The secret life of bats

Bat evolution, diversity and classification

Mammals have been in existence for over 200 million years, but began to diversify greatly around 65 million years ago. The earliest fossil insect-eating bat found to date is 50 million years old and is very similar to the species of bats that exist today, indicating that by that time they had already largely evolved. Perhaps the major part of bats' evolution occurred 70 million years ago or more. There were important influences at this time: flowering plants had also diversified between 70-100 million years ago, providing new foods for insects and other animals, and the rulers of the world, the dinosaurs, had died out by 65 million years ago. The mammals that began appearing on the scene were different. They had hair on their bodies instead of scales, gave birth to live young rather than laying eggs, and provided their young with milk from special glands.

Unfortunately, no 'missing links' have yet been found to show clearly the bats' evolutionary route - their bones are very thin and fragile, so bat fossils are rare. The most likely suggestion is that they all evolved from a shrew-like mammal that climbed trees, apart from the large group known as fruit bats, which seem to have had a different origin. The earliest known fossil of a fruit bat is only 35 million years old. Although fruit bats appear superficially similar to insect-eating bats, they are very different in a number of ways, including the shape of their skulls and teeth, their neck vertebrae and the bones in their hands. It is likely that they evolved along a very different path from the insectivorous bats, one that branched off from the primates, the group that contains monkeys, apes and humans. It is possible, therefore, that these bats are distantly related to us.

Present diversity

Flight gives bats the opportunity to go almost anywhere. Mountain ranges, seas or similar barriers that are obstacles to land-based mammals are far less of a restriction for bats. Bats have been able to reach isolated islands in vast oceans and to cross continents. Having arrived, they are affected by their new environment. Pressures of food, shelter and predators all have an influence. Over millions of years some failed to survive, but others adapted and flourished. After many, many generations, these survivors gradually changed by the process of natural selection into new species. Today's 1,100-odd species of bats live in habitats as diverse as deserts, riverbanks, forests and even cities. There have been records of bats found as high as 5,000 metres. Some bats have increased the range of food taken from insects and fruit to include nectar, fish, amphibians, birds, small mammals and even blood. Where there's a suitable niche, there's generally a bat to occupy it.

Classification of bats

The Solomons bare-backed fruit bat, Dobsonia inermis (left), is one of about 186 species of megabats. The common pipistrelle, Pipistrellus pipistrellus (right), is one of about 930 species of microbats. (Image: Phil Richardson)

As with other living things, biologists find it convenient to group similar species of bats together into a genus (plural: genera) and then group similar genera into families. The families are then grouped into sub-orders and then into an order. The order that incorporates all bats has the Latin name Chiroptera, which means 'hand-wing'. This is made up of 2 sub-orders, the Megachiroptera (popularly known as megabats) and the Microchiroptera (or microbats).

There is only one family of megabats, the Old World fruit bats, and it includes approximately 186 species. Species belonging to the most diverse genus, Pteropus, which makes up over one-third of all the megabats, are called flying foxes. Although many megabats are big, as the name suggests, the Megachiroptera also includes a large number of species that are smaller than some of the microbats. As their name suggests, most if not all of the Old World fruit bats feed on fruit, though for some pollen and nectar may be more important in the diet, and members of the genus Nyctimene may take some insects.

The approximately 930 species of microbats are grouped into 17 families. Again, the popular name of the sub-order can be misleading. Some of the microbats are large, with impressive wingspans, though none is as big as the largest megabats. Although most are insect-eaters, by no means all of them are, and the range of diets is very wide.

The 'local' name for a species may be completely different from the name given to the same bat in a neighbouring country. Each species is, therefore, given a Latinised scientific name to try to avoid such confusion. This consists of 2 parts. The first word, the generic name (which always has an initial capital), refers to the genus. The second, or specific name, indicates the particular species within that genus. In many cases, species are further divided into sub-species, or geographical races, involving a third name.

The words used are often descriptive, so Myotis macrotarsus means 'mouse-eared bat with big feet'. Some species, such as M. daubentonii, are named after people (Daubenton was a French naturalist) and some, such as M. atacamensis, which is a species found in the Atacama desert, after geographical regions.

As might be expected, new species of bat are being discovered occasionally in remote areas of rain forest or on isolated oceanic islands. Recently a new species was found in Britain, a country where bats are watched relatively intensively. The pipistrelle, the smallest European bat and the commonest species in Britain, was formerly regarded as a single species: Pipistrellus pipistrellus. Despite this bat having been studied for decades, it was not until the 1990s that bat researchers realised there are actually 2 different species. If a new species can be found in Britain, then it is likely there are other species in other parts of the world just waiting to be discovered.

So the total number of species worldwide is always changing, as new ones are discovered and as others become extinct. Another reason for the total frequently being altered is that different taxonomists decide to 'split' one species into two or 'lump together' two into one. An example is the round-eared tube-nosed bat, Nyctimene cyclotis, of New Guinea. It was thought to have a subspecies, Nyctimene cyclotis certans, but now some taxonomists think that this is really a full species, Nyctimene certans.

Classification of Chiroptera - megabats

Classification of Chiroptera - microbats

Bat distribution

Bats are found in many parts of the world in most terrestrial habitats, except in colder parts of the northern and southern hemispheres beyond the limit of tree growth or on some oceanic islands. The number of species increases towards the equator, where there is more available food of more varied types than in temperate regions. Thus about 120 species occur in the northern part of South America, but only 45 or so in the whole of North America. Some regions have a particularly great diversity. Approximately 100 species of bats are found in South-East Asia, but only about 60 in central Africa and a similar number in Australia. Fewer species are found on small islands than on larger continents, so Europe as a whole has about 30, Britain 16 and Ireland only 7 species.

Each species is restricted in its range due to the niche it has filled, governed by food, temperature and roosting-site availability. 

Some species have an extensive range, particularly those on large land masses. For example, Daubenton's bat, Myotis daubentonii, is found throughout Europe and eastwards as far as Japan. The range of the hoary bat, Lasiurus cinereus, extends from Canada south to Chile and Argentina. Schreiber's bat, Miniopterus schreibersii, occurs from southern Europe to southern Africa, and east to Japan and south Australia. 

Other species, by contrast, have very small ranges. A number of the flying foxes are restricted to a few tropical islands in the middle of oceans, such as the Rodrigues fruit bat, Pteropus rodricensis, which is found only on the island of Rodrigues in the Indian Ocean. Among the microbats, Kitti's hog-nosed bat, Craseonycteris thonglongyai, is found only in a few caves on the Thai-Burmese border.