The Hoolock Gibbon represents a significant component of Asia's biodiversity, inhabiting a distinctive ecological niche that spans across the continent's rainforests. Their habitats extend from the semi-evergreen rainforests north of the Isthmus of Kra to the dense evergreen rainforests of the Sunda Shelf, which constitutes the core of their habitat range. Moreover, substantial populations inhabit the more seasonal forests of mainland Asia, including evergreen forest enclaves under maritime climatic influences across India, Indochina, Thailand, Myanmar, and Bangladesh (Smith & Wang, 2023). Notably, Northeast India is recognized for its remarkable faunal and floral diversity, serving as a habitat for the highest diversity of primates in the country. This includes the Western Hoolock Gibbon (Hoolock hoolock), which is distributed widely across the northeastern states, from south of the Brahmaputra River to east of the Dibang River. Significant populations are found in several conservation areas such as the Hollongapar Gibbon Wildlife Sanctuary, Kaziranga National Park, Manas Wildlife Sanctuary, and Namdapha National Park (Das et al., 2022).
The Hoolock Gibbon's habitat is not restricted to a single forest type. These primates show adaptability by residing in both lowland and hilly forest regions, primarily within primary evergreen rainforests and semi-evergreen rainforests, and occasionally in semi-deciduous forests. Their habitats are characterized by a subtropical climate with hot, wet summers averaging temperatures around 30°C, and cool, dry winters with mean temperatures around 10°C (Kumar & Singh, 2021). This range of forest types occupied by the Hoolock Gibbon not only demonstrates their resilience but also highlights the fragility of their existence, reliant on specific climatic and ecological conditions. The discovery of the Eastern Hoolock Gibbon (Hoolock leuconedys) in Arunachal Pradesh in 2006 further broadened the recognized diversity and range of gibbons in India, underlining the region's significance as a biodiversity hotspot and underscoring the imperative for ongoing conservation efforts to safeguard these unique primates and their habitats (Gupta & Sharma, 2023).
The exploration of "dancing" behaviours in gibbons offers a compelling lens through which to examine the intricate social and communicative behaviours exhibited by these primates. Traditionally associated with human cultures, dancing involves rhythmic, patterned movements employed for expression, social bonding, and courtship. Intriguingly, similar manifestations observed in gibbons suggest that such behaviours are not confined to humans but are evident across the animal kingdom, underscoring a broader biological phenomenon. Gibbons, adept in brachiation or skilled tree-swinging, demonstrate behaviours that closely mirror dancing. These are not merely functional movements but are imbued with significant social and communicative meaning. Gibbons partake in rhythmic movements, such as swaying on tree branches, shifting body weight rhythmically, and engaging in repetitive runs, often accompanied by vocalizations like hoots, grunts, or squeaks, which serve to amplify the communicative dimension of these displays (Barelli et al., 2016). These dance-like behaviours, primarily observed in contexts of courtship and play, reveal the multifaceted roles they play within gibbon societies. For instance, female gibbons have been noted for their rhythmic movements and head bobs, aimed at sexually soliciting males. Similarly, Siamangs, which belong to the gibbon family, engage in distinctive movements such as upward thrusts and jerky body motions during sexual and playful interactions, maintaining intricate visual and vocal communication with their counterparts (Chivers, 2001). The functions of these dance-like displays are multifaceted, encompassing aspects of sexual attraction, social cohesion, and possibly serving as mechanisms for energy release or stress relief. The evolutionary origins of such behaviours remain a focal point of interest and debate among researchers, with some hypothesizing that the rhythmic movements observed in gibbons and other non-human primates might share a common ancestral root with human dance, suggesting a profound evolutionary link in the utilization of movement and rhythm for communication and social integration (Davila-Ross et al., 2014).
Moreover, the presence of dance-like behaviours in both birds and primates prompts a broader examination of the evolution of communication and mating strategies across species. Despite the evolutionary divergence and the likely distinct neurological pathways governing these behaviours in birds and primates, the similarities in gestural communication underscore a universal facet of social interaction and expression within the animal kingdom. This cross-species perspective not only enriches our understanding of the complexity and diversity of communicative behaviours in nature but also highlights the importance of continued observation and research efforts to decipher the nuances behind these fascinating displays (Fitch, 2016).
Gibbons, predominantly arboreal creatures, engage in a unique form of movement known as brachiation, accounting for 70–80% of their locomotion. This method entails swinging from one branch to another utilizing their extended arms, enabling them to traverse up to 15 meters in a single swing and reach speeds surpassing 55 kilometres per hour. Other forms of movement, including leaping, climbing, and jumping, make up 16–25% of their locomotion, with terrestrial bipedality occurring infrequently and usually in a clumsy manner. Their preference for elevated canopy living and avoidance of ground level underscore their adaptation to an arboreal lifestyle, characterized by remarkable dexterity (Cheyne, 2010).
In terms of social behaviour, gibbons allocate a small fraction of their day to activities such as play and grooming, which are essential for maintaining social bonds, mitigating aggression, and promoting group hygiene. Grooming, in particular, serves not only to cleanse and remove parasites but also to reinforce social ties within the group. This activity is believed to vary seasonally, potentially influenced by factors such as photoperiod or foraging pressures. Play, predominantly observed among juveniles, plays a significant role in social learning and development, with adult males occasionally participating, especially in the absence of other playmates (Palombit, 1997).
Gibbons' diet is chiefly frugivorous, focusing on small, dispersed sources of ripe fruits, which influences their territorial and monogamous family group structure. This dietary preference places them in direct competition with large birds for small, sugary fruits, necessitating their reliance on small fruiting trees to avoid competition with larger primate groups and orangutans. Their feeding behaviour is characterized by active foraging throughout the day, with a diet that includes a significant portion of plant reproductive parts, such as fruits and flowers, and a considerable amount of animal matter for protein (Bartlett, 2007). The reproductive cycle of gibbons features a gestation period of approximately seven months, with the norm being the birth of single offspring. The young are born with hairless ventral areas, heavily depending on maternal warmth and care. Nursing extends for about two years, with offspring remaining with their parents well into adolescence. Sexual maturity is attained around 6–8 years of age in captivity, although this can vary. The extended period of parental care and long birth intervals underscore the substantial investment gibbons make in their offspring, highlighting their complex social structures and the importance of familial bonds (Lappan & Whittaker, 2009).
(Table 3.)
Ecologically, gibbons play a pivotal role in seed dispersal, contributing significantly to the natural regeneration of forests. Their feeding habits and fruit consumption patterns facilitate the dispersal of seeds, impacting the distribution and diversity of forest plant species. The density of gibbon family groups is closely linked to food availability, reflecting these primates' adaptation to their environment and their integral role in forest ecology (McConkey & Chivers, 2007).