad, I contacted Brazilian researcher Wilson Uieda, who had been studying the hairy-legged vampire for years with his colleague, Ivan Sazima.

  Outside the capital city of Brasília, at a ranch where the cattle were commonly plagued with Desmodus bites, Wilson and I set up my infrared video camera at sunset. We certainly weren’t interested in the cows or even in Desmodus for that matter. Instead, we aimed our camera upward, into the branches of a fig tree, for it was there that the resident guinea fowl went to roost at dusk.

  Several hours after nightfall, as I stared bleary-eyed at the camera’s viewfinder, a pair of dark shapes flew past the sleeping birds.

  “Wilson, check this out,” I whispered.

  My friend, who had been dozing on the chair next to mine, was instantly alert.

  Less than a minute later, the aerial recon was performed for a second time.

  Wilson whispered a single word. “Diphylla.”

  After that we saw nothing for several minutes—until a tiny pair of glowing spots appeared beneath one of the roosting birds. I hit the zoom on the camera, focusing in on the twin points of reflected light.

  They were eyes!

  Wilson traced a dark silhouette on the screen and I could just make out Diphylla’s upside-down head peeking out from the guinea fowl’s feathery breast.

  “Dinnertime,” he said.

  “This is different from Diaemus,” I responded.

  Wilson replied with a smile.

  Rather than feeding from below the branch, Diphylla was actually hanging from the bird—and photographs taken by Wilson Uieda and his colleagues at another site clearly indicated that Diphylla was using its opposable calcar to get a grip on the body of its avian prey. Unlike the white-winged vampire, which generally fed from bites it inflicted on the toes of perching birds, many of Diphylla’s bites were made around the cloaca (the common opening for the digestive, urinary, and genital tracts found in many nonmammalian vertebrates, like birds).

  Several days later, we visited a cave that was home to a small colony of Diphylla. Using the infrared camera again, we recorded three hairy-legged vampires as they moved across the stony ceiling. Not only were the bats walking upside down, they were moving backward (not really strange since bat knees face backward). What was unique was the way that they led with their hind limbs—carefully seeking a secure purchase before taking a step—and using their “sixth digits” like a rock climber would use his thumbs. After scrambling around the cave ceiling for a few minutes, the vampire bats tired of our intrusion and disappeared into a narrow crevice.

  I left the cave elated that we’d been able to support my hypothesis with observations in the field. What had begun as a surprising observation back in New York City ended with the discovery that just like the panda’s radial sesamoid bone, the hairy-legged vampire bat’s calcar had been co-opted for a new role—as an opposable digit.

  Even more important, although local scientists in places like Trinidad and Brazil had been aware of it for years, it wasn’t until the very end of the twentieth century that the mainstream scientific community began looking at each of the three vampire bats as separate and quite unique. Thanks to researchers like Farouk Muradali, Wilson Uieda, and the late Arthur Greenhall, vampire bats are currently being studied with an eye toward variation rather than presumed similarity. By avoiding our tendency to lump things together, these scientists have increased our knowledge about these fascinating creatures and shifted the focus from flamethrowers and cave destruction to systematic control and, in the case of Diaemus and Diphylla, conservation efforts. Additionally, a better understanding of vampire bats has helped to dispel myths and misconceptions about the eleven hundred nonvampire bat species as well as blood feeders in general. We can now spend more time dealing intelligently with our attraction to nature’s vampires as well as the unique substance that ultimately led to adaptations like razor-sharp teeth and salivary anticoagulants. That substance is, of course, blood—to many, the source of life. But considering our seemingly innate feelings of attraction and revulsion toward blood, until recently, our relative lack of knowledge about the red stuff made us look positively erudite about vampire bat biology.

  I firmly believe that if the whole material medica as now used could be sunk to the bottom of the sea, it would be all the better for mankind—and all the worse for the fishes.

  —Oliver Wendell Holmes

  Nearly all men die of their remedies and not of their illnesses.

  —M
olière

  Several hours before his death, after repeated efforts to be understood, [he] succeeded in expressing a desire that he might be permitted to die without further interruption.

  —Drs. James Craik and Elisha Dick (December 31, 1799)

  4.

  EIGHTY OUNCES

  On the morning of Friday, December 13, 1799, the first president of the United States woke up with a sore throat. He had been riding at his farm the day before and the weather had been cold and windy—snow giving way to hail and finally rain. To make things worse, although his clothes had gotten soaked, he refused to change out of them before dinner. That evening, George Washington stayed up late, reading newspapers and asking his private secretary, Tobias Lear, to read him an account of the Virginia Assembly’s debates on the selection of a senator and governor. Washington, whose voice had become hoarse, took no treatment for what he perceived to be the start of a simple cold.

  Later, Martha, who could tell that her husband was starting to come down with something, chided him for coming to bed so late. “It has been my unvaried rule never to put off till the morrow the duties which should be performed today,” reportedly came Washington’s famous reply to his wife.

  At around 3 a.m., on December 14, the Founding Father woke with a fever. He also found it hard to speak and was having difficulty breathing. A mixture of molasses, vinegar, and butter was prepared, but Washington began choking violently when he tried to swallow it.

  The former president’s longtime physician, Dr. James Craik, was called in, but before the doctor arrived Washington sent for his estate overseer, Albin Rawlins, who appeared just after sunrise. Rawlins’s medical experience consisted of treating sick livestock, but that didn’t stop Washington from ordering the man to bleed him. Although this would seem to be a bizarre request, in Washington’s time bloodletting or “breathing a vein,” as it was called, was an extremely common treatment, comparable nowadays to popping a couple of aspirin. Still, after preparing the former president’s arm, Rawlins hesitated.