Vampire bats are bats whose food source is blood, a dietary trait called hematophagy. There are three bat species that feed solely on blood: the common vampire bat (Desmodus rotundus), the hairy-legged vampire bat (Diphylla ecaudata), and the White-winged Vampire Bat (Diaemus youngi). All three species are native to the Americas, ranging from Mexico to Brazil, Chile, and Argentina.
The fact that the three known species of vampire bat all seem more similar to one another than to any other species suggests that sanguivorous habits (feeding on blood) only evolved once, and that the three species share a common ancestor.
Unlike fruit-eating bats, the vampire bat has a short, conical muzzle. It also lacks a nose leaf, instead having naked pads with U-shaped grooves at the tip. The common vampire bat also has specialised infrared sensors on its nose (see ), which aids the animal in locating areas where the blood flows close to the skin of its prey. A nucleus has been found in the brain of vampire bats that has a similar position and similar histology to the infrared receptor of infrared-sensing snakes.
Vampire bats, generally have small ears and a short tail membrane. Their front teeth are specialised for cutting and their back teeth are much smaller than in other bats. Their digestive system is adapted to their liquid diet, and their saliva contains a substance, draculin, which prevents the prey's blood from clotting. The vampire bats do not suck blood, but rather lap the blood at the site of the haemorrhage.
The inferior coliculus, part of the bat's brain that processes sound, is well adapted to detecting the regular breathing sounds of sleeping animals that serve as their main food source.
If there is fur on the skin of the host, the common vampire bat uses its canine and cheek teeth like a barber's blades to shave away the hairs. The bat's razor-sharp upper incisor teeth then make a 7mm long and 8mm deep cut. The upper incisors lack enamel, which keeps them permanently razor sharp.
The bat’s saliva, which is injected into the victim, has a key function in feeding from the wound. The saliva contains several compounds that prolong bleeding, such as anticoagulants that inhibit blood clotting, and compounds that prevent the constriction of blood vessels near the wound.
The stomach lining rapidly absorbs the blood plasma, which is quickly transported to the kidneys from where it passes to the bladder for excretion. So within two minutes of feeding, a common vampire bat begins to expel urine.
While shedding much of the blood's liquid makes taking off from the ground easier, the bat still has added almost 20-30% of its body weight in blood. To take off from the ground, the bat generates extra lift by crouching and flinging itself into the air. Typically within two hours of setting out, the common vampire bat returns to its roost and settles down to spend the rest of the night digesting its meal. Excess urea from protein is thereby excreted via the urinary system of the vampire bat aided by hormones to make concentrated urine that consists of concentrated urea in small amounts of water.
The unique properties of the vampire bats' saliva have found some positive use in medicine. A study which appeared in the January 10, 2003 issue of Stroke: Journal of the American Heart Association, tested a genetically engineered drug called desmoteplase, which uses the anticoagulant properties of the saliva of Desmodus rotundus, and was shown to increase blood flow in stroke patients.
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