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).
Evolution of the Chiropterans
The scientific name for bats (Chiroptera) originates from the Greek words of kheir (meaning hand) and pteron (meaning wing).
They are warm-blooded and have fur or hair, and feed their young with milk.
- 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.
- 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.
- 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 animals 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.
- 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.
- 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.