Species Account: Common Vampire Bat,
Bats are classified as members of the Chiroptera order, which is often divided into two suborders: Megachiroptera (also known as Megabats) and Microchiroptera (also known as Microbats). American leaf-nosed bats are considered Microbats in the family Phyllostomidae. Further divisions are made: vampire bats are part of the genus Desmodus under the subfamily Desmodontinae. The scientific name for the common vampire bat, to which this paper is dedicated, is Desmodus rotundus. There are two other species in the Desmodus genus, but the common vampire bat is the most well-known and the most intriguing (Walker 1975).
D. rotundus is not a very large creature. A vampire bat could easily fit in the palm of your hand; it reaches a length from head to backside of 70-90 mm and weighs between 15 and 50 grams. The species is unique for several reasons. It has no tail and unusual dental features: the bat has only 20 teeth, two of which are sharpened incisors (Sedgwick County Zoo). The bats are several shades of brown, ranging from dark and grayish on the top to a much paler underside.
Vampire bats are New World bats. They thrive in climates that never reach extremely low temperatures; they can be found in the Americas from Mexico down to Chile and Argentina (Turner 1975). In many of these areas, they are extremely unwanted. This is perhaps most evident in some regions of Mexico where vampire bats and other bats caused an epidemic of cattle-killing rabies. In addition, vampire bats have also fed on some Mexican citizens; 45 people reported bat bites in the first three months of 2002 (Stevenson 2002).
Vampire bats generally inhabit caves, but can also flourish in hollow trees, wells, mine shafts, and abandoned buildings. The constant temperature and high humidity of these environments suit the vampire bats nicely (Wilkinson 1990). Bats tend to live in transient clusters, generally ranging from 20 to 100 individuals (although groups as large as 5,000 have been reported). Groups of females dominate the organizational structure of the habitats. Groups often move from tree to tree (or some other habitat) while remaining fairly intact. However, females can switch groups as well; a new female may join a group every couple years or so.
Grooming is a fairly prominent part of roost life. Self-grooming is the most common type, but social grooming also plays a major role. Social grooming generally occurs between a female and her offspring or two adult females, and can be a food-sharing method as well as a cleansing method (Wilkinson 1986).
Although humans are at times prey for vampire bats, their primary diet does not include humans. Vampire bats, like the myth, feed on blood. Other vampire bats have been known to feed on the blood of birds, but the common vampire bat has developed a preference for mammal blood (i.e. horses and cows primarily). Scientists often wonder why the thirst for blood only developed in the New World. If blood is such a good source of nutrition, why didnšt European bats develop a similar thirst? Fenton (2003) hypothesized that vampire bats initially sought out insects that were feeding on the wounds of large animals. During the proposed time period, large mammals could be found in greater numbers in the New World, leading to increased competition and therefore an increased number of wounded animals. Ergo, European bats did not have as abundant of a source to develop their taste for blood; it is a trait that only New World bats had the ability to develop. Over time, these New World bats must have decided the blood of the larger animal was a better meal than the insects themselves (Timson 1993).
Another interesting fact about the feeding patterns of the vampire bat is their lack of taste aversion, a common trait among animals. Taste aversion is the ability to avoid eating a type of food if it results in gastrointestinal malaise or more simply put, it makes the animal sick (Fenton 2003). Scientists experimented with four species of bats, three of which donšt feed on blood. Additional flavor was added to their food before toxins were injected to make the bats ill, as well. The three other species of bats learned to stay away from the cinnamon flavored food, while the vampire bat continued to ingest the citrus injected blood. Since taste aversion is a trait among all or nearly all other bats, scientists concluded the trait was lost by vampire bats. One of the scientists, John Ratcliffe, theorized that vampire bats got rid of this trait, because drinking blood probably initially made the bats vomit. Removal of this trait would allow the bats to take in increasing quantities of blood. Others, such as Laurence Nolan of Wagner College, disagree with this hypothesis. The vampire bats are unlikely to encounter toxins in blood that will make them ill, because those toxins would probably kill the mammals before bats could get to them. Therefore, Nolan and others have concluded this trait was lost as a result of it not being used (Fields 2003).
The common vampire bat has multiple means of motion. Flying is the usual method of travel employed by bats. However, D. rotundus has also developed the ability to walk and hop. These two traits have become as useful as they are unique; starting from a horizontal surface, the bats hop or leap into the air to initiate flight (Altenbach 1997). They maintain this flight at approximately three feet off the ground most of the time.
D. rotundus, after selecting a prey, jumps or flies to a spot near the victim. It moves nimbly, climbing the animal and making a small incision, from which it drinks up the blood.
Vampire bats have a lifespan of approximately 9 years in the wild, and have been know to live for as many as 20 years in captivity. They reach sexual maturity 9 or 10 months after birth.
The reproductive cycle of bats is a fairly constant process. Adult females give birth to a new pup about every ten months. Gestation length is about 7 months, a considerable amount of time for an animal of this size. Gestation is sometimes shorter, but in general females have roughly three months to focus on their newborn pup before conceiving another one (Sedgwick County Zoo). These newborns feed primarily on milk and regurgitated blood from their mother or some other adult.
D. rotundus is sexually active throughout the year, so young may be born at any time of the year. However, peak times for birth are before and after summer (April-May and October-November).
Blood regurgitation between a female bat and pups, whether their own pups or not, is a primary element of the self-preservation of vampire bats. Blood is vital if a vampire bat is to stay alive. Experiments have shown that when one of these bats is forced to fast, it is unable to maintain proper blood glucose levels, making death a likelihood (Freitas 2003). In fact, a bat will starve to death in sixty hours without the proper intake of blood. Thus, the vampire bats have developed the instinct to share blood by regurgitating it. This is commonly referred to as reciprocal altruism, because studies indicate donors often share a relationship with the recipient in which the donor can expect the favor to be returned when it is in similar trouble. These studies also suggest that blood regurgitation can be based on kinship as well (Wilkinson 1990). Most early experiments focused on reciprocity between adult females (or occasionally males) and pups, whether they are related or not. However, more recent experiments expand the bounds of this reciprocity, saying it can be witnessed between two adult males, two adult females, or one of each (DeNault 1995). Since this blood regurgitation increases the life span of struggling bats and does not significantly harm the life span of donor bats, it is perhaps the most important factor keeping the species in existence.
The common vampire bat is not endangered. In fact, the population of this species has experienced such a boom that they are now considered pests throughout the New World. This population explosion was caused primarily by the introduction of livestock in South America (Turner 1975). However, the vampire bat is still one of the bats that the Organization of Bat Conservation has targeted (Tomlinson 2004). The groupšs goal is to conserve bats through education and research.
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March 17, 2004