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|Common vampire bat, Desmodus rotundus|
|Common vampire bat, Desmodus rotundus|
Vampire bats are bats whose food source is blood, a dietary trait called hematophagy. Three bat species 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 New World, ranging from Mexico to Brazil, Chile, and Argentina.
Due to differences among the three species, each has been placed within a different genus, each consisting of one species. In the older literature, these three genera were placed within a family of their own, Desmodontidae, but taxonomists have now grouped them as a subfamily, the Desmodontinae, in the American leaf-nosed bat family, Phyllostomidae.
The three known species of vampire bats all seem more similar to one another than to any other species. That suggests that sanguivorous habits (feeding on blood) evolved only once, and 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, Desmodus rotundus, also has specialized thermoreceptors on its nose, which aid 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.
A vampire bat generally has small ears and a short tail. Its front teeth are specialized for cutting and the back teeth are much smaller than in other bats. The inferior colliculus, the part of the bat's brain that processes sound, is well adapted to detecting the regular breathing sounds of sleeping animals that serve as its main food source.
While other bats have almost lost the ability to maneuver on land, vampire bats can also run by using a unique, bounding gait, in which the forelimbs instead of the hindlimbs are recruited for force production, as the wings are much more powerful than the legs. This ability to run seems to have evolved independently within the bat lineage.
Vampire bats use infrared radiation to locate blood hotspots on their prey. A recent study has shown that common vampire bats tune a TRP-channel that is already heat-sensitive, TRPV1, by lowering its thermal activation threshold to about 30 °C. This is achieved through alternative splicing of TRPV1 transcripts to produce a channel with a truncated carboxy-terminal cytoplasmic domain. These splicing events occur exclusively in trigeminal ganglia, and not in dorsal root ganglia, thereby maintaining a role for TRPV1 as a detector of noxious heat in somatic afferents. The only other vertebrates capable of detecting infrared radiation are boas, pythons and pit vipers, all of which have pit organs.
Vampire bats tend to live in colonies in almost completely dark places, such as caves, old wells, hollow trees, and buildings. They range in Central to South America and live in arid to humid, tropical and subtropical areas. Vampire bat colony numbers can range in the thousands in roosting sites. The basic social structure of roosting bats is made of harems, which are composed of females and their offspring and a few adult males, known as "resident males" and a separate group of males, known as "nonresident males". In hairy-legged vampire bats, the hierarchical segregation of nonresident males is less strict than in common vampire bats. Nonresident males are accepted into the harems when the ambient temperature lowers. This behavior suggests social thermoregulation.
Resident males mate with the females in their harems, but it is common for outside males to copulate with the females. Female offspring usually remain in their natal groups unless their mothers die or move. Several matrilines can be found in a group, as unrelated females regularly join groups. Male offspring tend to live in their natal groups until they are about two years old, sometimes being forcefully expelled by the resident adult males.
Vampire bats are believed to be the only species of bats in the world to "adopt" another young bat if something happens to the bat's mother. Vampire bats also share a strong family bond with members of the colony, which is believed to be why they are the only bats to take up this adoption characteristic. Another unique adaptation of vampire bats is the sharing of food. A vampire bat can only survive about two days without a meal of blood, yet they cannot be guaranteed of finding food every night. This poses a problem, so when a bat fails to find food, it will often "beg" another bat for food. The "host" bat may regurgitate a small amount of blood to sustain the other member of the colony. This has been noted by many naturalists as an example of reciprocal altruism in nature. It was previously thought that food sharing depended equally on relatedness and reciprocation. However, it has recently been discovered that the predictive capacity of reciprocity surpasses that of relatedness. This finding suggests that vampire bats are capable of preferentially aiding their relatives, but that they may benefit more from forming reciprocal, cooperative relationships with relatives and non-relatives alike. Furthermore, a recent study demonstrated that donor bats were more likely to approach starving bats and initiate the food sharing. These findings contradict the harassment hypothesis—which claims that individuals share food in order to limit harassment by begging individuals. All considered, vampire bat research should be interpreted cautiously as much of the evidence is correlational and still requires further testing. For example, researchers question vampire bats’ ability to identify kin when past association, or interaction, is controlled. Similarly, scientists question if bats modify investments based on how other bats cooperate.
Another ability that some vampire bats possess is identifying and monitoring the positions of conspecifics (individuals of the same species) simply by antiphonal calling.
Vampire bats also engage in social grooming. It usually occurs between females and their offspring, but it is also significant between adult females. Social grooming is mostly associated with food sharing.
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 (occasionally including humans), 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, such as a sleeping mammal, it lands and approaches it on the ground. It then likely uses thermoception to identify a warm spot on the skin to bite. They then create a small incision with their teeth and lap up blood from the wound.
As noted by Arthur M. Greenhall:
|“||The most common species, the common 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 not been possible 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 wide and 8mm deep cut. The upper incisors lack enamel, which keeps them permanently razor sharp.
The bat’s saliva, left in the victim's resulting bite wound, 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.
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, and on to the bladder for excretion. A common vampire bat begins to expel urine within two minutes of feeding. While shedding much of the blood's liquid facilitates flight takeoff, 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 in search of food, the common vampire bat returns to its roost and settles down to spend the rest of the night digesting its meal.
Although rare, infection of humans by rabies from vampire bat bites has been documented; for example in 2010 four children in Peru died after being bitten. The highest occurrence of rabies in vampire bats occurs in the large populations found in South America. However, the risk of infection to the human population is less than to livestock exposed to bat bites. Only 0.5% of bats carry rabies, and those that do may be clumsy, disoriented, and unable to fly.
The unique properties of the vampire bats' saliva have found some positive use in medicine. A study 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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