vampire bat

vampire bat

vampire bat, name for the blood-drinking bats of the family Desmodontidae, found in the New World tropics. Vampire bats feed exclusively on the blood of living animals and are thus the only true parasites among mammals. There are three species ranging from Argentina to N Mexico. They are small (about 3 in./7.5 cm long), round-bodied bats with large, pointed ears and naked snouts. Unlike most bats, vampire bats can walk on all fours with the body lifted off the ground; it is in this manner that they approach their sleeping prey. The bat uses its razor-sharp incisors to make a neat incision, usually without waking the victim, then laps the blood with its tongue. Its saliva contains an anticoagulant that causes the wound to seep for several hours. Vampire bats parasitize a variety of animals, chiefly mammals. Although the quantity of blood they take is insufficent to harm a large animal, they are dangerous to livestock and humans because they transmit serious diseases such as rabies and Chagas's disease. Vampire bats live in caves, tree hollows, and houses. They are mutual groomers, and an effective method of reducing their numbers is to coat a captured bat with a sticky poison and release it; when the bat returns to its roost the poison will be licked by other bats. Members of another bat family, the Megadermatidae, of the Old World tropics, are known as false vampire bats. They are exclusively carnivorous but do not feed on blood. The generic name Vampyrus belongs to a large, fruit-eating bat of Central and South America that was once mistakenly believed to suck blood. True vampire bats are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Chiroptera, family Desmodontidae.

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.

Species

Because of differences between the three species, they have each been placed within a different genus, each consisting of one species. In the older literature, these three genera were placed within a family, Desmodontidae, but taxonomists have now grouped them as a subfamily, the Desmodontinae, in the American leaf-nosed bat family, 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 the three species share a common ancestor.

Anatomy

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.

Feeding

Vampire bats hunt only when it is fully dark. Like fruit-eating bats, and unlike insectivorous and fish-eating bats, they emit only low-energy sound pulses. The common vampire bat feeds mostly on the blood of mammals, whereas both the hairy-legged vampire bat and white-winged vampire bat feed on the blood of birds. Once the common vampire bat locates a host, usually a sleeping mammal, they land and approach it on the ground. They are very agile and a recent study found that common vampire bats can, in addition to walk, run at speeds of up to 7.9 km per hour (4.9 miles per hour). They locate a suitable place to bite using their infrared sensors.

"The commonest species, the South American vampire (Desmodus) is not fastidious and will attack any warm-blooded animal. The white-winged vampire (Diaemus) appears to have a special preference for birds and goats. In the laboratory it has been impossible to feed Diaemus on cattle blood.

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.

Digestion

A typical female vampire bat weighs 40 grams and can consume over 20 grams (1 fluid ounce) of blood in a 20-minute feed. This feeding behaviour is facilitated by its anatomy and physiology for rapid processing and digestion of the blood to enable the animal to take flight soon after the feeding.

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.

Habitats

Vampire bats tend to live in colonies in almost completely dark places, such as caves, old wells, hollow trees, and buildings. Colonies can range from a single individual to thousands, often roosting with other species of bat. They will almost always have only one offspring per breeding season. Each colony will typically have only one reproducing male, with around twenty females and their offspring. Each individual needs a blood meal at least once every few days. If a bat fails to get adequate food during its foraging, it may contact another vampire bat in its colony to induce a food donation. The food exchange occurs mouth-to-mouth in an activity similar to kissing. Vampire bats can live up to nine years in the wild and up to 19 in captivity.

Role in the spread of disease

Vampire bats are commonly infected with the deadly rabies virus which is responsible for the deaths of many thousands of farm animals each year in tropical and sub-tropical America. Humans bitten by these bats can likewise become infected with rabies and such infections are invariably fatal if left untreated.

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.

Footnotes

References

  • Greenhall, Arthur M. 1961. Bats in Agriculture. A Ministry of Agriculture Publication. Trinidad and Tobago.
  • Greenhall, Arthur M. 1965. The Feeding Habits of Trinidad Vampire Bats.
  • Greenhall, A., G. Joermann, U. Schmidt, M. Seidel. 1983. Mammalian Species: Desmodus rotundus. American Society of Mammalogists, 202: 1-6.
  • A.M. Greenhall and U. Schmidt, editors. 1988. Natural History of Vampire Bats, CRC Press, Boca Raton, Florida. ISBN 0849367506; ISBN 978-0849367502
  • Riskin, Daniel K. and John W. Hermanson. 2005. Biomechanics: Independent evolution of running in vampire bats. Nature 434: 292-292. Abstract, video.
  • Kishida R, Goris RC, Terashima S, Dubbeldam JL. (1984) A suspected infrared-recipient nucleus in the brainstem of the vampire bat, Desmodus rotundus. Brain Res. 322:351-5.
  • Campbell A, Naik RR, Sowards L, Stone MO. (2002) Biological infrared imaging and sensing. Micron 33:211-225. pdf.

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