The
Biogeography of Vampire Bat
(Desmondus
rotundas)
San
Francisco State University Department of Geography
http://bss.sfsu.edu/geog/bholzman/courses/fall99projects/vampire.htm
by
Nikki Michel, student in Geography 316, Fall 1999
Description
of Species
Vampire
bats are very ordinary looking, weighting about
one ounce and with a body the size of an adult's
thumb. They are 2 3/4 inches in length and have
an 8 inch wingspan. Please refer to Figure 1 for
a photograph of Desmondus rotundus. Their colonies
are quite structured with strong social bonds, grooming
each other and recognizing their fellows with voice
and smell. The structure is imperative to their
survival, as there are many nights when a bat may
not find a host to feed on. At that point, the hungry
bats are fed from others through a process of regurgitation.
In the wild, vampire bats live to about 9 years
old, but can reach 20 years in captivity. Vampire
bats mate all year round and usually have only one
offspring per year. Gestation is 6-8 months in length.
Natural
History
Bats
are the only true flying mammals, with about 925 different
species identified. Of all these species, only the common
vampire bat is able to maneuver on the ground as well as in
the air. According to Schutt (1998), vampire bats can move
side to side and backward, similar to a spider. Instead of
taking off in flight from the ground, these bats actually
launch themselves into the air with powerful pectoral muscles.
The force comes from the bat extending its hind knees, leaning
forward and using its forelimbs. The bat also invokes its
triceps muscle and very long thumb. While the jump only takes
about 30 milliseconds, the bat catapults itself about 4 feet
into the air. Altenbach (1979) comments, "although a few other
species of bats move readily on the ground and some take off
from the ground, no other species possess the extreme terrestrial
agility and jumping ability of Desmodus". Once in the air,
the transition into flight is basically one fluid motion.
Since the vampire bat feeds at ground level, their agility
and fast take-off is an amazing advantage.
Feeding
on the blood of animals like cows, pigs, and horses,
the vampire bat requires about two tablespoons of
blood each day. Locating their prey is a combination
of smell, sound, echolocation, and possibly heat
(Altringham 1996). While they do not actually suck
blood from their host, they make a small incision
and lap up the blood. Since they do not chew their
food, they have fewer teeth of any other bat. They
generally approach their prey from the ground. "They
have heat sensors on their noseleaf for locating
capillary-rich areas of the skin; modified canines
for fur clipping; long, sharp incisors for painlessly
opening a wound; anticoagulants to prevent clotting;
and a grooved tongue to help move blood rapidly
to the mouth" (Altringham 1996). While the bat may
consume up to 60% of its body weight in blood and
it only needs the red blood cells, it will begin
excreting plasma before its meal is over. With a
specialized stomach and kidneys, the vampire rapidly
removes the plasma as it may take up to twenty minutes
to the bat to finish its meal (Altringham 1996).
Due to length of time and the invasive nature of
its feeding, it is clear the vampire bat needs its
deftness and agility to be successful. "Observations
of Desmodus scrambling over the backs and necks
of animals prior to feeding (or to avoid movements
of the host animal to brush them off), and running
or hopping about on the ground while feeding, illustrate
the adaptive value of this effective terrestrial
locomotion" (Altenbach 1996).
The
unique social behavior of the vampire is most characteristic
in their reciprocal altruism, in which animals return
favors to their mutual benefit. If vampire bats
do not get their share of blood on a regular basis,
they rapidly deteriorate. A bat may be close to
starvation within 2-3 days (Altringham 1996). Within
social groups which largely refers to females as
the males roost separately to defend territories,
bats that successfully feed will regurgitate back
at the roosts to a hungry bat. Studies on the blood
sharing behaviors indicate bats will regurgitate
to related and unrelated bats within the group.
It is shown that they set up a buddy system, with
pairs of bats forming tight blood-sharing relationships
(Altringham 1996).
Vampire
bats are considered agricultural pests in many parts
of Mexico and Latin America where rainforests have
been cut down to make way for grazing cattle. Here,
control programs are initiated to cut down on health
risks to the cows. According to Anastasia Toufexis
(1995)," due to the fear associated with the vampire
bat, people routinely dynamite and burn caves or
roosts. Unfortunately, people also destroy very
helpful, fruit eating bats that occupy the same
areas. However, the vampire bat is not endangered
at this point.
Evolution
Currently,
bats are in a single order, Chiroptera (hand-winged)
and are thought to have appeared 65-100 myr, in
the late Palaeocene or early Cretaceous (Altringham
1996). They are divided into two subgroups, the
megabats and the microbats. Megabats refer to the
large, fruit-eating bats confined to Africa, tropical
Asia, and Indo-Australia (Altringham 1996). For
our purposes, we will focus on the microbats that
are found on every continent and house our common
vampire bat. The oldest fossil bat dates back 50
myr in the early Eocene period (Altringham 1996).
There is poor representation of bats in the fossil
record; however, some discoveries in Australia of
extensive fossil bat fauna dates back 55 million
years ago (Altringham 1996). Zimmer (1998) explains,
"bats are not good candidates for paleontological
study. When they die, they usually disappear…often
eaten by scavengers; if not, they decompose on the
ground."
Microbats
show no close affinities to any other mammalian
order, so this lack of a link suggests a very early
origin. "There are a number of cases where possibly
closely related microbat species live on once adjacent,
but now distant, fragments of Gondwanaland, the
supercontinent which broke up into fragments which
now make up the land masses of the southern hemisphere"
(Altringham 1996). When bats are thought to have
been evolving, the earth experienced dramatic diversification
of flowering plants that became dominant over primitive
plants of the Crenomanian period, 100-95 myr (Altringham
1996). Insects were also supported by these flowering
plants and insectivorous and frugivorous mammals
must have had to compete. The first bird dates back
to the early Cretaceous, 135 myr, so they were quite
abundant when bats appeared. Consequently, birds
were probably major competitors and/or predators
for early bat species, which may have led to the
nocturnal evolution of these small, nocturnal, tree-dwelling
mammals (Altringham 1996).
Flight
in bats is one of the most fascinating evolutionary
elements and currently has two theories on the table,
the arboreal and the cursorial (Altringham 1996).
The arboreal suggests that tree or cliff dwelling
ancestors evolved flight through a series of gliding
stages. The cursorial theory, which is more recent,
requires the animal take a running leap with wings
outstretched sending it into the air from a glide
to flight (Altringham 1996). With a basically absent
fossil record in regard to flight, there is room
for theory but little evidence.
To
be successful night fliers, bats needed a more sophisticated
system. Echolocation is orientation by analysis
of echoes from sound pulses. Due to the level of
sophistication, it is presumed echolocation evolved
alongside flight (Altringham 1996). Ancestors of
microbats probably emitted ultrasonic sounds in
a simple form of echolocation, which became more
sophisticated as bats became more adept fliers (Altringham
1996).
There
are some recent discoveries in bat evolution that
contradict common bat ancestry. The most widely
accepted theory linked megabats and microbats in
the same evolutionary line. Now, there is a body
of evidence suggesting convergent evolution, where
evolutionary change causes unrelated species with
different histories to acquire striking similarities.
The controversy has megabats, primates and dermopterans
closely related while microbats are independent
(Altringham 1996). Some compelling evidence is the
absence of echolocation in megabats and their limited
distribution to the Old World tropics. Altringham
(1996) expands, "this evidence is very diverse,
from factors as simple as a consideration of body
size ranges, to the analysis of the amino-acid sequence
of hemoglobin, obtained using modern molecular biology
techniques."
Desmondus
Rotundus evolved as singivores, a strange niche
that is unique to bats in the New World. Indeed,
all three members of the Desmodontinae family feed
on blood exclusively. Vampires may have evolved
from feeding on the insects and larvae on the wounds
of large mammals (Altringham 1996).
Distribution
The
common vampire, Desmodus rotundus, is widespread
in the tropical and sub-tropical areas of the Americas.
It occupies rainforests as well as deserts, making
its home in hollows, caves, trees, and even buildings.
Usually, they live in colonies of about one-hundred
bats but can reach up to two-thousand. Preferring
the blood of large mammals, the introduction of
domesticated horses, cattle, and pigs has meant
an increase in its numbers over the last three-hundred
years (Altringham 1996).
Vampire
bats move into subtropical Chile and Argentina,
but they are not excluded from temperate climates.
Indeed, where there is prey, there could be a population
of vampires. It is suggested, then, that the limits
to its distribution is due to the inability to transport
quantities of food sufficient to maintain a high
body temperature in cool to cold places (Kunz 1982).
Other
interesting issues
Central
and South America are alive with folklore about
the vampire bat. Legend has it that bats are filthy,
feeding on human blood or that they have supernatural
powers allowing them to change shapes from man to
bat. While these legends may sound strange, there
is recorded evidence of human hosts. Glover Allen
(1939) talks about bats feeding on humans, "while
travelling down the Amazon valley, he (Dr. William
Farabee) awoke one morning to find that a vampire
during the night had gouged a small piece of skin
from the tip of his nose and had evidently feasted
while he slept, for the wound was still bleeding
slightly" (98).
Not
all tales are negative; bloodletting has traditionally
held healing qualities. For example, the Mexican
monk who came down with a violent fever and was
given a death sentence by morning. But, the next
day, the monk was on his way to recovery. "It seems
that his feet had been left uncovered and that during
the night, a vampire bat had entered the room, which,
having bitten his toe and lapped his blood, had
so reduced the fever that the sick man recovered"
(Allen 1939).
While Western literature has embraced the vampire
bat making it almost cliché, they did not appear
in early vampire myth. The European folklore of
vampires did not incorporate the bat probably because
they did not occur in that region (McKaig 1999).
Some Gypsy folklore involved the vampire bat but
in a rather benign role; sometimes the bat's bones
would be carried in a small bag for luck (McKaig
1999).
Regardless
of the history, vampire bats appear in our modern
context as creepy blood-sucking creatures. They
are represented in Halloween festivities often with
blood dripping from their fangs.
The
most recent representation of our flying mammal
was the motion picture, Bats, where a colony of
bats wreaks havoc on a small Texan town. The trailers
showed a swarm of screaming bats busting through
car windows attacking the unassuming kids seated
inside.
