The world’s gliding mammals are an extraordinary group of animals that have the ability to glide from tree to tree with seemingly effortless grace. There are more than 60 species of gliding mammals including the flying squirrels from Europe and North America, the scaly-tailed flying squirrels from central Africa and the gliding possums of Australia and New Guinea.
The fossil record and the locations of the gliders we see today offer important information on the past and present distribution, or zoogeography, of this group of mammals. Zoogeography is the science that deals with the distribution of animals and their forerunners (both in time and space) and considers geology and geography through the examination of the fossil record, past and present climates, changes in vegetation, changes in sea level and continental movement.
Zoogeographic regions are areas of the world with distinctive fauna. They are based on the taxonomic or evolutionary relationships of animals, and not the adaptations of animals to specific environments. Each region maintains a level of homogeneity within its borders that differs from adjacent areas, so the animals within each region are likely to display similarities to each other. Therefore the boundaries between zoogeographic regions are drawn according to the distribution of vertebrate taxa, especially families.
There are six zoogeographic regions of the world:
Although the system of identifying regions around the globe according to the relative uniqueness of their fauna was initially developed for birds by Philip Sclater in 1858, the first to associate mammals with different regions of world was Andrew Murray in 1866 and subsequently by Alfred Wallace (1876), Richard Lydekker (1896), and William and Phillip Sclater (1899). Of the different regions that are recognised, it is that which separates the placental mammals of the Oriental region with the marsupials of the Australasia region that is probably the most famous. The first zoogeographical study of this region was undertaken by Salamon Müller in 1846, with a more thorough assessment undertaken by Alfred Wallace (1863) who placed a line between Borneo and Sulawesi in the north and, in the south, between the tiny islands of Bali and Lombok, separated by a mere 32 kilometres, but for the most part inhabited by different families of mammals and even birds that have the powers of flight. There are several later variations to the Wallace Line that place it closer to New Guinea.
Although gliding mammals occur in all zoogeographic regions, the diversity of the different groups of gliding mammals shows some clear regional differences. The marsupials for example are restricted to the Australasian region where all eight species are found. The highest diversity of gliding marsupials is evident in Australia where six species occur. All six are found in Queensland, five occur in New South Wales and Victoria, four in South Australia, and only the Sugar Glider is found in the Northern Territory and Tasmania (where it appears to have been introduced). Outside Australia the diversity of gliding marsupials is limited though very widespread, being confined to the Sugar Glider that occurs throughout New Guinea and nearly 26 surrounding islands (making it the most widespread of all Australasian mammals), the Biak Glider (Petaurus biacensis) that is limited to Biak and Supiori Islands off the north-west coast of New Guinea, and the Northern Glider that occurs only in the Torricelli Mountains in northern New Guinea.
Throughout the Australasian region all marsupial gliders, with the exception of the Northern Glider, occur primarily within open woodland and forest. The Northern Glider appears to be restricted to closed rainforest on Mount Somoro in northern New Guinea. Some populations of Sugar Gliders have been found to occur in Australian rainforest; however, studies in northern Queensland have found that Mahogany Gliders and Sugar Gliders live in open woodland and rarely enter the adjacent rainforest.
The colugos are restricted to the Oriental region where one species occurs in the Philippines, while the other occurs on mainland Asia and on more than 40 islands within this region, including Borneo, Sumatra and Java. The Sunda Shelf, which is an extension of the continental shelf of mainland South-East Asia (adjacent to the Malay Peninsula, Thailand, Cambodia and Vietnam) links this region with Borneo, Sumatra, Java, Bali and the smaller surrounding islands. During the glacial maxima of the Pleistocene, sea levels were as much as 120 metres below those we see today and the entire Sunda Shelf was exposed as dry land. This exposure during glacial periods and submersion during interglacial periods may be the mechanism that allowed the broad island distribution of colugos throughout the region.
The flying squirrels have the greatest distribution of the gliding mammals and are represented in four zoogeographic regions. There are two species in the Nearctic region of North America, with the distribution of one of these extending slightly into the extreme north of the Neotropical region. Only one species of flying squirrel occurs in Europe while the rest of the Palearctic region includes 21 species.
The highest diversity of flying squirrels occurs in the Oriental region where 41 species are known to occur, of which 13 species also occur in the Palearctic region.
Within the Oriental region the greatest diversity of flying squirrels is in Borneo where 15 species can be found. Sumatra and Thailand have 13 species; Burma, Sumatra and the Malay Peninsula have 12 species; Vietnam and Bhutan have eight species, while Java has seven species. No species of flying squirrel occurs east of the Wallace Line in the Australasian region. Many species found in this region also have populations or subspecies in western and northern Asia including southern China, India, Sri Lanka, Nepal and Pakistan. Another five species (two woolly flying squirrels, Kashmir Flying Squirrel, Hodgson’s Giant Flying Squirrel and Bhutan Giant Flying Squirrel) are restricted to north-western Asia in the region of Bhutan, Sikkim in north-eastern India, Nepal, north-western India, Kashmir, Pakistan and Afghanistan.
The North Chinese Flying Squirrel, Complex-toothed Flying Squirrel, Red and White Giant Flying Squirrel and Chinese Giant Flying Squirrel are restricted to the southern parts of the Palaearctic region in southern and central China and eastern Tibet. Within China there are 16 species. The provinces of Sichuan and Yunnan have the greatest diversity of flying squirrels, with nine species and 11 species respectively. The Siberian Flying Squirrel is the most northerly flying squirrel and occurs throughout Eastern Europe, Russia, China, Korea and Japan.
All six of the scaly-tailed flying squirrels occur in the Afrotropical region where they are restricted to central and western Africa. The highest centres of diversity are in Cameroon, Congo, Equatorial Guinea, Gabon, Liberia and Zaire, each of which has five species.