by Bill Schutt
With less plant cover in which to hide, longer limbs became important for moving quickly over open ground. Basically there are only two ways to augment running speed: by increasing stride frequency and by increasing stride length. Longer limbs contributed to the latter since each time the limb moves forward during a stride, more ground is covered. As the limbs lengthened, toes that were once on the ground either disappeared or remained as vestiges, like the splint bones found in the front legs of the modern horse, Equus cabalis.*22
Protohorse skulls became longer as well, with the eyes set farther back from the mouth. Longer snouts (rostrums) allowed these creatures to graze while simultaneously watching for predators.
In addition to looking more and more like modern horses, these ungulates became extremely diverse—with up to fifteen North American species living at the same time (around ten million years ago). For whatever reasons, though, by roughly five million years ago, only the modern horse remained, spreading into Asia and Europe across a land bridge that spanned what is now the Bering Strait (separating Russia from Alaska). By about thirteen thousand years ago, climate changes, humans, or perhaps, as hypothesized by American Museum of Natural History Curator of Mammalogy Ross MacPhee, a rabieslike hyperdisease drove many large North American mammals to extinction.*23 While it is commonly known that creatures like woolly mammoths and saber-toothed cats went extinct at this time, it’s perhaps a bit more surprising to learn that modern horses also vanished completely in the New World and did not reappear until the Spanish conquistadores reintroduced them in the early 1500s.
Sadly, of the thirty-four recognized genera in the family Equidae, only one survives. What does remain, however, from this once diverse and widespread group, is a transitional fossil record that is unsurpassed in its ability to shed light on the relationship between environmental change and the accompanying structural modifications that can accumulate in generations of creatures living in those changed environments.
Unfortunately, no such easy-to-interpret transition exists for vampire bats or many other organisms, for that matter. Compounding the fact that bat bones are extremely delicate, fossils from creatures that inhabited tropical regions are relatively rare. This is primarily because the remains of the newly dead in such environments are usually dismantled, eaten, and destroyed—with little chance of preservation in the fossil record. The vast majority of vertebrate fossils come from creatures that lived near shorelines—beaches, rivers, or even ponds. Here, rapid sediment deposition could give the dead at least a small chance at becoming fossilized.
Regrettably, this phenomenon, along with the fact that most fossilized creatures had hard parts like shells or bones, led some paleontologists to describe the fossil record as “biased.” Not a bad description, really. But problems arose when deceptively named creation scientists intentionally took the term (and others) completely out of context in an effort to discredit the theory of evolution and insert their own faith-based beliefs.*24
So how do scientists think vampire bats evolved? In cases like this one, where the fossil record isn’t very helpful, researchers often rely on knowledge of what works for organisms living today—preferably those that are closely related to the ancient creatures in question. Prehistoric environments are also important since they provide information on the climate and surroundings in which the ancient critters existed. For the most part, this technique has led to the following hypotheses on the origin of blood feeding in bats.
In one scenario, protovampires fed on blood-engorged ectoparasites like ticks that were feeding on large mammals. Seemingly, the ectoparasite hypothesis was founded upon the knowledge that roughly 70 percent of bats are insectivores (although ticks are certainly not insects), combined with purely anecdotal reports that vampire bats consume parasitic moths. During my graduate studies, I added a modification to this hypothesis by suggesting that if protovampires had in fact gotten their first blood meal by dining on ectoparasites, then blood feeding might actually have originated during mutual grooming behavior. Vampire bats are extremely social animals and studies have shown that they spend approximately 5 percent of their time grooming each other. During such behavior, protovampires may have obtained their first taste of blood from the very same tick and bed bug species that commonly parasitize modern vampire bats (and, indeed, most bats).
Bat biologist Brock Fenton suggested that the small size of ectoparasites, combined with the difficulty of locating them on another animal, made the ectoparasite hypothesis improbable. He was also troubled by the fact that ectoparasites have a worldwide distribution, yet vampire bats are restricted to three New World species. In other words, if ectoparasites were found pretty much everywhere, feeding on all sorts of vertebrates, then why weren’t there more species of vampire bats in existence? I’ll address this question momentarily.
Another hypothesis on the origin of vampire bats was proposed by Fenton, who suggested that blood feeding might have evolved from protovampire bats feeding on insects and their larvae present in and around wounds on large mammals. These wounds, some of which can be quite gruesome, are the result of aggressive social behavior, thorns, or unsuccessful predation.*25 However wounds are inflicted, they can quickly become beacons for swarms of insects (like screwworm flies) searching for a meal or a warm, moist place to lay their eggs. According to Fenton, insectivorous bats feeding at wound sites may have received additional nourishment from the blood and flesh of the wounded animal itself—and at some point, these protovampires would have switched to feeding solely on blood. Fenton strengthened his case by citing the feeding behavior of oxpeckers, a pair of African bird species (genus Buphagus) related to the omnipresent starlings (family Sturnidae). Oxpeckers glean ectoparasites like ticks off large mammals and they’re also reported to feed at wound sites and festering sores. Similarly, certain finches (Geospiza) remove ticks from giant Galápagos tortoises, which elevate their massive bodies on fully extended limbs to allow the tiny birds total access to the blood-engorged pests.
In any event, my problem with the wound-feeding hypothesis is that it proposes that vampire bats evolved in the face of environmental pressures that would have seemingly acted against the development of such behavior. Not only would potential prey need to be wounded but it would also have to be of conspicuously large size and relatively immobile. Because vertebrate blood is basically made up of water and protein, vampire bats cannot store energy in the manner of non-blood-feeding mammals (as fat, for example). This requires vampire bats to consume approximately 50 percent of their body weight in blood each night. Failing to do so, they can starve to death within two or three days. Now that is an extremely tough way to make a living—and studies have shown that vampire bats (especially young adults) may fail to obtain a blood meal one out of every three nights that they hunt. I estimate that this figure would be prohibitively higher if the prey were required to have existing open wounds. Just as important, there are no living bats (nor mammals, for that matter) that are reported to feed at nonlethal wound sites.
Ultimately, it’s extremely difficult to imagine what would have driven protovampires to abandon an insect-eating lifestyle for one dependent on locating large wounded animals on a nightly basis. I can’t envision the selective pressure that would have led to this behavioral transition. As we’ll see a bit later, the wound-feeding hypothesis also flies in the face of modern vampire bat behavior (sorry about that) since these bats can forage only for short periods of time each night. Finally, echolocation (highly evolved in vampire bats and all of their relatives) would have been useless in differentiating wounded from unwounded prey.
In the frugivore hypothesis, well-developed incisors used to slice through thick fruit rinds would have evolved in fruit-eating protovampires into the bladelike teeth that characterize modern vampire bats. Those who proposed this alternative scenario never discussed how or why this transition from fruit to blood might have occurred and the hypothesis remains undeveloped.
Some critics rejec
ted the frugivore hypothesis on the grounds that vampirism never evolved in the Old World fruit bats*26 even though they too are known to possess large upper incisors. This reasoning is similar to rejecting the wound-feeding hypothesis on the grounds that worldwide distribution of ectoparasites fails to explain why there isn’t a worldwide distribution of vampire bat species. Both of these arguments fall short because they suggest that evolution is somehow completely predictable (i.e., “If vampires evolved from fruit eaters in the New World, they must also have evolved from fruit eaters in the Old World”). In reality, the exact set of circumstances that led to the evolution of blood feeding in New World bats (things like habitats, prey, and predators) was not present for the Old World bats. And even if those circumstances had been present, there would be no guarantee that blood feeding would have evolved again. As Stephen J. Gould explained in his outstanding book Wonderful Life, if we could somehow rewind the tape of the earth’s history and then allow it to replay, there would be no guarantee that evolutionary outcomes would turn out exactly the same. Gould’s point was that contingency (i.e., chance occurrences) had a great deal to do with which organisms survived to evolve over historical time. If, for example, the climate shifts that led to a reduction of North American forests had never occurred (or differed in some slight way), then, quite possibly, the modern horse as we know it would never have evolved. Similarly, if a meteor had missed the earth some sixty-five million years ago instead of slamming into an area near the current Yucatán Peninsula—maybe small, bipedal ostrich dinosaurs (Ornithomimosaurs) would now be trashing the earth’s resources, instead of humans. With regard to the evolution of vertebrate vampires, far more subtle changes might have produced Old World vampire bats, vampire birds, or even blood-feeding rodents. For whatever the reasons, though, in the conditions that actually existed, a single group of New World leaf-nosed bats underwent the evolutionary changes that would ultimately result in the only obligate mammalian sanguivores.
As an alternative to previous speculation on vampire bat origins, I proposed the arboreal-feeding hypothesis. Basically, this suggests that protovampires may have been foraging in much the same manner as several species of vampire bat relatives do today, that is, by feeding in the trees on small vertebrates like birds, bats, lizards, rodents, and marsupials.
In that regard, Diaemus youngi and the hairy-legged vampire bat, Diphylla ecaudata, both hunt in trees—feeding primarily on perching birds. There are, however, significant anatomical and behavioral differences between them that provide hints about the evolution of their feeding behavior. While a number of primitive anatomical features indicate that Diphylla originated as an arboreal blood feeder, evidence points to a recent return to the trees for Diaemus, where its ability to prey on birds would have reduced competition with the wildly successful, terrestrial hunter Desmodus rotundus.*27
While the fossil record for bats is scanty, it does indicate that there were carnivorous members of the Neotropical bat family Phyllostomidae present ten million years ago, right around the time vampire bats are thought to have evolved. There were also major climactic changes occurring in South America at this time, with evidence suggesting that formerly vast tracts of forest became isolated islands (refugia) surrounded by grassland. Similar to the horse evolution story in North America, these forest refugia and their surroundings may have become perfect staging grounds for evolutionary change—this time among the phyllostomids.*28
Evidence indicates that at least one phyllostomid alive at this time was a carnivore. Because of its size, Notonycteris may have stalked its prey through the branches before subduing it with bites, in much the same manner as its oversized modern counterpart, Vampyrum spectrum. It is very likely that Notonycteris and other ancient phyllostomid relatives would have encountered an increasingly diverse arboreal fauna, as marsupials like opossums, as well as primates, sloths, and larger forms of birds, took up residence in the trees during this time. Some of these new inhabitants would have been too large for carnivorous bats to stalk and kill using previously existing attack strategies. Over time, isolated populations of some carnivorous phyllostomid may have undergone a behavioral shift that allowed them to exploit these larger animals as a food source.†29 Maintaining a stealthy approach to their potential prey, these protovampires might have started biting larger species as they slept in the branches at night. Similar to Brock Fenton’s wound-feeding hypothesis, these early protovampires may have supplemented their normal diets with flesh and blood, in this case from the bitten animal. The tendency for creatures sleeping in the trees would have been to remain quiet and stationary—even after a bat bite. Sudden relocations or frantic movements by the stricken creatures would have attracted other, even more dangerous, nocturnal predators. For the protovampire, natural selection would have favored adaptations that maximized the nutritional payoff, while minimizing the danger and the likelihood that the prey would move off. In that regard, teeth that could inflict painless bites and salivary anticoagulants to keep the prey’s blood flowing would have been key adaptations, as would an ability to move nimbly along and under the branches where their prey slept. Anatomical modifications that allowed for spiderlike terrestrial locomotion may have evolved as the ancestors of the common vampire bat, Desmodus rotundus, and the white-winged vampire bat, Diaemus youngi, moved down from the trees. Quite possibly these bats would have modified their arboreal blood-feeding techniques to exploit a new source of blood—ground-dwelling vertebrates like procyonids (raccoons and their cousins) or cow-sized, herbivorous tanks called glyptodonts (which are related to modern armadillos).
In nature, this type of coevolution between parasites and their hosts (or predators and their prey) is the rule rather than the exception.*30 In this case, these early vampire bats were simply filling an open niche by exploiting a previously unexploited resource—vertebrate blood.
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
