Agile Gibbon, Hylobates agilis
AGILE GIBBON
Hylobates agilis
Geographic Distribution and Habitat
Agile gibbons, sometimes called black-handed gibbons, are widespread throughout Sumatra, northern Malaysia, and Southern Thailand. They live in evergreen and semi-deciduous forests, from lowland swamps to higher-elevation montane forests. They prefer high canopy elevation, sometimes as high as 4,593 feet (1,400 m).
Gibbons are an extremely varied primate group with many species and potential subspecies. Their taxonomic classification has been revised multiple times, and extensive genetic studies have been undertaken to clarify the differences between gibbon species. Some researchers think agile gibbons should be reclassified to have subspecies with individuals based on their geographical location and fur color variation. Therefore, sometimes their lineage is divided into two subspecies—the mountain agile gibbon, Hylobates agilis agilis, and the lowland agile gibbon, Hylobates agilis unko.
Currently, agile gibbons are considered monotypic, without any subspecies. However, hybrids (offspring resulting from the mating of different species) can confuse this classification structure. Agile gibbons often inhabit areas close to other gibbon species, leading to hybrids between them and the lar gibbon (H. lar) and Müller’s gibbon (H. muelleri) in the wild.
Size, Weight, and Lifespan
Agile gibbons are tailless primates that measure 17-25 inches (44–63.5 cm). Males are larger than females. Males weigh 9.7 to 16.3 pounds (4.42 to 7.37 kg.), and females weigh 10 to 15 pounds(4.5- 6.8 kg). The oldest recorded age of an agile gibbon was a female kept in captivity who was 49 years old.
Appearance
These tailless primates have the typical gibbon appearance of long arms (1.5 times longer than their legs) and large, curious brown eyes that peer from a hairless face. Their body is covered in short fur, and agile gibbons have a distinctive pale or white fur band across their brow. Agile gibbons can have different color forms or are polymorphic, with fur colors ranging from light buff to dark brown. Blond coloration is typical in Sumatra, while darker forms are common in Malaysia. Blonde forms have a line of dark brown fur along their spine that sometimes carries through their inner thighs. Dark forms typically only have contrasting pale colors on their face, and the rest of their body is covered in dark brown fur. Agile gibbons are sometimes called dark-handed gibbons because both color morphs have dark hands, which stand out in contrast in the case of the blond morph.
Males and females can be differentiated by their facial markings. Males have pale cheeks and sometimes a pale beard that meets together at their chin. Males have long tufts of fur covering their genital areas, which can be a lighter color in dark morphs. Females also have hair tufts, but they are less prominent and are usually the same color as the rest of their belly fur.
Young agile gibbons have a white face ring that connects their brow and chin. As they age, the coloring changes. Males retain the white brow and cheek patches. Adult females’ white brows become divided over each eye, and they lose the pale coloring on their cheeks.
Diet
They primarily eat high-energy fruits and flowers that allow gibbons to be active throughout the day. They also eat leaves and insects when fruit sources are low and if they need additional protein. About 60% of their nutrition comes from fruits and 39% from young leaves.
Behavior and Lifestyle
They are arboreal (tree-dwelling), and as their name suggests, they are incredibly skilled at moving fast through the canopy. Agile gibbons make Tarzan look slow and clumsy. Their arms are 1.5 times longer than their legs, and their first and second fingers have webbing near their palms. This trait helps them firmly grasp branches above their head and swing forward to catch the next branch. This distinctive movement pattern in gibbons is called brachiation. Using this technique, they can travel 35 miles per hour (56 kph) through the canopy without ever coming to the ground.
Gibbons’ long arms are amazing at aerial acrobatics in trees but are not ideal for walking on the ground. Gibbons walk on two feet (bipedal) and hold their arms above their head for balance because their arms are too long and would drag on the floor if they were to walk on the ground on all four limbs (quadrupedal). This awkward posture makes them walk in a slow, swinging motion, and they avoid this inefficient method of travel when possible.
Gibbons are diurnal and usually awake before sunrise. They reach their sleep sites high in the canopy before sunset.
Agile gibbons are territorial, and mated pairs aggressively call and approach intruders to chase them away. If the intruder is a solitary male, then only the male of the mated pair acts territorial. If the intruder is a solitary female, then both the male and female of a pair will approach and start calling at the stranger. Researchers think these territorial strategies help maintain the monogamous status of agile gibbons, particularly in the case of mated females chasing away strange female intruders. Males lead most of the territorial activities to protect food resources and the family’s safety. Mated females are the primary caretakers of young gibbons, so mothers will not risk an aggressive encounter that may put their child in danger.
Agile Gibbons have arms 1.5 times longer than their legs and webbed fingers, which help them swing through the forest canopy at high speeds.
Females are famous for their showy “great call” songs, which can be heard from half a mile away.
Their development from infant to adult is slow and can take up to 8 years.
They can be long-lived. The oldest recorded agile gibbon was 49 years old.
Agile gibbon groups typically consist of about four individuals: a monogamous male and female pair, an infant, and a juvenile.
An agile gibbon’s day starts before sunrise with the male’s solo songs from their sleeping trees. This solo bout signals the family members to wake up. Female agile gibbons tend to lead the family as they search for food.
Much of their daily energy is used to find food and defend their territory. They travel as far as 0.83 miles (1.34 km) daily for food. Their ability to travel large distances in the forest means they have an extensive home range and can defend a large area. Their home range can be between 40 and 106 acres (16 and 43 ha). Up to 75% of their home range can be considered a territory the gibbons will defend. Smaller family groups hold on to smaller territories. They will tolerate other agile gibbons outside their territory, and multiple family groups can have overlapping home ranges outside the territory.
During the fruiting season, food sources are clumped together, and gibbons stay in one area longer to feed. During the drier months, they must travel further to find enough food and adjust their diet to incorporate more leaves and insects.
These highly energetic primates are most active in the mornings. They spend the cooler mornings and early afternoons foraging for food, with a period of rest during the hottest time of the day. Their activity level tapers towards the evening, and they select adequate sleeping trees to rest in for the night hours before sunset.
Gibbons prefer sleeping high in the canopy, where there is less competition with other primates, and predators are unlikely to find them.
Vocal communication is efficient for gibbons because they live in dense canopies where it isn’t easy to see each other from afar. Gibbon voices can be used to distinguish an individual, their sex, and even their mated status. This is important because gibbons can tell when a caller is a close relative (such as a sibling) and avoid aggression or mating interest.
Gibbons are famous for their spectacular song, “The Great Call,” which consists of a series of musical notes that change in pitch and frequency and can be heard from about 0.5 miles (1 km) away.
The female gibbon’s great call is stunning, with a quiet start and a longer crescendo towards the end of her call. Males will abruptly stop calling and listen when their mated female starts her great call. Towards the end of her call, he will rejoin in a “coda call,” a distinct phrase of notes that acts as the concluding part of the great call. The coda call is a way to let listeners know that the singing female is mated and their territory is jointly defended.
Interestingly, immature male agile gibbons have been observed making sounds like the female’s great call, which suggests that males can develop a great call (like females) and learn to practice their calls from their mother. In the wild, adult males let the female take center stage while making her great call.
Great calls act as a territorial signal that lets other gibbons know that a strong and experienced gibbon lives there. Gibbon songs are learned and modified through individual interactions with other gibbons. The more complex calls come from listening to different songs and changing them. Older and more experienced gibbons tend to have more complex vocalizations. Louder calls usually signify a larger animal with a strong lung capacity. Therefore, gibbon vocalizations are a product of their genetics, environment, and conspecific interactions.
Males and females make sounds such as simple hoots and more complex “whoo-aa” phrases. Males also have solo songs, usually sung in the early morning, to display their presence and let the family know it is time to wake up. Male and female gibbons also participate in duets after the first feeding round. These duet calls help pairs contact each other when they get separated while they forage. Duetting also has the social benefit of strengthening the pair bonds, similar to how human relationships improve through better communication.
Facial expression is an important part of primate communication. Still, it is difficult to decode the different meanings behind subtle changes in species that are not human. Studies in agile gibbon infants’ ability to recognize other faces have shown that at 4 weeks, they can tell apart faces and even prefer more familiar faces. This may be similar to “stranger anxiety” in human babies and indicates an evolutionary development of facial recognition traits in primates.
Hormones and urine are common ways primates communicate their general health and reproductive status to each other (e.g., females who are ready to mate will have more estrogen in their urine, which is readily detected by males).
Gibbons have a sternal gland, which is a primitive feature of apes. The function of the sternum gland is not clear.
Agile gibbons form monogamous pairs, in which one male and female mate and stay with each other for their whole lives, a rare case for primates.
When females are ready to mate, their genitalia or external reproductive organs swell. This is a visual clue to the male that mating will result in successful offspring. Females are pregnant for 7-8 months (the gestation period) and will give birth to a single infant at a time. Gibbon development is slow; it usually takes about 3 years for an infant to be completely independent from their mother. As a result, mated females typically give birth only once in 3.2 years.
Infant gibbons can take about 8 years to reach sexual maturity. Agile gibbon babies cling to their mother’s belly and are considered infants for the first two years. Between two and four years of age, they are juveniles and become more independent and leave their mother’s side for longer. When they are four to six years old, they are about half the size of adults and are considered adolescents. Sub-adult gibbons are about the same size as the adults but are not mated yet. Once they reach sexual maturity, male and female sub-adult gibbons will disperse from their natal troop (where they were born) to find mates in another forest area.
Agile gibbons share their habitat with other gibbon and primate species with similar diets. In forests, fruits are high-value food sources often fought over by many species. Agile gibbons are quick and do not depend on strength to compete with larger species, like orangutans. They avoid competing for food by utilizing the higher canopies, which larger primates tend to avoid because of their size and less acrobatic abilities.
As frugivores that travel long distances, they play a role in dispersing seeds throughout the forest. This way, fruiting trees can regenerate far away from the parent plant and avoid competition for space and water.
Agile gibbons are charismatic and adapt to different habitats in their geographical region. Their quick movements, showy songs, and human-like walking behavior capture the imagination of people, making these agile gibbons important flagship species for the gibbons and primates of Asian forests.
The International Union for Conservation of Nature lists the agile gibbon as Endangered (IUCN, 2019), appearing on the IUCN Red List of Threatened Species.
Agile gibbons are widespread and are found in many places. Still, they are endangered because their population numbers are low in these habitats. Their main threat is deforestation caused by logging and mining activities that have cut down forests for decades. Habitat loss also happens due to human encroachment, clearing forests to build houses, roads, and other infrastructure (for electricity, transport, agriculture etc). Deforestation reduces food sources (fruiting trees) for agile gibbons, and it fragments the forests, which means that agile gibbons cannot travel from one open part of the forest to another using the canopy alone. Forest fragmentation and roads force gibbons to the ground, where they are slower and more prone to accidents, car collisions, and exposure to ground predators.
The unique appearance and cute, childlike face make gibbons valuable to illegal pet traders. Forest fragmentation makes gibbons easier to spot, especially when forced to the ground, which makes them more vulnerable to trappers.
Agile gibbons have a slow reproduction rate, and young gibbons take a long time to mature and form mating pairs. So, gibbon populations have difficulty recovering from threats such as reduced habitat quality and high mortality from illegal hunters.
The agile 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.
National and international laws in Indonesia, Malaysia, and Thailand protect agile gibbons. Larger national parks such as Bukit Barisan Selatan National Park in Sumatra and Ulu Mudah Wildlife Reserves in Malaysia are their best hope of habitat protection. However, illegal trade and deforestation are still rampant in these regions.
Conservationists have tried to increase public involvement and awareness through programs such as the New Scientist and Lego “Build the Change” competition, inviting young inventors to help solve forest connectivity issues that gibbons face.
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Written by Acima Cherian, December 2024