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Premolars (located in back of the incisors and canines but in front of molars) usually see action before molars, shearing large bits of food into smaller ready-to-be-mashed bits. But in vampire bats, they act like a barber's razor, clearing away tiny patches of their prey's hair, feathers, or scales, in preparation for a bite.

Not long after I began studying these creatures, I started to wonder who their ancestors might have been and what they were doing before they made a switch to full-time blood feeding. Although vampire bats clearly had their origins in what is now South America, there was little in the way of a fossil record indicating how they might have evolved. With no fossils to point the way, a couple of hypotheses had been proposed, each suggesting how vampire bat ancestors (protovampires) might have been feeding.

Zoologist Dennis Turner addressed the question briefly in 1975. "Perhaps a progenitor of today's vampire bats specialized on the ectoparasites of larger wild animals," he wrote, likely referring to blood-filled ticks as a food source before a transition to blood obtained directly from animals that the ticks were feeding on.

In the wound-feeding hypothesis proposed by renowned bat biologist Brock Fenton, protovampires likely fed at previously existing wound sites commonly found on large animals. There, the stealthy fliers munched on the maggots and other insect larvae that called those festering gashes home. Importantly, though, they would have also gotten a taste of blood from the injured animals—eventually making the switch from feeding on wounds to feeding on blood obtained from bites they themselves had inflicted.

Primarily because neither of these feeding styles is exhibited by any known bat species, I came up with an alternative hypothesis on vampire bat origins. This one was based on a form of feeding behavior that is present in extant (i.e., currently living) bats, including species that are closely related to the three vampires. The arboreal-feeding hypothesis posits that vampire bat ancestors living in what is now South America were carnivores dedicated either full-or part-time to feeding on victims they attacked in the trees. Back in the ancient forests, roughly ten million years ago, protovampires would have conducted "sneak up and pounce" attacks upon smaller, tree-dwelling prey (like large insects and frogs), but they later tweaked their attacks to allow them to feed upon much larger arboreal species like marsupials, primates, and tree sloths, which also happened to be evolving at the same time and place. Since these animals were too large for the bats to overpower using previous attack strategies, this new predation technique could be described as "sneak up and bite."

 

According to the arboreal-feeding hypothesis, some ancient vampire bats, having evolved a strictly blood-feeding diet, would have moved down from the trees (stealthily, of course) to take advantage of terrestrial prey, which may have included megamammals like giant sloths and heavily armored armadillo relatives known as glyptodonts. These four-footed blood banks had been around for roughly twenty million years before the first vampire bats are thought to have evolved. Perhaps reflecting the presence of larger prey, several extinct vampire bat species, including Desmodus stocki and the wonderfully named Desmodus draculae, were significantly larger than the three extant vampires, with D. stocki inhabiting what is now Florida.

In each of these three hypotheses, natural selection would have acted to transform protovampires into the ultimate stealth hunters. Their arsenal included dental modifications that made their bites painless, thus preventing their prey from fleeing or biting back. Other adaptations functioned to keep a victim's blood flowing for as long as possible before clotting. Changes to teeth centered on the evolution of the ultrasharp incisors and canines that characterize living vampire bats. The anticoagulants currently found in vampire bat saliva, which keep the prey's blood flowing, may have evolved from the previously existing substances used by the bats themselves to prevent their own accidental blood-clot formation. As we'll soon see, this form of chemical weaponization would mimic the evolution of other naturally occurring substances, like those involved in lowering blood pressure. In many species of snakes, fish, and even a few mammals, these compounds an others were modified through natural selection into venom. Defined as "a toxic substance delivered by a bite, sting, or barb," venom would provide a survival edge to the creatures possessing it—over those that didn't.

Sadly, there are no transitional fossils showing a clear link between non-blood-feeding leaf-nosed bats and their vampiric descendants. But whether vampire bat ancestors were wound nibblers, tick gleaners, or prey-switching carnivores, one certainty is that their three living descendants are smaller than most people think. A while lot smaller, in fact. With a body length averaging about 3.5 inches and a wingspan of 7 to 8 inches, vampire bats generally weigh in at two ounces or a shade less—before settling in for a blood meal, that is. The premeal weigh-in is important, since bats in each of these species can consume up to 50 percent of their body weight in blood each night.

To reduce competition where the species overlap (in places like Trinidad and Brazil, where I studied them), vampire bats divvy up the resources, with the common vampire bats feeding primarily on large terrestrial imports like cattle, horses, and pigs, and the white-winged and hairy-legged vampires preferring to dine on bird blood. Given the human-generated flood of livestock into South America, populations of the common vampire bat exploded during the early twentieth century.