What Vampire Bat Teeth Tell Us About Their Evolution
Author Bill Schutt writes about vampire bats’ unusual teeth—and how scientists puzzled over their evolutionary history.
The following is an excerpt from Bite: An Incisive History of Teeth, from Hagfish to Humans by Bill Schutt.
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Bite: An Incisive History of Teeth, from Hagfish to Humans
One of my favorite things about social media is connecting with people I haven’t seen or heard from in decades. When some of these old acquaintances contact me and we get around to career talk, I tell them that I’ve been teaching anatomy (though I recently retired), working as a research associate at the American Museum of Natural History, and studying bats for over thirty years. With my long-standing love of animals and the macabre, no one seems surprised that with roughly 1,470 species of bats to choose from, I picked the three species of vampire bats as my research subjects. And with their unique feeding behavior (they’re the only vertebrates that feed solely on blood) and just-as-unique dental anatomy, these fascinating and long-misunderstood members of the mammalian order Chiroptera have had a lot to tell us about teeth.
In the spring of 1991, and not long after entering my doctorate program in zoology, I started thinking about potential topics for thesis-related research. As these sorts of things tend to go, I was expected to pick a topic that fell under the expertise of the graduate committee chairperson. In my case, it was John Hermanson, a young faculty member whose only other grad student had departed the year before. Hermanson, who taught anatomy at Cornell University’s veterinary school (though he was not himself a vet) had a strong background in muscle biochemistry, especially variants of the famous contractile protein duo actin and myosin. In the presence of calcium and adenosine triphosphate (the energy currency of the cell, better known as ATP), the chain-like molecules of actin and myosin ratchet past each other. Occurring simultaneously millions of times over, this molecular movement produces the muscle contraction responsible for actions like walking and galloping—activities formerly popular with the horses whose leg muscles Hermanson kept stored in an ultracold freezer.
Hermanson was at the forefront of the relatively new field of functional morphology, a modern take on classical anatomy. Practitioners of “func morph” studied the relationship between the structures of an organism (i.e., its anatomy) and the function of those structures. Hitting the literature hard, I quickly became intrigued. It also became apparent that Hermanson’s favorite study animals were clearly off the beaten horse track—some of them in fact flitting over said tracks after dusk.
Having spent at least a third of my childhood free time peering under rocks and logs for snakes, salamanders, and other creepy-crawlies, another third maintaining a collection of exotic pets (I had a monkey named Guggie and, later, a boa constrictor named Alice), and the remainder of my time watching horror movies, it took me approximately five seconds to choose bats over biochemistry.
When the fifteenth- and sixteenth-century Europeans returned from their flag-planting exploits in the New World, along with the loot they had pillaged and the Indigenous people they had kidnapped and enslaved, they also brought back tales of strange creatures—of sea monsters, of cyclopean humans with tails, and of bats that attacked men in the night and drank their blood. The latter were generally described as hideous creatures sporting five-foot wings. With these Westerners far more concerned with gold, God, and geography, it would take another 250 years for the first naturalists to begin closely examining and classifying the natural wonders they either encountered in places like Central and South America or pulled out of bottles and shipping crates returned from those regions.
When it came to unmasking the identity of the vampire bats, though . . . they blew it. Undoubtably, most of these scientist types saw only the results of vampire bat attacks, which took place in the dead of night, but not the attacks themselves. And the majority of the accounts they collected were likely secondhand tales or worse. Many of the bats they did see had spear-shaped nasal structures projecting vertically from their snouts. Unfortunately, this led the naturalists to the mistaken belief that these nose leaves were wielded like fleshy stilettos. The thought was that after impaling their victims, the vampires would then drain their blood through the resulting gash.
Taxonomists began assigning scary-sounding names to any specimens with a nose leaf, names like Vampyrops, Vampyressa, Vampyrodes, and Vampyrum, though, in truth, none of these bats were vampires. Along the way, the group picked up the common name New World leaf-nosed bats. Things went further off the rails as the presence of nose leaves in two Old World bat families (Rhinolophidae and Megadermatidae) likely contributed to the incorrect belief that vampire bats could also be found throughout Europe, Africa, the Indo-Pacific, and Asia.
Today, vampire bats belong to the large (with approximately 170 species) and diverse family Phyllostomidae (in which all but three species do not feed on blood). Though they are still commonly referred to as New World leaf-nosed bats, we now understand that nose leaves aren’t weapons but are instead employed, megaphone-style, to direct the larynx-generated echolocation calls that characterize the bats that possess them.* I’ve always found the earlier interpretation of nose-leaf function more than a bit odd, basically because nose leaves are soft and pliable, and in no way capable of inflicting wounds of any kind. Perhaps it was the permanently stiffened condition they were in when they arrived at museums and universities across Europe that misled taxonomists.
It is nearly always a surprise to non–bat biologists that there are but three species of blood-feeding bats alive today. Their ranges are confined to parts of Mexico, Central and South America, and two Caribbean islands (Trinidad and Margarita). The fact that they lack elongate nose leaves, which aren’t a modern-day requirement for inclusion in the Phyllostomidae family, but instead possess pad-shaped versions of these structures, goes a long way to explain the nonvampiric scientific names the trio was assigned in the nineteenth century: Desmodus rotundus, Diphylla ecaudata, and Diaemus youngi. But even with- out sinister-sounding scientific names, each of these bats is wonderfully equipped for a life of obligate sanguivory (blood feeding only)—and several of their adaptations are related to their choppers.
Vampire bats have the fewest teeth of any bat species: twenty-six in Diphylla ecaudata, the hairy-legged vampire bat; twenty-two in Diaemus youngi, the white-winged vampire bat; and twenty in Desmodus rotundus, the common vampire bat. Desmodus is also the only bat with a single upper and lower molar on each side of the jaw. Interestingly, the fact that vampire bat molars are few in number and tiny in size makes perfect sense. Molars generally serve as the functional equivalent of grinding mills. In many mammals, their job is to smash food items into near pulp—which not only increases the surface area available for diges- tive enzymes to start doing their jobs but makes it easier to swallow that saliva-moistened mouthful of whatever. Vampire bats consume no solid food, so molars are not required—especially given the metabolic expense of growing and maintaining them.
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 ecto-parasites 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 terres- trial 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 drac- ulae, 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 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 and 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.
Bite: An Incisive History of Teeth, from Hagfish to Humans by Bill Schutt. Used with permission of the publisher, Algonquin Books. Copyright © 2024 by Bill Schutt.
Meet the Writer
About Bill Schutt
Bill Schutt is a zoologist and author of Bite: An Incisive History of Teeth from Hagfish to Human. He’s based in New York, New York.