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Rainforest Bats
Order Chiroptera
Bats are one of the
successful groups of mammals of all time. Of the living mammals today, one fifth
of all species are bats (Strahan 1998). Bats are the only mammals capable of
true, sustained flight and this has enabled them to colonise many parts of the
planet before other mammals. Subsequently, they are found all over the world. As
with many groups of animals, there are many more species in the tropics.
The ability of bats to fly
is connected to a suite of physical features that are often their most
distinguishing characteristics. The most obvious of these is of course the wing.
Unlike birds, bat wings are actually stretched skin. It is an elastic membrane
that stretches from the shoulders, along the arm, is supported by the elongated
fingers, and completes the wing by reaching to the ankle (Strahan 1998).
This design has certain
consequences for the bat. While being efficient climbers, their hip arrangement
restricts the movement and support of the hind legs under the body (Strahan
1998), therefore they are rather clumsy on the ground, limited to scurrying
along. This also means that when resting, all bats tend to hang rather than sit
or stand. The arrangement of the bones and ligaments in the foot is thus that it
does not require any effort to 'lock' onto a branch or ledge.
The bat order is comprised
of two major groups, the suborder of the Microchiropterans, and the suborder of
the Megachiropterans. The former group includes the smaller insecteating,
echo-locating bats that most people are familiar with, while the latter consists
of the larger fruit and nectar bats that are found predominantly in the old
world tropics. Because these two types of bats are so different, there has been
some interesting debate over the last few decades as to whether they are even
related.
Bats in Australia
There are at least 70
species of bats in
Australia, with perhaps as many as 90 (Egerton 1997). This is almost a quarter
of Australia's mammals. Despite this large proportion, they did not evolve here
from the original Gondwana stock. Rather, they 'invaded' the continent
relatively recently. Because they have the ability to fly as no other mammals
can, they were able to colonise Australia millions of years ago when it was
still quite a relatively remote island. Therefore, in contrast to Australia's
marsupials, the bats of Australia are not seen as unique. It is generally agreed
that bats colonised the Australian continent from South-east Asia, and this idea
is confirmed by the similarity between the Asian and Australian bat fauna. The
invasion was probably mostly via New Guinea and then by way of Cape York, as
this is presently where most species are found (Strahan 1998). Thus, Australian
bats are not seen as particularly unique and are considered closely related to
their ancestral Asia stock. Nevertheless, of the numerous species found here
about half are endemic to the continent (Strahan 199).
Additional Information:
Evolution of the
Chiropterans
Rainforest Bats
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The scientific name for bats (Chiroptera) originates from the Greek words of
kheir (meaning hand) and pteron (meaning wing).
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They are
warm-blooded and have fur or hair, and feed their young with milk.
Distribution:
- Queensland is home to a recorded 52 species of bats.
- Present and past distributions of bats indicate that they originated in tropical
forests. They probably reached Australia about 26 million years ago when land connections
existed between some of the islands between New Guinea and north Queensland.
- In general, tropical non-hibernating bats are restricted to north of the Tropic
of Capricorn. Colonisation of temperate climates depends on the ability to hibernate
during seasonal food shortages such as winter.
Flight:
- They are the only mammals capable of sustained flight.
- Bats’ forelimbs have undergone dramatic changes during their evolution to
allow them to function as wings. They have the same basic parts as the human hand and arm,
only in very different proportions. Their upper arm is shorter, their forearm is longer,
and all the toes except the thumb are greatly elongated.
- Their wing consists of a thin membrane with a leading edge extending from the
shoulder to the base of the thumb and to the tips of the second and third toes and a
trailing edge extending from between the tips of the third, fourth and fifth toes to the
ankle. The thumb is free of the wing membrane and bears a claw for use in crawling,
support and grooming.
- Muscles that pull the forearm towards the chest mainly power flight and, to
provide a surface for attachment of these large muscles, the breastbone is keel-shaped, as
in flying birds.
Temperature Control:
- As most bats are small, they have relatively large body surface areas from which
body heat can be lost by radiation. Their extended wings can act as radiators. Warm
surroundings reduce heat loss too. Humid tropical conditions and lots of drinking water
prevent potential water loss problems.
- Bats may become torpid (dormant) in cold regions when resting by switching off
their internal thermostat and letting their body temperature drop to that of their
surroundings. This reduces the animal’s rate of energy consumption as its metabolism
slows and less heat is lost by radiation to the environment.
Energy and Metabolism:
- A bat must ingest larger quantities of food than a terrestrial mammal of similar
size as much more energy is spent in flying than in other modes of locomotion.
- The rate of metabolism may be as little as one percent of the rates when flying
and the heart rate may be correspondingly slower. As this state also requires some energy,
a store of fat laid down in the body of the bat in the time just prior to hibernation is
needed.
Breeding:
- Bats have two alternative reproduction cycles associated with hibernation. The
first is the storage of sperm in the female reproductive tract from autumn to spring. This
allows the female to fertilise an ovum as soon as hibernation ends. The second method is
for an ovum, usually fertilised in autumn, to develop to an early embryonic stage and then
remain at ease until spring, when development continues.
- Gestation period is between 50 and 240 days – relatively long for small
mammals.
- The period of maternal dependence is also long – 3 to 10 weeks.
- Usually one
young is born each breeding season.
- It is blind (except in fruit bats) and hairless.
- Females suckle their young from two nipples, one under each armpit. In some
groups there is a false (non-lactating) nipple in each groin, onto which the young may
attach itself by its teeth.
Additional Information:
- Bats have been considered as objects of mystery, superstition, fear and worship.
They are important in medical research when studying problems like hypothermia, survival
in extreme environments, and topics of comparative anatomy, embryology, histology and
cytology.
- Bats make a practical contribution to the environment in the form of tree
pollination and insect control. Flying foxes are the primary agents for pollination of
many Australian hardwood forests.
- Many bat species need protection if they are to be maintained in a sufficiently
large area, in numbers high enough for reproduction. Australia protects bats by law.
- Bats are second only in number to the rodents: about 40 percent of living mammal
species are rodents, and about 20 percent are bats.
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