ANTHROPOID APES Among the Old World primates one group is distinguished from the rest by its far closer resemblance to man, a likeness rec ognized in the name anthropoid (man-like) apes. These include the gibbons, the orang, chimpanzee and gorilla (qq.v.). The an thropoids have usually been collectively grouped in a single family Simiidae, but this name is now frequently restricted to the orang, chimpanzee and gorilla, a second family, Hylobatidae ("tree-walk ers"), comprising the much smaller gibbons.
The range of this family is now restricted to southeastern Asia, especially the Malay peninsula, Borneo, Su matra and some other East Indian islands, though in the Miocene gibbons extended into Europe and in the Pliocene as far north as central China. They are slender, long-limbed, monkey-like apes, the most striking feature being the enormously elongate arms, which reach the ground when the animal stands erect. Like other anthropoids, they are tailless. They differ widely from the tailed monkeys in their mode of progression, as they do not run on all fours, either in the trees or on the ground, but hang and swing from branches by their long arms and hook-like fingers, making almost incredibly long leaps in the manner of a trapeze gymnast. This mode of arboreal locomotion has been called "brachiation" by Sir Arthur Keith. While travelling thus by the hands, gibbons use the feet for prehension and carrying food. They sit upright and when on the ground run swiftly in an erect position, holding the arms out as balancers. Gibbons differ from most Old World monkeys in the absence of cheek-pouches and in the pattern of the molars, which is essentially that of the great apes and man. They also in common with these forms have a vermiform appendix and flattened sternum. They nevertheless retain some Old World monkey characters; for example, the small ischial callosities are present, and the central bone in the wrist is retained as a separate element. Air-sacs, formed as extensions of the laryngeal ventricles, well-developed in the Simiidae, are present only in the siamang.
About a dozen species of Hylobates have been described, a few well-defined, but most of them, based chiefly on hair colour (which is a variable character even in the same species), are of doubt ful validity. Among the more distinctive may be mentioned H.
hoolock which extends northwesterly as far as Bhutan, H. lar in Burma and Siam, and H. leuciscus in Java. The colours in gen eral are black, fawn or grey, sometimes with white around the face or on the hands. The fur is soft and woolly. A considerably larger black gibbon, the siamang, found in Sumatra, is now sepa rated as Symp/ialangus, the name referring to the fusion of the second and third digits of the foot. This animal has a better devel oped chin than any other anthropoid, but its most conspicuous character is the possession of a large laryngeal air-sac capable of inflation. The siamang when standing erect may exceed three feet in height. Forms closely related to the Sumatra siamang, S. syn dactylus, occur in adjacent regions. The vocal ability of gibbons is most remarkable, their high-pitched cries being audible for a mile or more.
The three genera of great apes constituting the family Simiidae, the orang-utan of Borneo and Sumatra and the chimpanzee and gorilla of the African forests, are by far the closest of all animals to man. Their divergences from man, striking though some of them are, are nearly all differences in degree rather than in kind, and these apes are nearer anatomically and physiologically to man than to any of the tailed monkeys. Their differences from man are largely correlated with habit. Man has become terres trial, while the apes have retained their primary arboreal habit, and have even developed further arboreal adaptations in varying degrees. In the orang these have become greatly exaggerated, while in the gorilla, which has become partly terrestrial, they are less marked. The chimpanzee is intermediate. The arms have become long and the relatively short legs retain the opposable great toe. This disproportion of limbs results in a peculiar secondary type of quadrupedal progression on the ground, the hands resting on the knuckles and the fore-part of the body somewhat elevated.