Apes, in the past, have been classified as a single group of primates. The current nomenclature divides apes into two distinct families: the greater apes and the lesser apes. The lesser apes are in the family Hylobatidae, consisting of 11 species that are currently recognized in the family. Hylobates, the single genus of Hylobatidae, is divided into 4 subgenera: the Hoolock gibbon (Bunopithecus), the Lar gibbon (Hylobates), the Concolor gibbon (Nomascus), and the Siamang (Symphalangus). The term lesser ape implies that while the gibbons are apes, they did not pursue the same evolutionary line that gave rise to humans. While gibbons stayed in the trees, great apes engaged more time on the ground.
The English translation for Hylobatidae is “tree walker.” Gibbons live in the forest canopy and infrequently descend to the ground. Because they are above everyone in the forest, they never battle with large predators or compete with other apes on the ground. This lifestyle has enabled them to remain petite and agile. Morphologically, their arms are the most exaggerated and the longest of all the primates. The arms end in long, slender hands, and the thumb is attached to the wrist instead of the palm. They use their hands like hooks to move through the forest. This type of locomotion is called brachiation. The arm-over-arm movement enables the gibbon to gain great speed in the trees. By using their weight like a pendulum, a gibbon can easily overtake a person running on the forest floor. They can cover over 3 m in a single swing. These lesser apes can also leap from a standstill from branch to branch, sometimes more than 9 m in a single leap. With their extremely long arms and their hooklike hands, gibbons are magnificent acrobats, perfectly adapted for a life in the forest canopy.
Lesser apes are similar to monkeys, with longer canines, slender bodies, and two hard buttock pads, called ischial callosities. They are similar to the great apes because they do not have a tail. Their skulls resemble those of hominids, with very short rostra, enlarged braincases, and large orbits that face forward. Their teeth are similar to hominids, as well, with similar grinding molars. The canines are prominent but not sexually dimorphic.
At night, the gibbon sleeps high up in the trees, sitting down on the ischial callosities or curled up on its side. It is the only ape that does not build a nest for sleeping. All gibbons are monogamous, and their social group is based on a mated pair and their offspring, averaging 3 to 4 members. A family may have up to 3 to 4 juveniles aged 2 to 3 years apart. All species of Hylobates are highly territorial. Defense rarely involves physical contact, but instead calling, chasing, and a whole host of theatrics, such as stick breaking and thrashing of vegetation. The spacing of different groups is accomplished by loud vocalizations that can carry for several kilometers. Siamangs call out about 30% of the day, and other gibbons 80% to 90% of the day.
References:
- Meyers, P. (2000). Hylobatidae. information/Hylobatidae.html
- Nowak, R. M. (1999). Walkers mammals of the world (6th ed. Vol. 1). Baltimore: Johns HopkinsUniversity Press.
- Rowe, N. (1996). The pictorial guide to the living primates. East Hampton, New York: Pogonias Press.
