Introduction

In India, gibbons are restricted to tropical and subtropical forests of the southern bank of the Dibang-Brahmaputra river system in the seven states of the Northeast. Western hoolock gibbons (Hoolock hoolock) occur in all of the states: Assam, Arunachal Pradesh, Meghalaya, Mizoram, Tripura, Nagaland and Manipur. The eastern hoolock gibbon (Hoolock leuconedys) occurs in two: Arunachal Pradesh and a small part of Assam (Chetry and Chetry 2010).

Seven other primates occur in Arunachal Pradesh besides the two hoolock gibbons (Borang et al. 1993; Singh 2001). Chetry (2002) and Chetry et al. (2003) reported on the primates in Namdapha National Park and Pakke National Park, respectively, and Chetry (2004) described the diversity and status of the primates in the Eagle Nest and Sessa Orchid wildlife sanctuaries. Sinha et al. (2005) described a new macaque from the Tawang district in the eastern part of the state, the Arunachal macaque (Macaca munzala), and Kumar et al. (2005) indicated the presence of the Tibetan macaque (Macaca thibetana), although this has yet to be confirmed. There are also reports on the status of primates in Dibang Valley Wildlife Sanctuary (Chetry and Medhi 2006; Chetry and Chetry 2009). There have been studies on the distribution of the western Hoolock gibbon in the districts of Tirap, Changlang and Lohit (Mukherjee et al. 1988, 1991, 1992; Choudhury 1991; Biswas et al. 2007). Das (2002) studied the behavior of H. hoolock in Namdapha, and Das et al. (2006) it was who reported finding the eastern hoolock (H. leuconedys) in Arunachal Pradesh, the first record for India (see also Chetry et al. 2007, 2008). Here we present our findings on the status and distribution of the eastern hoolock gibbon in the Mehao Wildlife Sanctuary in the Lower Dibang Valley district of Arunachal Pradesh. We also report on the anthropogenic pressures that the hoolock gibbon and its habitats in the sanctuary are facing.

Study Area

The Mehao Wildlife Sanctuary (281.5 km²) is in the Lower Dibang Valley district of Arunachal Pradesh (93°30′– 95°45¸, 28ï05′–8°15′N) (Fig. 1). The topography is undulating and hilly, and altitude ranges from 400 to 3,568 m above sea level. The forest types change with altitude: tropical evergreen forest, tropical semi-evergreen forest, subtropical evergreen forest, temperate broad leaf forest and temperate conifer forest. The human population living around the sanctuary is primarily of the local Idu and Padam tribes of Arunachal Pradesh. Mehao is a sanctuary for numerous threatened species of the Indian flora and fauna, and much has yet to be explored and properly documented.

Methods

Line transect survey

A population survey was carried out in the Mehao Wildlife Sanctuary from October, 2008 to April, 2009. The trails were set up to cover all representative areas of the park (Mueller-Dombois et al. 1974; Kent et al. 1994). The total length of the trails was 800 km. Three observers walked the trails, covering 10–12 km per day from 06:00 to 15:00 h, during 80 days of field surveys. Observers walked slowly along the transects, pausing at intervals of 500 m. When gibbons were seen, the observers recorded the location using a Global Positioning System (GPS), and noted the group size and composition. At 500-m intervals, and at every location where gibbons were encountered, the observers estimated the tree height and canopy cover in a circle of radius 10 m, and also took note of the evidence and degree of grazing and logging in the study area.

Figure 1.

Location of the Mehao Wildlife Sanctuary, Arunachal Pradesh, Northeast India.

Results

Population size

We saw 88 individuals in 28 groups in the sanctuary at altitudes of 300 m to 1,713 m above sea level (Table 1). Locating calling gibbons, we estimated a further 129 groups at altitudes ranging from 142 m to 1,865 m above sea level (Table 2, Fig. 2).

Seventy-five percent of the 28 groups seen were at altitudes below 1,000 m; 25% above 1000 m. Of the 21 groups seen below 1,000 m, most (16 groups) were at or below 500 m. For the call counts, 69% of the calls were recorded at altitudes below 1,000 m, while 31% were above 1,000 m. Of the calls recorded from below 1,000m, 60% were at altitudes of 500 m or lower, and 40% were at altitudes of 500 m to 1,000 m (Tables 1 and 2).

Figure 2.

Location of eastern hoolock gibbon (Hoolock leuconedys) groups seen and recorded by their calls in the Mehao Wildlife Sanctuary, Arunachal Pradesh, October, 2008 to April, 2009.

Sighting time and calling times

We saw 25 of the 28 groups before 12:00, and only three groups after mid-day. Sightings were evenly distributed through the morning—52% of them between 06:00 and 09:00 and 48% between from 09:00 and 12:00.

Of the 129 groups registered from calling bouts during the survey, 92.2% called before 12:00, and only 7.8% of the calls were heard after 12:00. Of the morning calls, 42.4% were between 07:00 and 09:00, and 57.6% between 09:00 and 12:00. The average duration of the calls was 18.26 minutes, with a range of 5 to 35 minutes.

Table 1.

Sightings of eastern hoolock gibbons (Hoolock leuconedys) in the Mehao Wildlife Sanctuary, Arunachal Pradesh, October, 2008 to April, 2009.

Threats

During the survey, we also tried to identify the threats to the hoolock gibbon population and other wildlife of the sanctuary. The conversion of forest for the commercial cultivation of orange, ginger and cardamom is increasingly widespread in the area and is undoubtedly a serious threat to the gibbons, reducing habitat and fragmenting the remaining forest. All the primates there are hunted for their meat and skins and as agricultural pests, except for the hoolock gibbons (the Idu people of the area do not hunt gibbons although, elsewhere, other communities do). The local communities at Mesao have a long tradition of hunting, which is a major threat to wildlife in the sanctuary. The survey team recorded several incidences of hunting of such as takin, serow, musk deer, sambar and barking deer, and bears by the different local communities.

Table 2.

Eastern hoolock gibbon groups located by their calls in the Mehao Wildlife Sanctuary, Arunachal Pradesh, October, 2008 to April, 2009.

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A newly constructed, high-tension, electric power line along the southern boundary of the sanctuary, running parallel to the Roing-Koronu-Bhisma Nagar road, is emerging as a potential threat to the wildlife of the sanctuary, especially the arboreal species. A male gibbon was killed due to electrocution during the survey.

There is widespread encroachment of the forest for human settlements and for small-scale agriculture (horticulture, tea plantations, ginger cultivation, jhum cultivation), along with illegal felling of select trees, and the extraction of non-timber products such cane, bamboo, and medicinal plants. Cane and bamboo are also extracted commercially from the sanctuary.

In practice, the sanctuary does not have a well-marked boundary. Other than declaring the sanctuary, the forest department seems to have not taken any initiatives to convey the message to the local people. Most of the local people have no clear understanding of the existence of the sanctuary or its boundaries, and still think that the land belongs to their forefathers and they have the right as such to hunt and to carry out their day-to-day activities as they always have.

Discussion

This is the first assessment of the eastern hoolock gibbon population in the Mehao Wildlife Sanctuary. Das et al. (2006) had first reported the species from the district of Lohit, Arunachal Pradesh, and Chetry et al. (2008), discovered it also in the district of the lower Dibang Valley. With an estimated 157 groups, the Mehao Wildlife Sanctuary is clearly an important stronghold for this species.

Das et al. (2006) estimated an average group size of 3.37 for the eastern hoolock gibbon. The average size of the 28 groups we observed was slightly smaller at 3.14. Groves (1971) reported that eastern hoolock gibbons can be found between 1,067 m and 1,219 m above sea level in Myanmar and China. The Indian populations, however, also occupy the lowlands. Das et al, (2006) found them occurring at altitudes of 122 m to 1,075 m, and in the Mehao Wildlife Sanctuary they range from 142 m to 1,865 m; higher and lower elevations than had been recorded previously, although in Mehao the majority of the groups we saw were below 500 m. The gibbons were found to occupy primarily the subtropical evergreen and semi-evergreen forests that are predominant in the lower elevations of the sanctuary.

During the surveys, we recorded four other primates besides the eastern hoolock gibbon. We saw rhesus macaque (Macaca mulatta), Assamese macaque (Macaca assamensis), and the capped langur (Trachypithecus pileatus), and the slow loris (Nycticebus bengalensis) was reported to occur there by the local people and rangers. It is possible that stump-tailed macaques (Macaca arctoides) and pigtail macaques (Macaca leonina) may also occur the sanctuary, but they may have been extirpated or reduced to extremely low numbers by hunting. This assemblage of five, possibly seven, species of non-human primate establishes the sanctuary as one of the most primate-rich areas in Arunachal Pradesh (and the entire country), but only the hoolock gibbons can be seen and heard regularly. The other primates were very scarce. Biswas et al. (2007) also found stump-tailed and pigtailed macaques to be very rare in the Lohit and Changlang districts of Arunachal Pradesh. The low numbers of primates other than Hoolock gibbon in the Mehao sanctuary indicates regular hunting. Only the centuries-old traditional belief of the local “Idu Mishimi” tribe is protecting the hoolock gibbons there. Else-where in Northeast India, the gibbon populations are declining severely due to hunting (Das et al. 2005), habitat loss and habitat fragmentation (Chetry et al. 2007). Habitat loss and fragmentation resulting from encroachment, jhum cultivation (traditional slash and burn cultivation) and other horticultural and agricultural practices (especially ginger, cardamom, orange and tea cultivation) are major threats to the eastern hoolock gibbons and to other wildlife of the sanctuary.

Figure 3.

Adult female (left) and male (right) eastern hoolock gibbons (Hoolock leuconedys). Photo by Dilip Chetry.

Effective conservation measures involving the local community and with a long-term vision are needed, along with regular population monitoring and ecological studies, if the eastern hoolock gibbon is to continue to thrive in this sanctuary. Infrastructure and facilities, and strict vigilance on the part of forest department is also vital for protecting and conserving all the rich wildlife there.