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Vampire bats feed only on blood, a fact that sets the human imagination racing. The three species are medium-seized, with adult wingspans of 320 to 350 mm and weights of about 40 grams. While they are larger than most of the bats of the temperate zones of the world, vampire bats are much smaller than Gigantic Flying Foxes, which weigh in at 1,500 g. Horror movie depictions of vampire bats often use flying foxes as models because larger bats are easier to photograph. This contributes to the public perception of vampire bats as large, terrifying animals. But animals that feed on blood, whether they are insects, leeches or bats, tend to be small because blood is a precioous commodity and hard to obtain in large amounts.

Vampire bats are members of the New World Leaf-nosed bats, the Phyllostomidae. All three species of living vampire bats occur in South and Central America. Fossils of three other species reveal that several thousand years ago vampire bats were more widespread. They are known from Cuba and from as far north as West Virginia and the northern Califfornia, places where they no longer occur. With the exception of captive amimals, vampire bats have never been found outside of the New World.

People are often surprised to learn that vampire bats are not found in central Europe. It seems common to suppose that human myths about vampires and stories about Dracula somehow involve vampire bats. The truth is that vampire bats got their names from human myths about vampires. In many human cultures, vampires are people who return from the dead to feed on the blood of living people. After the bats were discovered by European explorers, they were given the name vampire, denoting blood-feeding. The blood-feeding were well known to many human inhabitants of South and Central America well before their discovery by Europeans. Bram Stoker, intrigued by the publicity surrounding bats that fed on blood, included bats in his book Dracula.

Naturalists returned from South and Central America with many fascinating plants and animals. Among the collections were many bats, which were described and given scientific names. Tooday in South and Central America bats in several genera have names reflecting an earlier preoccupation with vampires. There are bats in genera called Vampyrum, Vampyressa, Vampyrodes and Vampyrops. Linnaeus' False Vampire Bat (Vampyrum spectrum) is a versatile predator that takes a variety of prey. True, whether it eats a bat, mouse or bird it gets some blood, but Vampyrum spectrum is neither a vampire nor a blood-feeder. The others, Vampyressa, Vampyrodes, and Vampyrops, are fruit-eating bats that occasionally may take an insect. Together these threee genera house about 11 species but none of them is blood-feeder.

Vampire bats are among the most fascination of mammals, although we know relatively little about the details of their lives. Common Vampire Bats, as their name implies, are the most widespread of vampires. They adapt well to captivity and often are exhibited in zoos. These bats may be among the best studied in the world. Hairy-legged Vampire Bats and White-winged Vampire Bats are less common and are less often caught by biologists. While Common Vampire Bats may feed on the blood of mammals or birds, the White-winged Vampire Bat and the Hairy-legged Vampire Bat are thought to prefer bird blood.

Origins
Biologists have argued for years about whether vampire bats should be housed in their own family (the Desmodontidae) or included among the New World Leaf-nosed Bats (the Phyllostomidae). People on both sides of this argument agree that the New World Leaf-nosed Bats are the vampire bats' closest relatives. Vampire bats are highly specialized for feeding on blood. This situation raises several questions: How did blood-feeding arise in bats? Why is it restricted to the New World tropics? When did it appear?

Three theories account for the origin of vampire bats. The first proposes that vampire bats originated from fruit-eating bats. This theory suggests that large, strong upper incisor teeth would make fruit bats well suited to switching to blood. This theory does not explain why blood-feeding did not also appear among the Old World Fruit Bats, the Pteropodidae.

The second theory suggests that the ancestors of vampire bats acquired a taste for blood by feeding on ticks and other blood-feeding ectoparasites of large mammals. Today in Africa, birds known as Ox-peckers make their living by feeding on ticks. It appears to be a viable life-style. But as the Ox-peckers show, both ticks and blood-feeding ectoparasites and large mammals occur in Africa. Again we are left with the question, why did blood-feeding bats not appear in the Old World tropics?

The third theory proposes that the ancestors of vampire bats began to feed on insects and insect larvae they found in wounds on large animals. This theory notes that insectivorous bats often feed where there are many insects and some of them adjust their hunting style according to the situation. Throughout the tropics, flies known as screwworms lay their eggs in wounds and their larvae develop into large masses. This theory identifies strong, sharp upper incisor teeth as the key to why blood-feeding only appeared in New World bats. Many New World Leaf-nosed Bats have large, strong upper incisors. These teeth are lacking from those Old World bats with flexible foraging behaviour, namely the Slit-faced Bats and the False Vampire Bats.

None of these theories about the origins of vampire bats has been proved. Evidence from proteins suggests that vampire bats have been around for 6 to 8 million years. These are the dates when the Hairy-legged Vampire Bats separated from the White-winged Vampire Bats and the Common Vampire Bats.

Blood as Food
Even though living off of blood is a challenge, it is the metier of many species of animals from different evolutionary backgrounds. In many ways blood is an ideal food. Within the bodies of animals with backbones, the vertebrates, blood is the central ingredient of the circulatory system. It flows in vessels, dispensing food and oxygen through the body and collecting wastes for removal. Blood consists of water-like plasma, chemicals, red blood cells and white blood cells. The blood cells represent the nutritious part of blood. Keeping blood in vessels, arteries, veins or capollaries ensures that it flows more efficiently. The systems that protect blood illustrate its importance to animals. A variety of mechanisms such as clotting and constriction of veins, keep an animal from bleeding to death through a small wound. Blood is constantly manufacutured in the body, so it is a renewable resource and a complete food.

In mammals and birds, which are thought to be the usual prey of vampire bats, blood amounts to 6-10% of the animal's weight. This means that a 100-kilogram person (220 pounds) would have no more than 10 kilograms of blood, or a 1,000-kilogram moose would have 100 kilograms of blood. A 450-gram (1-pound) rat would have no more than 45 g of blood, and the vampire bats themselves have only 4 g of blood.

Each vampire bat, whatever the species, needs about two tablespoonful of blood every day. This represents about 60% of the bat's body weight, or 20 g of blood. The bats extract this blood through a wound they make with their front (incisor) teeth. The wounds are approximately 5 mm deep and 5 mm in diameter and do not cut arteries or veins. If you made a wound this size on your body, it would produce about one drop of blood or less than a gram. It appears that vampire bats are "one stop shoppers," feeding on one victim each night. Getting 20 g of blood from a wound that normally produces just one drop is a specialized business.

When you remember how much blood is available in different sized mammals and birds, it is obvious that one stop shopping for vampire bats will only work with large prey. The availability of large prey and the difficulty of obtaining large amounts of blood probably explains why vampire bats are no bigger than 40 g. Fossil species that probably weighed about 60 g may have had more large mammals and birds to tap.

Let's follow the process as a Common Vampire Bat sets out for a night's foraging.

The Hunt
Common Vampire Bats usually roost in hollows, so they may be found in caves, hollow trees and buildings. The bat leaves its roost just after dusk, setting out to find a meal. The relatively long and narrow wings--providing a high aspect ration on high wing loading--make it possible for the Common Vampire Bat to fly rapidly and consume relatively little energy in doing so.
Radio-tracking studies suggest that a foraging Common Vampire Bat returns to a general area where it has found prey before. Having reached its foraging area, the bat must find and select a victim. The fine details of its search and selection behaviour remain unknown. However, the inferior coliculus, part of the bat's brain thatt processes sound, is specialized for detecting the regular breathing sounds of a sleeping animal such as a cow. The bat lands on the ground near its intended victim and approaches on foot. Among bats, Common Vampires are the most agile on the ground, hopping about like ballet dancers.

There is a heat (infrared) sensor on the nose-leaf of Common Vampire Bats, permitting them to locate an area where the blood flows close to the skin. 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 are then used to make a quick cut, leaving the 5-mm wound described above. The upper incisors lack enamel, making it easier to keep them razor sharp.

The bat then begins to use its tongue in the wound as well as its saliva. The action of he tongue keeps blood flowing, while grooves on the underside of the tongue draw blood toward the bat's mouth. Meanwhile, the saliva has at least three active ingredientts that promote bleeding. One is an anticoagulant that counters the clotting defences. A second keeps red blood cells from sticking together and a third inhibits the constriction of veins near the wound. It may take the bat about 20 minutes to fill its tank; then it is time to take off and return to its roost.

The tank is the bat's stomach, and its 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 2 minutes of beginning to feed, a Common Vampire Bat begins to urinate. The urine is very dilute--no wonder, it is the plasma from that blood meal. The plasma is heavy but contains no nutritive value, so the bat benefits from leaving it behind.

Shedding the plasma makes taking off from the ground easier. But the bat still has added almost 60% of its body weight in blood. To take off from the ground the bat must generate lots of lift. Common Vampire Bats have very long thumbs. As the bat prepares to take off it crouches close to the ground and then, by contracting its chest muscles, flings itself skyward. The thumbs provide extra leverage for takeoff. Usually 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.

Risks and Social Support
Studies of Common Vampire Bats in Costa Rica indicate that it is risky to depend upon blood as the only source of food. Each adult Common Vampire Bat has a 7% chance of not feeding on any night. This means that about once every 25 nights the bat will miss its meal and go hungry. Common Vampire Bats cannot survive two nights without meal, so fasting is dangerous. For young bats, the risk of not finding food is higher, 33%. So, twice a week each young Common Vampire Bat will probably miss a meal.

Other blood-feeding animals such as ticks, insects and leeches do not face the same problem as vampire bats because they can go for weeks, months or even years without a meal. Vampire bats, however, are warm-blooded, and the cost of staying warm means that fasting is soon fatal. The costs of keeping warm account for the absence of vampire bats from cooler parts of North, Central and South America.

Common Vampire Bats live together in structured societies that provide a network of social support. Like other bats, Common Vampires are long-lived. Banding studies suggest that some survive almost 20 years in the wild. Banding studies also reveal that individuals remain in their roosting groups for at least three years and probably for their entire lives. Colonies of Common Vampire Bats usually include one adult male with several females and their young. The bats may not all roost together at any one time, for they move between several roosts within the home range of the colony. Females that roost together often forage in the same general area, and several bats may line up, feeding in succession at a wound.

When a Common Vampire Bats rerturn to the roost, they often meet face-to-face and groom one another. A bat that fails to feed uses this face-to-face contact to beg blood from a roostmate. The successful bat may then regurgitate some blood to the unsuccessful one. The cost of the doner is relatively small, particularly since before the month is out it will need a donation itself. The benefit to the receiver is high, for it is survival.

Genetic analyses have revealed that colonies of Common Vampire Bats are mixtures of relatives and nonrelatives. This means that the social support provided by the colony transcends the business of helping relatives. Cooperation may be one of the central keys to the success of Common Vampire Bats. We would expect that Common Vampire Bats that are not part of the colony would not get blood from the members of the group. Giving blood appears to depend upon the prospect of a donation in return.

Blood-feeding is a risky business, particularly for a warm-blooded animal. Among bats, the vampires are exceptional because they spend so much time caring for their young. Young Common Vampires nurse for up to nine months, three months longer than flying foxes, which are many times their size, and at least six months longer than most other bats. Female Common Vampire Bats show no seasonal pattern of reproduction. But even though they may have young at any time of the year, the long period of nursing means that each female produces just one young a year.

Whether you consider their anatomical or physiological specializations or their amazing social structure, the vampires are among the most exciting of bats.

(Fenton, Brock M. Bats. Oxford and NY: Facts on File, 1992. pp.149-55)