8. Only 0.5% of bats carry rabies. Although most bats do not have rabies, those that do may be clumsy, disoriented, and unable to fly, which makes them more likely to come into contact with humans.
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 precious 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 California,
places where they no longer occur. With the exception of captive animals,
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. Today 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
three 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 behavior, 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 mightier 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 capillaries 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
manufactured 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 behavior remain unknown. However, 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. 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 ingredients that promote 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. 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 return 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 donor
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 non-relatives. 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)
Copyright © 1999 by Helena Goscilo ([email protected]) and Petre
Petrov ([email protected]).All
rights reserved.
