Bats are the only flying mammals in the world that roost upside down. Their wings are made of excessively long fingers between which membranous skin called patagium is stretched. Vampire bats are blood sucking or sanguivorous flying mammals that live solely on blood of vertebrates. Three species of vampire bats occur in central and South America: the common vampire bat (Desmodus rotundus), the hairy-legged vampire bat (Diphylla ecaudata) and the white-winged vampire bat (Diaemus youngi).
Unlike other bats, vampire bats can walk, jump and even run by using both forelimbs and hind limbs. A recent study found that common vampire bats can, run at a speed of up to 72 meters per minute.
Contrary to the prevailing stories about the large blood sucking bat-man or Dracula, vampire bats are tiny creatures, the size of human thumb that emerge from their roosting places in the darkness of night to suck blood of sleeping cattle and horses which are their usual victims.
Common vampire bats are tailless, medium-sized mammals, with a length of 7–9 cm and weighing only 50 grams but can double their weight in one feeding. They have very strong hind legs and a strong thumb of fore limb that helps them in running as well as in taking off after feeding.
HUMAN VAMPIRE MYTH
The excessively cruel Romanian General, Prince Vlad III, the Impaler (1431-1476) was the original “human vampire” and was also called Dracula (son of dragon). Bram Stoker’s (1897) novel Dracula combined the discovery of vampire bats in South America with Vlad the Impaler horror stories, to create the main character of his novel, Count Dracula who was a century old blood thirsty vampire. Stoker had read newspaper articles on vampire bats and decided to include the blood-sucking creature in his book. Vampire bats rarely bite humans but once they suck human blood, they are known to come back the next night to feed again.
In Transylvania, the area of Bram Stoker’s Dracula, there are stories of sightings of the vampires which are believed to hang around crossroads on St. George’s Day, April 23, and on the eve of St. Andrew, on November 29, when people are known to have close encounters with them. Some believe the Dracula hallucination happens because of the fact that the area is situated on one of Earth’s strongest magnetic fields and people there have some kind of extra-sensory perception.
SPECIES SIMILAR TO VAMPIRE BATS
The lesser false vampire bat (Megaderma spasma) is an insectivorous bat found in Southeast Asia from India to Indonesia and the Philippines. It has a wing span of 30 cm and body length of 10 cm.
The greater false vampire bat (Megaderma lyra) is native to Asia. It is also known as the Indian false vampire bat that preys upon other bats, small birds, reptiles, fish and insects.
Desmodus draculae is an extinct species of vampire bats that inhabited South America during the Pleistocene. The species has also been called the giant vampire bat because of its large size.
The genus Vampyrum contains only one species, the spectral bat or false vampire bat (Vampyrum spectrum). This species is the largest bat native to the Americas and also the largest carnivorous bat in the world. The wingspan of this bat can reach up to 100 cm.
METHOD OF BLOOD FEED
Vampire bats locate a suitable host using infrared sensors which lead them to their victims on the ground. They land near their prey and walk stealthily using fore as well as hind limbs till they find a suitable spot to suck blood. Vampire bats have heat sensors on the nose that help it to locate warm blood just beneath victim’s skin. The bats have razor-sharp teeth with which they make a painless incision to draw blood. Vampire bats’ saliva contains a substance called draculin which is a plasminogen activator that ensures that the blood from the wound of their prey doesn’t clot either while flowing from the wound or in the stomach of bat.
Contrary to popular belief, Vampire Bats do not suck blood but lap it up using their grooved tongue as it flows out of the wound. Certain chemicals in the Vampire Bat’s saliva numb the area of skin around the bite to prevent the host from feeling pain.
White-winged vampire bats also suck blood of birds that are perched on trees. The bat approaches the bird by crawling on the underside of the branch thus staying out of sight of bird and then makes a cut on the hind toe to suck blood. Vampire bat can suck half of its body weight in blood during 20-30 minutes of feeding session.
SOCIAL BEHAVIOUR AND TROPHOLAXIS
Vampires are known to share meals with one another since they roost in colonies either in caves or in tree holes. Mother bats regurgitate blood from their stomach for their offspring to lap it up. If some bats can’t find meal some day, they may beg others to regurgitate some blood for them. Such favours are always reciprocated for the social system to work and to ensure survival of the colony. Vampire bat colony typically has one reproducing male, about 20 females and their offspring. Vampire bat female almost always has only one offspring in a breeding season.
UNUSUAL RESTING POSTURE OF BATS
Bats rest by hanging on a branch of tree or on a rock cliff because their legs are not strong enough to support their body weight in an upright posture. When bats relax, the weight of their body pulls down the tendons of muscles that are connected to digits, making the digits to clench tightly around the branch. The talon joints lock in that position preventing the bat to fall. This system is so effective that even if a bat dies while roosting, it continues to hang for quite some time.
Why bats roost in hanging position? Hanging posture is an ideal position for takeoff because the bat simply falls down in air to fly rather than jumping like birds. Contrary to birds, bats can’t jump into the air from the ground or branch to take off because they have weaker hind limbs that are attached to patagium of the wings. Their wing muscles are also not strong enough to produce sufficient lift to take off from the ground upward. Owing to their weak hind limbs they can’t even run to build up necessary lift force. Instead, they use the claws of fore limbs to climb to a higher spot and then drop down into flight.
By perching in hanging position, bats hide effectively among foliage and escape from predators by quickly launching into flight. In hanging position they also remain close to the hanging fruits on which the fruit-eating bats feed. However, unlike other species of bats, vampire bats can walk on the ground, can also run and jump using both fore limbs and hind limbs, which is a blood sucking adaptation.