gibbons and siamangs
This family contains 1 genus, Hylobates,
and approximately 11 species (some authorities recognize a second genus, Symphalangus)
. They are found only in the tropical forests of southeastern Asia,
including on Sumatra, Borneo, Java, and other islands as well as the
mainland. These apes are of medium size (3.9-12.7 kg). They lack tails.
Their forearms are remarkably long, and both forefeet and hindfeet are
long and thin. The forefeet have a deep cleft between first and second
digits. The pelage of these apes is usually black, gray, or brownish,
often with white markings on hands, feet, and facial region. Ischial
callosities are present. Some gibbons and siamangs have an enlarged throat
sac, which inflates and serves as a resonating chamber when the animals
call. This structure is enormous in a few species, equalling the size of
the animal's head.
The skulls of
hylobatids resemble those of hominids, with very short rostra,
enlarged braincases, and large orbits that face forward.
Hylobatds are catarrhine primates; that is,
their nostrils are close together and face forward and slightly downward.
They lack cheek pouches and their stomach is not sacculated. Their
teeth are similar to those of hominids. The molars are bunodont
and lack lophs. The upper
molars usually have a cingulum, which is
sometimes large. The canines are prominent
(more so than in most hominids) but not sexually dimorphic. The dental
formula is 2/2, 1/1, 2/2, 3/3 = 32.
Gibbons and siamangs are monogamous, living
in small family troops of usually 2-6 related individuals. These groups
are territorial, maintaining their territories partly by conspicuous
vocalization, which may involve loud duetting by group members. They are
primarily vegetarian, feeding on figs and other fruit, leaves and shoots.
They are remarkably active and agile brachiators, capable of exceeding 3m
in a swing. Gibbons leap from branch to branch, sometimes travelling more
than 9 m in a single leap. They also sometimes walk on large branches or
on ground, assuming a bipedal stance with their arms raised for balance.
Gibbons are active during the day.
Fossil hylobatids are known from the
Miocene. |
