Contents
BAT,
the only mammal capable of sustained flight. Bats make up
a unique group, the order Chiroptera (Gr. cheir, “hand”; pteron “wing”).
It is divided into 2 suborders, Megachiroptera (“larger
bats” or “megabats”) and Microchiroptera
(“smaller bats” or “microbats”),
and 17 families. One of these families includes all megabats (more
than 150 species); the other 16 are microbats. In all, 850 to 900
species of bats exist, far more than in any other mammalian order
except Rodentia, and bats exceed even rodents in total abundance.
Bats occur worldwide, except in the Arctic Zone and certain
remote oceanic islands. All megabats and some families of microbats
are confined to the eastern hemisphere; other microbats are confined
to the western hemisphere, and a few families are worldwide in distribution.
Bats are believed to have originated in a warm climate, probably
in the early Eocene geologic epoch (the oldest known fossil bat, Icaronycteris
index, is about 60 million years old), and the center of
their distribution and abundance remains the tropical and subtropical
areas. Members of only four families, all of them microbats, live
in temperate regions, and the number of resident species diminishes
poleward. Members of only two families reach subarctic regions,
and then only in the summer.
The megabats include the largest bats, the giant “flying
foxes” of Africa, India, and Australo-Malaysia. They are
so called because of their long, doglike muzzles. The biggest, Pteropus vampyrus of
Java, achieves a wingspan of 1.4 m (nearly 5 ft) and a body length
of 42 cm (16.7 in). Many species of megabats are smaller than the
largest microbats, however, the distinction between the suborders being
based on tooth form and major skeletal differences rather than size
alone. The smallest microbat, Pipistrellus nanus of
Central Africa, is only 4 cm (1.5 in) long, has a wingspan of only
12.5 cm (about 5 in), and weighs less than 4 g (0.14 oz), making
it one of the tiniest of mammals. The differences between microbats
and megabats are sufficient, in fact, to suggest that they are of
different evolutionary origin, with the megabats appearing much
more recently than the microbats.
Among living vertebrates, true flight is unique to bats and birds.
Unlike most birds, however, bats are able to fly at relatively low
speeds with extreme maneuverability. The wing is a thin, fleshy
membrane supported near its leading edge by the greatly elongated
bones of the forelimb and second finger, and toward the tip and
rear by the even more attenuated third, fourth, and fifth fingers.
It is attached along the midline of the trunk and outward-directed
legs, and in various species it extends between legs and tail. Only
the first finger, or thumb, is free, and in most bats it alone is
clawed, together with the toes. This structure enables bats to vary
dramatically the convexity of the wings and thus their aerodynamic
lift.
All microbats navigate—and most insectivorous species
also target their prey—by echolocation. This is the pulsed
emission of high-frequency sounds that are reflected back as echos
to a bat’s ears from surrounding surfaces, indicating the
position, relative distance, and even the character of objects in
its environment. In this sense microbats “see” acoustically.
This is the basis for their ability to navigate in total darkness.
The physical properties of the emitted sounds vary in characteristic
ways among species. The sound pulses are generated in the larynx,
and in different species are emitted either from the mouth or nostrils.
By contrast, megabats use vision rather than acoustics for
orientation. Only one genus (Rousettus of Africa,
the Middle East, and Asia) has evolved an echolocation mechanism,
involving the emission of audible “clicks,” and
it is used only when the bats fly in darkness. The eyes of megabats
are also relatively larger than those of microbats. No bat is blind,
however, and even echolocating microbats may use gross visual landmarks
for homing during flight.
With a few exceptions, such as Saccopteryx of
the western hemisphere and the African Lavia, all
microbats are nocturnal. During the day they may rest in a variety
of roosting places, such as caves, crevices, hollow trees, foliage,
hiding places beneath rocks or bark, and in buildings. They may even
roost in exposed situations; certain larger megabats hang upside
down in enormous aggregations from tree branches, like so many gourds.
Nocturnality gives bats many advantages, such as greatly reduced competition
for insects and other food items, substantial freedom from attack,
and protection from overheating and dehydration, to which bats are
especially liable because of their enormous skin area relative to
their size.
A few species of bats live solitarily, such as Lasiurus
cinereus of North America, but most are gregarious. Aggregation
during the day may vary from small groups consisting of a single
male and a dozen or more females to enormous assemblages numbering
many thousands or even millions of individuals (for example, Tadarida
brasiliensis of the southwestern U.S.). Aggregations of
members of specific species may show seasonal variation and sexual
segregation in varying combinations. Mixed-species associations
of a casual sort are common among bats using protected shelters
such as caves.
Certain species of Temperate Zone bats are migratory to some
degree, and movements of nearly 1600 km (about 1000 mi) between
summer and winter quarters have been recorded in Tadarida
brasiliensis. Others may fly only a few or up to 40 km
(about 25 mi) or more daily between roosting and feeding sites,
but the majority forage within more restricted home ranges.
Most bats are insectivorous and are able to hawk their prey
on the wing or to seek out stationary insects on foliage or other
surfaces. Most megabats, and many species of leaf-nosed bats of
tropical America (so named for the remarkable folds of skin projecting
upward from the nose), are fruit eaters. Still others in both groups
consume flower parts or extract the nectar from flowering plants
by means of greatly elongated tongues, aiding cross-pollination
of the plants in the process. Some of the larger leaf-nosed bats
as well as members of the Old World family Megadermatidae are carnivorous
or omnivorous; they attack small amphibians, lizards, birds, mice,
and even other bats, in addition to consuming insects and fruit.
Closely related to the leaf-nosed bats are the three genera of true
vampires of the American Tropics (Desmodus, Diphylla, and Diaemus),
which subsist entirely on blood freshly drawn from small wounds
inflicted on mostly warm-blooded prey such as fowl, cattle, horses,
swine, and occasionally human beings. At least three species of
bats supplement their diets with small fish, which are caught as
the flying bats drag their enlarged feet and claws just beneath
the water surface.
The gestation periods of bats are relatively long, ranging
from about 44 days to 8 months in various species. Few produce more
than a single offspring each year, and the young tend to mature
slowly. The reproductive cycles of most species follow general mammalian
patterns, but exceptions exist. Most interesting are Temperate Zone
species such as Myotis, Rhinolophus, and Eptesicus, which hibernate
during the winter months. Copulation occurs prior to hibernation,
and sperm are retained within the females throughout the hibernation
period. Fertilization occurs when the ovum is released from the
ovary after the bats arouse in the spring. This is called delayed
fertilization. A variant of this pattern is observed in the European
genus Miniopterus, in which copulation, ovulation,
and early development of the embryo all occur in normal sequence
immediately prior to hibernation, but the embryo experiences developmental arrest
before uterine attachment. It remains free and undeveloped until
after the torpid mother resumes normal metabolism in spring. This
is called delayed implantation.
The abundance of bats despite their low individual reproductive
performances is attributable not only to the survival value of their
habits but also to their remarkable longevity. Some larger species
of megabats (Pteropus) and the smaller vampire
bat (Desmodus) have survived in zoos for 20 years.
Among various species of microbats banded and released in the wild,
many have been recaptured after years of freedom. The record is
a specimen of Myotis lucifugus recovered 31 years
after it was initially marked and released in New England.
Insect-eating bats can be considered generally helpful to
humans, and many bat species play an important role in plant pollination
and seed dispersal. Because of their size and numbers, the larger
fruit bats of the Old World can be an economic menace when they
invade fruit orchards, but perhaps the greatest adverse effect of
bats is the transmission of disease, especially rabies, to domesticated animals.
This is a particular problem in the American Tropics, where local
outbreaks of rabies among cattle, stemming from the bites of infected
vampire bats, have decimated whole herds. Bats other than vampires
can also contract and transmit rabies; most species of bats inhabiting
the U.S. and Canada have been reported at one time and place or
another to be infected. The prevalence of infection, however, has
been low, outbreaks seldom, and unprovoked attacks on humans rare.
Many bats have low tolerance for certain insecticides such
as DDT, which they can concentrate through the food chain. Some
populations in the U.S. may have been adversely affected, inasmuch
as three species on the list of endangered species have suffered
from the agricultural use of pesticides.