Whether in the folklore of Central and South America, where the vampire bat is mainly found, as well as the mythology of European cultures and various others too, or in popular film and fiction, vampire bats have always been depicted as monstrous blood-sucking creatures. It was the movie 'Dracula', based on Bram Stoker's novel, featuring Count Dracula, with his bat-like cape and fangs, prowling for victims to suck blood from in the dead of the night, while he slept all day long, which also featured bats, that has left lasting images of the vampire bat.
Vampire bats do indeed feed on blood, however, rather than being the huge monstrous creatures depicted in fiction and film, they are actually not larger than a mouse. There are only three species of bats that have this feeding habit―The Desmodus rotundus, or the common vampire bat, the Diphylla ecuadata, or the hairy legged vampire bat, and the Diaemus youngi, or the white winged vampire bat. And all these species are in the Americas, in the regions of Mexico, Chile, Brazil, and Argentina. Amongst these, as is evident from its name, the common vampire bat is the species that is most widespread. These bats feed on cattle, birds, horses, pigs, donkeys, goats, and so on.
The muzzle is short and flattened, and it does not have a nose leaf, like other species of bats, with its nose being padded and a V-shaped groove at the tip. There is also a specialized infrared sensor on the nose, which helps it to locate areas where blood is close to the skin.
These creatures have a short tail membrane and small ears. They have a furry body, and their wings are basically made up of their long fingers, which are covered by thin skin. The wingspan measures just 20 cm, or 8 inches. The size of their body is just about that of a human thumb. Their legs are very weak; hence, they are not able to walk very well.
However, the common vampire bat does have the ability of maneuvering on the ground as well as in the air. And they have strong pectoral muscles which they use, along with their long thumb and their hind knees to launch themselves into the air, catapulting about 4 feet high. Once airborne, it transitions into flight in a single fluid motion, all of which takes just 30 milliseconds. As they need to feed close to the ground, this rapid take-off is highly advantageous.
They feed only when it is completely dark. They use echolocation, as well as sound and smell in order to locate prey. They fly close to the ground, at a height of about one meter, and once they do find their prey, they use their specialized heat sensors on their nose to locate veins close to the skin.
They have specialized razor-sharp front teeth which they use for cutting into the skin, and since their diet is exclusively liquid, they have fewer teeth for chewing compared to other species of bats. In case the skin has fur, the common vampire bats have cheek and canine teeth which they use to clip off the hair, like a barber's shears.
The saliva is one of the most important elements used in their system of feeding. It contains a number of ingredients which help in prolonging bleeding. One of them is an anticoagulant, called Draculin, that prevents clotting. Another ingredient prevents red blood cells from sticking together, while a third prevents the veins at the wound from constricting. It then uses its tongue to lap up the blood, and not suck it, as is the common misconception. Each bat requires about 2 tablespoons of blood per day.
Although the average bat weighs just about 40 grams, it usually drinks more than 20 grams of blood in a 20-minute feeding session. This adds another element of complexity, since this weight would make flying after a session of feeding quite impossible. But, the bat's digestive system has adapted itself to process its food rapidly and digest it.
The blood plasma, which has no nutritive value, is absorbed rapidly by the lining of the stomach, which is then swiftly taken to the kidneys by the circulatory system. From there, it goes into the bladder and is excreted. In fact, the common vampire bat begins expelling extremely dilute urine, mainly comprising blood plasma, within 2 minutes of starting to feed.
Then it goes back to its roost, settling down for the remaining part of the night to digest its meal of blood. But, it now faces a new digestive problem. Since blood is basically protein, it creates a large amount of urea which has to be eliminated. Therefore, the bat's urinary system utilizes a number of hormones which makes the urine very concentrated, containing less water and more urea.
Unique Social Behavior
According to studies, it has been found that their dependence on blood as their only source of food is highly risky. This is because if they do not happen to find their food, it can lead to rapid deterioration. They cannot go without food for more than two to three nights, hence, it can be highly dangerous for them to fast.
Other animals that feed on blood, such as leeches and ticks, do not have the same problem, because they can go for weeks, or months, and sometimes even years without feeding themselves. However, since the vampire bat is a warm-blooded animal, the necessity of it staying warm means that if it fasts it can die soon. This is one of the reasons why they are not found in the cooler regions of North, or Central, or South America.
The vampire bat has developed a unique social behavioral pattern which can be seen in their reciprocal altruism, wherein, bats that feed successfully come back to their roost and regurgitate some of the blood to a hungry bat. According to studies, it has been found that this behavior of blood sharing by regurgitation occurs amongst both related as well as unrelated bats in a group. In fact, they even set up a buddy system, wherein, pairs of bats form relationships based on blood sharing.