ABBOTT'S GRAY GIBBON
Geographic Distribution and Habitat
Abbott’s gray gibbons, also called western gray gibbons, are endemic to a small part of Borneo, an island shared by Malaysia, Brunei, and Indonesia. These gibbons occur only in the southwest Indonesian part of the island, specifically in West Kalimantan Province and South Sarawak State. The species can only be found on the northern side of the Kapuas River, as the river creates a geographical barrier. They thrive in rainy, semi-deciduous, and tropical forests. They have also adapted to live in secondary forests that grow in areas that were once logged. The presence of fruiting trees seems to be the major factor that determines their presence.
Their common and species names draw inspiration from Dr. W. L. Abbott, a 19th-century explorer who studied many wildlife species in Indonesia and collected samples for museums. The genus name Hylobates means “forest wanderer,” likely because of the gibbon’s distinctive swinging movements in the forest.
The Abbott’s gray gibbon was classified as a subspecies of the Bornean Gibbon (Hylobates muelleri). Further studies have revealed that the Abbott’s gray gibbon displays physical characteristics that are typically smaller and possess notable genetic variations, justifying its classification as a separate species. The occurrence of hybridization in these gibbon species poses challenges for accurately classifying their genetics in the future.
Size, Weight, and Lifespan
Abbott’s gray gibbons have the typical body proportions of other species in the genus, where they have arms much longer than their torso, slightly shorter legs, and a small head. They weigh approximately 14 pounds (6.3 kg), which is smaller than other closely related gibbons in the region. Their body length (head to the base of their tail) measures about 19.5 inches (49 cm). There is not enough information on wild or captive individuals to know their lifespan. Wild gibbons live to about 25. However, there is a remarkable case of a captive Bornean gibbon (Hylobates muelleri), which is closely related to the Abbott’s gray gibbon, living for 60 years!. This record is astonishing because scientists expect larger primates to live longer and smaller primates, like gibbons, to live shorter lives, closer to 20 years. This highlights how much we still have to learn about the effect of the environment on the biology of a species.
The colors of gibbons in Indonesia and Borneo are often similar. This has caused a lot of confusion for biologists in this region who try to figure out what species a gibbon belongs to just by observing them. Abbott’s gray gibbons blend well with their environment and do not stand out as much as other gibbon species. The gibbons have short brown-gray fur that can become longer around the ears. Sometimes, their face, genital area, and lower back can have darker fur. Gray gibbons are distinguished by their absence of beards and dark caps of fur on their heads. Unlike gibbons with white rings around their faces or eyes, Abbott’s gray gibbons have only a white streak on their brows.
Males and females look similar, which is referred to as sexual monomorphism. Infant gibbons are usually much lighter colored than adults. The young are so small and cling to their mother’s chest and belly for about a year. The light color blends with the bright sky above, which makes the young one almost invisible to a predator on the ground. As the young mature, their fur color changes to match the adult’s mouse-brown color.
Fruits, young leaves, flowers, and some insects are the main diet of these gibbons. Gibbons are highly energetic primates and they need to have food that is high in sugar to help them maintain energy as they swing from tree to tree. The gibbons look for ripe fruits that have high sugar content. When fruits are not available, they prefer to eat younger leaves that are sweeter and have more protein than mature leaves.
Behavior and Lifestyle
Gibbons are diurnal primates that are most active during the day when they travel and forage for food. They are usually most active just after dawn and their activity slowly decreases towards the afternoon. Then they seek out resting sites to bed down for the night. This is unusual for tropical primates who usually have two spikes of activity—one after dawn and one just before dusk.
Gibbons are built to glide through the tree canopy where they spend most of their time. This means that they are arboreal. This agility through the forest is most clearly seen through a specialized traveling behavior called brachiation, in which they use one of their long arms to grasp a branch in front of them, and they hang from two branches forming a “Y” shape with their arms and body. They then hang on one arm and stay suspended from that branch for a bit before reaching out to the next branch to form the “Y” shape again. Gibbons can travel long distances quickly, through the canopy, using this technique.
Gibbons are territorial over a small area which is usually defended using song duets. Longer and louder duets indicate the presence of a stronger gibbon, which should discourage neighbors from foraging or stealing their territory.
Abbott’s gray gibbons are endemic to only a small southern part of Borneo island.
They depend mostly on ripe fruits that they forage for throughout the year.
They are smaller and more mousey-gray colored than similar species that live close by.
They sleep high in the canopy, and on branches, and change their sleeping sites every night.
Most of Abbott’s gray gibbons’ time is spent foraging and eating food (between 20-50% of the time they are active) and a lesser amount of time is spent traveling (12-33% of their active time). Gibbons can spend a lot of time suspending their body weight from branches using one or two of their long arms. This behavior is useful because they can eat “on the go”. Gibbons can grab fruit when they find it and eat it as they travel to the next patch of food, or if they need to travel to a rest site before it gets dark then they can still get in a last-minute meal as they move through the trees.
They sleep on bare branches at night. These sleeping sites can change daily and it is likely that these gibbons, like Bornean gibbons, can use close to 100 trees over their lifetime.
In dense forests, where it is difficult to see each other through the leaves, gibbons primarily communicate by vocalizing, an efficient way to get another gibbon’s attention. Humans have researched gibbon vocal signals, especially their songs because they are loud and easily detected. Facial expressions are more subtle and probably used for close-range communication among family members, which makes them less noticeable to researchers and therefore they are largely understudied.
Like all gibbons, Abbott’s gray gibbons also make loud calls to communicate with other members of the species. These calls often trigger a response resulting in back-and-forth singing sessions called “duets”. There is no specific research on the songs and their variations in this species, but there are some general call behaviors among closely related gibbon species in the same geographical region that we can use to help understand overall gibbon communication behavior.
Both males and females have distinctive calls and researchers often use these calls to identify both species and gender of the caller. Males especially use loud calls to declare territory to neighbors. When a male gibbon hears another male calling, he will wait and respond interactively. These interactive duet calls may be a way for gibbons to communicate aspects about troop size or troop activity.
In one study, a recording of a mated female’s call was played to a troop. The troop’s female responded with her own calls and she investigated the area for the potential intruder. However, males tended to be silent during the playback. This study suggested that this behavior helped maintain monogamous relationships between mated males and females, and prevented intruding females from approaching mated males.
Gibbons form monogamous pairs, which means one male and one female will mate and form a family troop. Monogamy in gibbons is observed as a general rule, but we need more long-term research of wild populations to determine if monogamy occurs throughout the gibbon’s lifetime, if it persists if one of the adults dies, or if mating can occur outside of the monogamous pair when environmental conditions change. Researchers think that monogamy could be a flexible social condition and there may be some gibbon species that do not live in true monogamous pairs. Gibbons consistently form small family units or troops. A family group can consist of 4 or 5 individuals with the main adult pair, an infant, and juvenile, and a subadult from previous births.
Gestation (the period a female is pregnant) is most likely about 190 days or 6 months after which she gives birth to a single offspring. She keeps her baby close to her for about a year until the infant becomes stronger and more independent. It is important that the young remain attached to the adult because the troop has to move quickly through the forest and young gibbons would find it difficult to keep up.
Both male and female sub-adults leave the natal group when they become sexually mature, usually between the ages of 6 and 8. Sub-adults that leave will travel away from the natal group to find a mate to form their own troop with. But this behavior depends on factors such as availability of food, space, and size of the natal troop.
Due to their demand for soft and ripe fruit, gibbons mainly compete with squirrels and birds for food. Gibbons manage this competition by traveling across the canopy quickly to find food, eating while traveling, and having a short feeding window where they can gorge on food before other animals get to it before moving on to the next food patch.
Gibbons are known seed dispersers because they mostly eat the ripe flesh of the fruit and throw away the seeds. They also eat as they travel and end up dropping the seeds further away from the parent trees, which gives the seeds a chance to grow in areas without competition from the same tree species. This relationship between the gibbon and fruiting trees helps maintain the tree population and forest regeneration.
The International Union for Conservation of Nature lists the Abbott’s gray gibbon as Endangered (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Habitat loss is the primary threat to the gibbon’s survival. Borneo is a major source of timber products and they have lost about 1% of their forest every year since 1973! Gibbons are completely dependent on forests with large fruiting trees for their survival, so deforestation has a direct effect on their population numbers.
These gibbons are hunted for bushmeat and are sometimes captured to be kept as pets. Logging activities (e.g. cutting down trees, making roads to transport timber) have exposed the gibbons to open areas and humans, which makes capturing the gibbons easier.
There has been a decreasing population trend for the Abbott’s gray gibbon because of habitat loss, human population growth, and the conversion of forests into agricultural land to feed the human population. Existing populations are fragmented or limited to pockets of forest that are still intact and can provide an adequate amount of food for the troops.
The Abbott’s gray gibbon is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. This listing gives the maximum level of protection under this treaty.
National laws in Indonesia and Malaysia are the main conservation tools to protect the species from extinction. National parks remain important refuges where logging and hunting are restricted. Given the low numbers and the taxonomic reclassification of Abbott’s gray gibbons, there is a lack of research on the number of individuals still in the wild. The decreasing population of Abbott’s gray gibbons makes further studies on their feeding and ecology even more difficult.
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Written by Acima Cherian, January 2024