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ZooNooz Article - september 2004
Focus on Silvery Gibbon
Story by ANDREW PARNELL
Photograph Derek Smith
Illustration Katerina Sakkas
Silvery Gibbons are small apes,
about the size of a domestic cat, and
weighing around 8kg. Both males and
females have the same dense, silvergrey
fur with a white or light grey
fringe surrounding a darker face. The
fur is quite long, giving them a fluffy
appearance, and they have a dark
grey cap and chest.
Gibbons' scientific name, Hylobates, means, appropriately, 'dweller in the trees'.
There are nine species of gibbons,
all living in south-east Asian forests.
The Silvery (Javan or Moloch) Gibbon
lives in the few remaining tropical
rainforests in the lowlands and hills
on the western side of the Indonesian
island of Java. It is the only species of
gibbon living on the island.
Critically endangered
Silvery Gibbons |
It is estimated that Silvery
Gibbons have already lost 98
percent of their original forest
habitat, and the remaining forest
faces considerable pressure from
Java's highly concentrated human
population of about 115 million.
Combined with the lesser problem
of the young being captured as pets,
this ongoing habitat loss has meant
the Silvery Gibbon is now classified
as critically endangered. Fewer than 2,000 animals are left, spread
across about 21 small areas isolated
from each other by rice fields and
villages. The habitat of only about
eight of these areas is considered to
be viable. The current combined subpopulations
are not sufficiently large
to be considered an evolutionally
viable population, and so these
fragmented groups will require active
conservation management for longterm
survival. The only known large
populations, each of less than 100
animals, live in three national parks.
Of these, Gunung Halimun
National Park, is considered to be
the best available habitat and is the
only area capable of supporting
1,000 gibbons. Currently it is home
to around 100 animals, whose
conservation is supported by the
work of the Silvery Gibbon Project.
Aerial acrobats
Silvery Gibbons |
Gibbons' scientific name,
Hylobates, means, appropriately,
'dweller in the trees'. Like all
gibbons, Silvery Gibbons rarely go
down to the ground. They live in the
upper canopy of the forest and move
around by 'brachiation': swinging their lean bodies from branch to
branch and using their extremely
long arms and fingers as hooks
(gibbons, being apes, do not have
tails). Their shorter thumbs extend
from near their wrists, allowing a
wide range of movement.
They are able to reach incredible
speeds through the tree-tops - vital
in defending their families, their
shelter and their food supply - and are
considered to be the most acrobatic
of all the tree-dwelling animals. They
confidently swing through many wellworn
tree-top routes, a single swing
sometimes carrying them three metres
through the air. On the rare occasions
when they walk, they will cover only
short distances, holding their arms in
the air to maintain their balance.
The domestic scene
One of the unique characteristics of Silvery Gibbons is the haunting and beautiful song of the adult female, sung at about the same time every day, usually just after dawn.
Unlike many other primates,
gibbons form permanent and
apparently close-knit families
comprising the mother and father
(all gibbons are monogamous) and
up to four immature offspring.
Silvery Gibbons breed at any
time of the year, with a single baby
being born after a pregnancy of about seven months. On average a
pair will produce five or six offspring
over 10 to 20 years. When the young
adults leave their family they roam
by themselves until they can find a
partner and establish a territory of
their own.
Their primary food is fruit,
although they sometimes eat leaves,
flowers, insects and small animals.
To find enough ripe fruit all year
round, families range over large areas
of forest, within which is a strongly
defended territory. Families tend
to stick to their own territories so
direct border conflict is rare, usually
confined to periods when food is
scarce. When faced with intruders,
the adult male chases the male of the
intruding group, while all members
of the family call and scream loudly.
Physical conflict is uncommon.
The fact that they spend most of
their time high in the dense canopy
and have such an intimate knowledge
of escape routes within their territory
means they are relatively safe from
potential predators, such as snakes,
birds of prey and leopards. They are
also very alert and have the ability to
escape at great speed.
Hear my song
Her song primarily warns other gibbons to stay away from her territory, reducing incidences of border conflicts.
Because they live high in
the dense trees and often have
considerable distances between
them, brightly coloured or patterned
fur like that of some monkeys
would be of little use. Consequently
they primarily use vocalisation to
communicate with each other, both
friends and foes.
One of the unique characteristics
of Silvery Gibbons is the haunting and
beautiful song of the adult female,
sung at about the same time every
day, usually just after dawn. Lasting
for several minutes at a time, she can
be heard up to 1.5km away. While
she is sometimes accompanied by an
immature daughter, other members
of the family remain quiet, grooming,
feeding or resting in nearby trees.
Her song primarily warns
other gibbons to stay away from
her territory, reducing incidences
of border conflicts. It is also an
important means of communicating
with the rest of the family, confirming
and enhancing their bonds.
Unlike other species of gibbon,
solo singing by the male is rare and
is primarily used by single males
seeking partners.
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