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Generally, gliding is used for some animal species as a mean of fleeing from the predators since it enables them to move between trees without the need to descend to the ground and it also is an energy-efficient way to travel long distances between scattered food resources. For scientist, gliding animals (flying squirrels, flying frogs, and flying lizards with wings of skin that allow them to glide through the tropical forest) have long been the intriguing subject of study. Recently, researchers have found that Southeast Asia has a unique abundance and diversity of these animals. This observation leads them to the following questions. What could be an explanation about biological diversity of these animals found in the forest of Southeast Asia and what could explain the scarcity of gliding animals in other regions? Most of all, what makes Southeast Asian rain forests unusual? Several theories have been proposed by many scientists to explain the diversity of gliding animals in Southeast Asia. The first theory might be called the tall-trees hypothesis. According to this theory, taller trees in Southeast Asia could offer longer glides as well as the opportunity to boost in a dive before gliding because the forests of this region are taller than any other forests in the world, which comes from the domination of dipterocarp family, a family of tall, tropical hardwood trees. And by providing a more advantageous situation for gliding between tall-trees, the lower wind speeds might also contribute to the great number of appearance of gliding animals. This speculation, however, has several flaws. First, gliding animals are found throughout the Southeast Asian region, even in relatively short-stature forests located in the northern area of the rain forest in China, Thailand and Vietnam. Also, some gliders thrive in low secondary forests, plantations, and even city parks. It is obvious that gliding animals do not need tall trees for their activities. In addition, many gliding animals initiate their glides from the middle of tree trunks, not necessarily ascending to the tops of trees to take off. Another theory, known as broken-forest hypothesis, speculates that animals in Southeast Asia must risk descending to the ground or glide to move between trees because the top layer of the forest – the tree canopy – has fewer woody vines connecting tree crowns in Southeast Asian forests than in New World and African forests. It also presumes that the tree canopy in Asian forests is more uneven in height, due to the existence of tall dipterocarp trees with lower trees between them, and this imbalance is favored by gliding animals. But it is observed by ecologist working in different regions of world that, depending on the site conditions of soil, climate, slope elevation, and local disturbance, there is a tremendous local variation in tree height, canopy structure, and abundance of vines. Indeed, we can find many locations with abundant woody vines and numerous connections between trees in Southeast Asia and similarly many Amazonian forests with few woody vines. A last theory differs from the others in suggesting that it is the presence of dipterocarp trees themselves that is promoting the evolution of gliding species. According to this theory, dipterocarp forests can be "food-deserts" for the animals that live in them. The animals living in dipterocarp forests that have developed gliding divide into two main groups: leaf eaters and carnivores that eat small prey such as insects and small vertebrates. For leaf-eating gliders the problems is not the absence of any leaves but the desert-like absence of edible leaves. Dipterocarp trees often account for 50 percent or more of the total number of canopy trees in a forest and over 95 percent of the large trees, yet dipterocarp leaves are unavailable to most vertebrate plant eaters because of the high concentration of toxic chemicals in their leaves. Many species of gliding animals avoid eating dipterocarp leaves and so must travel widely through the forest, bypassing the dipterocarp tree, to find the leaves they need to eat. And gliding is a more efficient way of traveling between trees than descending to the ground and walking or else jumping between trees. Since there is the lower abundance of prey and other insects, many carnivorous animals also may need to forage more widely for food. This scarcity of food source is caused by dipterocarps' irregular flowering and fruiting cycles of two- to seven-year intervals, resulting in a shortage of the flowers, fruits, seeds, and seedlings that are the starting point of so many food chains. The lower abundance of prey in dipterocarp forests forces animals such as lizards and geckos to move between tree crowns in search of food, with gliding being the most efficient means.