25-08-2017, 09:32 PM
Bat evolution
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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.
Classification of bats
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 genusNyctimene 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.
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.