Gibbons are the smallest of the extant ape species and are, with the exception of the Genus Homo, the most numerically successful. There are four extant taxa, all of which are found in tropical forests of south-eastern Asia: Symphalangus, Nomascus , Hylobates and Hoolock. Only the siamang (Symphalangus syndactylus) and two hylobatids – the white-handed (H. lar) and agile gibbon (H. agilis) – have overlapping distributions.
Morphologically, gibbons are very different to their ‘great ape’ counterparts. Not only do they share several primitive characteristics with old-world monkeys but they exhibit exaggerated limb proportions and extreme basal separation of the first digit of both hands and feet. Like most primates, gibbons are predominantly arboreal. Unlike other arboreal primates which are largely obligate quadrupeds, gibbons have evolved to travel and forage using three main modes of horizontal locomotion: leaping, bipedalism and, largely and primarily, brachiation.
Brachiating is, put simply, a method of locomotion whereby the animal is suspended and travels hand-over-hand, much like a child on the monkey bars in a playground. Gibbons are the only ape – and one of only two primate taxa – which utilise brachiation as the primary mode of locomotion. The other primates said to brachiate are the spider monkeys (Ateles spp.) though their reliance on their prehensile tails disqualifies them as true brachiators. Using this method, gibbons are able to move through the canopy at great speed, often using their momentum to cross large gaps between trees. They have even been reported to snatch birds from the air. Brachiating also allows the species access to relatively inaccessible food as they are able to pull thinner fruit-laden branches, which would otherwise be inaccessible, down and towards themselves.
To facilitate such a lifestyle, gibbons have evolved several adaptations, primarily in the forelimbs. Their arms are greatly elongated and their hands permanently hooked, allowing the animal to swing like a pendulum, greatly increasing the efficiency of the movement. Uniquely, gibbons also have a ball-and-socket-type joint in the wrist, granting increased agility and allowing sudden changes of direction, even while moving at high speed.
Though gibbons are famed for brachiating, there were few studies conducted which examined their musculoskeletal structure and sought to link that data to the mechanical action of brachiation. Michilsens et al. (2009) compared gibbons to ‘occasional brachiators’ such as chimps and bonobos, the semi-brachiating spider monkeys and non-brachiating species. They found that while the organisational structure of gibbon musculature is broadly similar to that of other primates, there are differences in shoulder flexors, extensors and rotator muscles which result in an increased capacity for power generation. Further, gibbon wrist and elbow flexors are capable of generating a high degree of force and the species exhibit exaggerated muscular proportionality with the greatest density around the shoulders. This adds to the pendulum effect and can be used by a flexing of the arm to change various facets of the swing. Tellingly, muscle dimensions in different gibbons species are directly comparable when body mass is normalised, meaning that the largest gibbon (the siamang) exhibits the same relative muscular dimensions as the smallest (Nomascus sp.).