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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