Feeding on non-plant food items by Western hoolock gibbon ( Hoolock hoolock )

The distribution and quality of food resources are generally recognized as the pre-eminent factors explaining much interspecific and intraspecific variation in the behaviour of non-human primates. The diet of non-human primates is characterized by large portion of plant items such as fruits, flowers and leaves and also contributes a small portion of nonplant food items. Primates that live in seasonal forests often show predictable responses of feeding to fluctuating resources. Generally during the dry season, when preferred food resources become a limiting factor, primates often consume non-plant food items. These mainly include insects, caterpillars and larvae as well as other items like bird’s eggs, soil from salt-licks, termite nests, etc. which are seldom eaten by primate species, including gibbons and contribute only a small part of their diet. Although the quantity of these food items is usually small in size, they provide larger amounts of energy, protein and fat per unit mass than most other food items of primates, such as fruits, flowers and leaves. But, while studying the ecology and feeding behaviour of western hoolock gibbon (Hoolock hoolock) in an isolated forest patch of Hollongapar Gibbon Wildlife Sanctuary (HGWLS), Assam, we recorded high-scale feeding evidences on non-plant food items, particularly on insects by the species during the hot premonsoon (March–May) and monsoon (June–September) seasons, besides the presence of ripped fruits in their major food plants. This uncommon food items consumption may form a regular part of their diet in small fragmented and isolated forest patches. H. hoolock is distributed in the monsoon evergreen and semi-evergreen rainforests of North East India, south of the Brahmaputra, eastern Bangladesh, northwest Myanmar and west of the Chindwin River. In NE India, gibbons are distributed in 22 protected areas, including 6 national parks and 16 wildlife sanctuaries in addition to several populations in non-protected areas. Due to large-scale deforestation in primary habitats of H. hoolock in the entire distribution range, particularly in NE India, most of the gibbon population has become fragmented and isolated in small forest patches and the population has been declining drastically in the last two decades. As a result, the western hoolock gibbon has become endangered under the IUCN category in India and critically endangered in Bangladesh. It is also included in Appendix-I of CITES and listed in Schedule I of Wildlife (Protection) Act, 1972. HGWLS is a small isolated forest patch covering an area of 20.98 sq. km and situated between 2640–2645N and 9420–9425E at an elevation of 100– 120 m amsl. The sanctuary is surrounded by tea gardens, agricultural fields and small villages and is located in the south bank of the mighty Brahmaputra River system in Mariani area, Jorhat district, Assam, India. The forest type of HGWLS is ‘Eastern Alluvial Secondary Semi-Evergreen Forest (1/2/2B/2S2)’ under Moist Tropical Forest of India. This is the only sanctuary in NE India which provides habitat for seven primate species, viz. Trachypithecus pileatus, Macaca assamensis, Macaca arctoides, Macaca leonina, Macaca mulatta, Nycticebus bengalensis and Hoolock hoolock. Twenty-five groups of H. hoolock comprising 101 gibbons (mean = 4.4  SE 1.1 individuals per group) were reported from sanctuary. We conducted a study on feeding ecology and behaviour of H. hoolock in two groups (group A – one adult male and one adult female; group B – one adult male, one adult female, one sub-adult male and one infant) for a period of one year from January to December 2011 in HGWLS after initial habituation of both the groups for three months. Selected groups were followed from dawn to dusk between 0600 and 1600 h for 12 days every month to collect field data on their basic feeding ecology, diet composition and behaviour via focal animal sampling. Instantaneous sampling (every five minutes) was used to quantify activity budget and behaviour. Continuous sampling was used every time the focal animal began to feed. The total observation time was 1440 h during the one-year study period, accounting 720 h for males and females belonging to both the groups. The different behavioural activities recorded were categorized as feeding, resting, travelling, calling and social behaviour. In the present study, ‘feeding’ refers to catching followed by chewing and then ingestion of plant and non-plant food items and this excludes the time for foraging, as it has been included under travelling. The focal animal was rotated between the adult male and female of the group every one hour. There was no significant variation in feeding time on nonplant food items between the selected groups and also between the sexes. Therefore, data of both groups were pooled together for final analysis. We calculated the percentage of the daily feeding time on different food categories to the total feeding time for each month, according to Gupta and Kumar.