Bats are
mammals of the order
Chiroptera (
//; from the
Greek χείρ -
cheir, "hand"
[2] and πτερόν -
pteron, "wing"
[3]) whose forelimbs form webbed
wings, making them the only mammals naturally capable of true and sustained
flight. By contrast, other mammals said to fly, such as
flying squirrels,
gliding possums, and
colugos, can only
glide for short distances. Bats do not flap their entire forelimbs, as
birds do, but instead flap their spread-out
digits,
[4] which are very long and covered with a thin
membrane or
patagium.
Bats represent about 20% of all classified mammal species worldwide,
with about 1,240 bat species divided into two suborders: the less
specialized and largely fruit-eating
megabats, or flying foxes, and the highly specialized and
echolocating microbats.
[5] About 70% of bats are
insectivores. Most of the rest are
frugivores, or fruit eaters. A few species, such as the
fish-eating bat, feed from animals other than insects, with the
vampire bats being
hematophagous.
Bats are present throughout most of the world, performing vital ecological roles of
pollinating flowers
and dispersing fruit seeds. Many tropical plant species depend entirely
on bats for the distribution of their seeds. Bats are important, as
they consume insect pests, reducing the need for pesticides. The
smallest bat is the
Kitti's hog-nosed bat, measuring 29–34 mm (1.14–1.34 in) in length, 15 cm (5.91 in) across the wings and 2–2.6 g (0.07–0.09 oz) in mass.
[6][7] It is also arguably the smallest extant species of mammal, with the
Etruscan shrew being the other contender.
[8] The largest species of bat are a
few species of Pteropus and the
giant golden-crowned flying fox with a weight up to 1.6 kg (4 lb) and wingspan up to 1.7 m (5 ft 7 in).
[9]
Classification and evolution
Bats are
mammals. In many languages, the word for "bat" is cognate with the word for "mouse": for example,
chauve-souris ("bald-mouse") in French,
murciélago ("blind mouse") in Spanish,
saguzahar ("old mouse") in Basque, летучая мышь ("flying mouse") in Russian,
slijepi miš ("blind mouse") in Bosnian,
nahkhiir ("leather mouse") in Estonian,
vlermuis (winged mouse) in Afrikaans, from the Dutch word
vleermuis. An older English name for bats is
flittermice, which matches their name in other
Germanic languages (for example
German Fledermaus and
Swedish fladdermus).
[10] Bats were formerly thought to have been most closely related to the
flying lemurs,
treeshrews, and
primates,
[11] but recent molecular cladistics research indicates they actually belong to
Laurasiatheria, a diverse group also containing
Carnivora and
Artiodactyla.
[12][13]
The two traditionally recognized suborders of bats are:
Not all megabats are larger than microbats. The major distinctions between the two suborders are:
- Microbats use echolocation; with the exception of Rousettus and its relatives, megabats do not.
- Microbats lack the claw at the second toe of the forelimb.
- The ears of microbats do not close to form a ring; the edges are separated from each other at the base of the ear.
- Microbats lack underfur; they are either naked or have guard hairs.
Megabats eat fruit,
nectar, or
pollen, while most microbats eat
insects; others may feed on the
blood of animals, small mammals, fish, frogs, fruit, pollen, or nectar. Megabats have well-developed
visual cortices and show good
visual acuity, while microbats rely on
echolocation for navigation and finding prey.
The phylogenetic relationships of the different groups of bats have
been the subject of much debate. The traditional subdivision between
Megachiroptera and Microchiroptera reflects the view that these groups
of bats have evolved independently of each other for a long time, from a
common ancestor
already capable of flight. This hypothesis recognized differences
between microbats and megabats and acknowledged that flight has only
evolved once in mammals. Most molecular biological evidence supports the
view that bats form a single or monophyletic group.
[14]
Researchers have proposed alternative views of chiropteran phylogeny and
classification, but more research is needed.
In the 1980s, a hypothesis based on
morphological evidence was offered that stated the Megachiroptera evolved flight separately from the Microchiroptera. The so-called
flying primates theory proposes that, when adaptations to flight are removed, the Megachiroptera are allied to
primates
by anatomical features not shared with Microchiroptera. One example is
that the brains of megabats show a number of advanced characteristics
that link them to primates. Although recent genetic studies strongly
support the monophyly of bats,
[15] debate continues as to the meaning of available genetic and morphological evidence.
[16]
Genetic evidence indicates megabats originated during the early
Eocene and should be placed within the four major lines of microbats.
Consequently, two new suborders based on molecular data have been proposed. The new suborder
Yinpterochiroptera includes the
Pteropodidae or megabat family, as well as the
Rhinolophidae,
Hipposideridae,
Craseonycteridae,
Megadermatidae, and
Rhinopomatidae families
[17] The new suborder
Yangochiroptera
includes all the remaining families of bats (all of which use laryngeal
echolocation). These two new suborders are strongly supported by
statistical tests. Teeling (2005) found 100% bootstrap support in all
maximum likelihood analyses for the division of Chiroptera into these
two modified suborders. This conclusion is further supported by a
15-base-pair deletion in BRCA1 and a seven-base-pair deletion in PLCB4
present in all Yangochiroptera and absent in all Yinpterochiroptera.
[17]
The chiropteran phylogeny based on molecular evidence is controversial
because microbat paraphyly implies one of two seemingly unlikely
hypotheses occurred. The first suggests laryngeal echolocation evolved
twice in Chiroptera, once in Yangochiroptera and once in the
rhinolophoids.
[18][19]
The second proposes laryngeal echolocation had a single origin in
Chiroptera, was subsequently lost in the family Pteropodidae (all
megabats), and later evolved as a system of tongue-clicking in the genus
Rousettus.
[20]
Analyses of the sequence of the "vocalization" gene,
FoxP2
were inconclusive as to whether laryngeal echolocation was secondarily
lost in the pteropodids or independently gained in the echolocating
lineages.
[21] However, analyses of the "hearing" gene,
Prestin seemed to favor the independent gain in echolocating species rather than a secondary loss in the pteropodids.
[22]
In addition to Yinpterochiroptera and Yangochiroptera, the names
Pteropodiformes and Vespertilioniformes have also been proposed for
these suborders.
[23][24]
Under this new proposed nomenclature, the suborder Pteropodiformes
includes all extant bat families more closely related to the genus
Pteropus than the genus
Vespertilio, while the suborder Vespertilioniformes includes all extant bat families more closely related to the genus
Vespertilio than to the genus
Pteropus.
Little fossil evidence is available to help map the
evolution of bats, since their small, delicate
skeletons do not fossilize very well. However, a
Late Cretaceous
tooth from South America resembles that of an early microchiropteran
bat. Most of the oldest known, definitely identified bat fossils were
already very similar to modern microbats. These fossils,
Icaronycteris,
Archaeonycteris,
Palaeochiropteryx and
Hassianycteris, are from the early
Eocene period,
52.5 million years ago.
[14] Archaeopteropus, formerly classified as the earliest known megachiropteran, is now classified as a microchiropteran.
Bats were formerly grouped in the superorder
Archonta along with the
treeshrews (Scandentia),
colugos (Dermoptera), and the
primates,
because of the apparent similarities between Megachiroptera and such
mammals. Genetic studies have now placed bats in the superorder
Laurasiatheria, along with
carnivorans,
pangolins,
odd-toed ungulates,
even-toed ungulates, and
cetaceans.
[1]
The traditional classification of bats is:
Megabats primarily eat fruit or nectar. In New Guinea, they are
likely to have evolved for some time in the absence of microbats. This
has resulted in some smaller megabats of the genus
Nyctimene
becoming (partly) insectivorous to fill the vacant microbat ecological
niche. Furthermore, some evidence indicates that the fruit bat genus
Pteralopex from the
Solomon Islands, and its close relative
mirimiri from
Fiji, have evolved to fill some niches that were open because there are no nonvolant or nonflying mammals on those islands.
Fossil bats
Fossilized remains of bats are few, as they are terrestrial and
light-boned. Only an estimated 12% of the bat fossil record is complete
at the genus level.
[25] Fossil remains of an Eocene bat,
Icaronycteris, were found in 1960. Another
Eocene bat,
Onychonycteris finneyi, was found in the 52-million-year-old
Green River Formation in Wyoming, United States, in 2003.
[26][27]
This intermediate fossil has helped to resolve a long-standing
disagreement regarding whether flight or echolocation developed first in
bats. It had characteristics indicating it could
fly, yet the well-preserved skeleton showed the
cochlea
of the inner ear lacked development needed to support the greater
hearing abilities used by modern echolocating bats. This provided
evidence flight in bats developed well before echolocation. The team
that found the remains of
O. finneyi recognized it lacked ear and throat features present not only in echolocating bats today, but also in other known
prehistoric species.
The appearance and flight movement of bats 52.5 million years ago were different from those of bats today.
Onychonycteris
had claws on all five of its fingers, whereas modern bats have at most
two claws appearing on two digits of each hand. It also had longer hind
legs and shorter forearms, similar to climbing mammals that hang under
branches such as
sloths and
gibbons.
This palm-sized bat had short, broad wings, suggesting it could not fly
as fast or as far as later bat species. Instead of flapping its wings
continuously while flying,
Onychonycteris likely alternated
between flaps and glides while in the air. Such physical characteristics
suggest this bat did not fly as much as modern bats do, rather flying
from tree to tree and spending most of its waking day climbing or
hanging on the branches of trees.
[28]
Habitats
Flight has enabled bats to become one of the most widely distributed groups of mammals.
[29] Apart from the Arctic, the Antarctic and a few isolated oceanic islands, bats exist all over the world.
[30]
Bats are found in almost every habitat available on Earth. Different
species select different habitats during different seasons, ranging from
seasides to mountains and even deserts, but bat habitats have two basic
requirements: roosts, where they spend the day or hibernate, and places
for foraging. Bat roosts can be found in hollows, crevices, foliage,
and even human-made structures, and include "tents" the bats construct
by biting leaves.
[31]
The United States is home to an estimated 45 to 48 species of bats.
[32][33] The three most common species are
Myotis lucifugus (little brown bat),
Eptesicus fuscus (big brown bat), and
Tadarida brasiliensis
(Mexican free-tailed bat). The little and the big brown bats are common
throughout the northern two-thirds of the country, while the Mexican
free-tailed bat is the most common species in the southwest.
[34]
