Blood Thirsty: Vampires, From Myth to Reality (part II).
The Origin of Real Vampires
∘ Blood Thirsty
∘ How did vampires start eating blood?
∘ Vermin snacks and large prey
∘ Terrible wounds and razor-teeth
∘ Vampires of the New World
Blood Thirsty
It is a gloomy and quiet night, the moonlight barely illuminates an unsuspecting victim resting on a tropical rainforest. Meanwhile, a night-bound creature bites its victim with sharp, razorlike teeth and then, laps up some blood. It is a feast for a tiny bat vampire enjoying its bloody meal.
In the first chapter of this story, I told you about the biology behind real vampires. But some questions remained unanswered: how did these amazing creatures acquire blood as a diet? Why do they only exist in the New World, even if they are culturally related to Eastern Europe? As Dracula would say: Enter freely and for your own will. I pray you, be seated and drink how you pleased, cause I will tell you the story for these answers [1].
How did vampires start eating blood?
Bob Slaughter in the 70s used morphological dentition patterns of bats to explain the origins of hematophagy (diet based on blood). He thought that teeth were the key to understanding it, so he suggested bats evolved from specialized teeth for feeding on fruits to sucking blood. A similar case happens on the genus Calyptra, a peculiar moth native to Malaysia and southern Europe. Calyptra males usually feed on fruits but also have the capacity to suck the blood of mammals, even humans [2, 3]. Researchers revealed that this behaviour appeared from ancestors who perforated fruit shells to suck the juice inside [4]. So, this seemed to be an analogous explanation for this spooky behaviour of bats, from biting fruits to biting animals [5, 6].
However, this assumption had issues. If this hypothesis was correct, why is it that none of the frugivorous bats of the Old World evolved to a blood-based diet? Finally, Slaughter´s idea was rejected after genetic studies on Phyllostomidae bats (a huge group of bats where hematophagy appeared). Those studies concluded that the ancient blood diet on vampires indeed came from feeding on insects, not fruits [7].
Five years later, other studies supported this idea suggesting that vampire bat’s ancestors would have fed on ectoparasites like fleas or bed bugs of large mammals. Then, they would acquire blood as a by-product of this diet. The unsolved problem was how prehistoric bats found at night small prey like those [5, 8, 9]. The topic was left for a while until 1992, when Canadian researcher Brock Fenton proposed a little plot twist, with a hypothesis for wounds as the main origin of blood-feeding on bats. Fenton´s hypothesis was based on four principles under the idea that wounds attracted insects and, consequently, bats [7].
Vermin snacks and large prey
- From insects to blood: It is well known that phyllostomids have the highest diversity of diets between bats: insects, small vertebrates, fruits, nectar, and blood [5, 7]. This huge variety of diets provided an evolutionary white canvas for moulding the distinct digestive functionalities for each specific diet [7]. At the perfect time, the same year as Fenton´s paper, a study found that enzyme activity (digestive capacity) differs between the diets of Phyllostomidae bats, linking the lacking capacity of vampires to digest maltose (a sugar that is found in grains and fruits) with the digestive peculiarities of their insectivorous ancestors.
2. Large prey of the past: There is an interesting relationship between vampires and megafauna that inhabited the south of the Americas [5]. Considering the limitations imposed upon a blood-feeder, vampires must drink nearly 60% of their weight daily. To get this, they prefer large prey to obtain a sufficient amount of blood without investing time looking for other victims [5, 8, 9]. This fits like a glove for the evolution of hematophagous bats and the diversity of mammals and birds of considerable size in South America, like prehistoric armadillos (Glyptodon) or giant sloths (Megatherium), for instance [5, 10].
Terrible wounds and razor-teeth
3. Bleeding wounds: The third principle is that proto-vampire bats would have learned that wounds were a good spot for eating insects and incidentally blood. In tropical zones, wounds often are infested with screwworms and fleas laying eggs (for some development stages of screwworms, infected wounds are warmer than surrounding areas) [11]. These could have been used by proto-vampires as a valuable and detectable source of food even at night [5].
4. Razor-teeth, all better to eat blood: Finally, morphology dentition must have played an important role for feeding on blood, since most phyllostomid bats developed razor upper incisor teeth. These characteristics would have permitted bats easy frontal access to wounds. In comparison, animal-feeding bats of the Old World bear small to minute incisors impeding a proper bite [5].
Vampires of the New World
Certainly, the reason for the restricted distribution of hematophagous flying mammals to the Neotropics appears to be related to their diet and other inherent particular characteristics. The first is to be a mammal and in consequence, a homeothermic organism (an animal that can regulate its body temperature). In comparison, reptiles and amphibians are ectothermic, so they rely on external sources such as the sun to regulate their temperature.
Being a mammal comes with a high cost for thermal regulation, so they need to consume sufficient energetic resources, which blood does not fully provide [12]. In addition, flight is a costly attribute that limits the amount of blood a vampire can eat at night [13]. These main reasons constricted the development of blood-feeding in colder regions outside the Neotropics, just because the higher energetic cost would be unsustainable. In the end, the mere existence of hematophagous mammals is an intricate and fine balance of factors between ideal temperatures, food resources, and the associated cost to flight and thermoregulation.
As far as we know, evidence points that a small group of bats evolved from insectivorous diets to blood by metabolic affinities and morphological adaptations. They took advantage of an empty niche in their environment. All these to shape and to embody one of the most prolific, ancient, and fascinating myths such as the creatures of the night, the immortal vampires.
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