Vampire bats are bats that feed on blood (hematophagy). There are only three bat species that feed 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.|
They are therefore now grouped as a subfamily, the Desmodontinae within the Phyllostomidae. 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 all three species share a common ancestor.
They have small ears and a short tail membrane. Their front teeth are specialized for cutting and their back teeth are much smaller than in other bats. Their digestive systems are also specialized for their liquid diet. The saliva of vampire bats contains a substance, draculin, which prevents the victim's blood from clotting. They, therefore, lap blood rather than suck it as most people imagine.
The inferior coliculus, part of the bat's brain that processes sound, is specialized for detecting the regular breathing sounds of a sleeping animal such as a cow.
If there is fur on the skin, the Common Vampire Bat uses its canine and cheek teeth like a barber's shears to clip 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, making it easier to keep them razor sharp.
The batís saliva is a key element in feeding from the wound. While grooves on the underside of the tongue draw blood toward the bat's mouth, the saliva has several ingredients that prolong bleeding. One is an anticoagulant that counters the clotting defenses. A second keeps red blood cells from sticking together and a third inhibits the constriction of veins near the wound.
The stomach lining rapidly absorbs the blood plasma. In turn, the circulatory system shunts the plasma to the kidneys. From there it passes to the bladder and out of the bat. Within two minutes of beginning to feed, a Common Vampire Bat begins to expel highly dilute urine containing the plasma (which has no nutritive value).
While shedding the plasma 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 must generate extra lift which it does by crouching and flinging itself in 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 blood meal. However, when the bat is resting, a new problem is faced. The large protein intake creates excess urea and must also be disposed of. The urinary system of the vampire bat then uses various hormones to make concentrated urine -- consisting of more urea and less water.
Role in the spread of disease
The unique properties of vampire bats do, however, also have some positive value 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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