Response of hoolock gibbons (Hoolock hoolock) to habitat degradation in the Borajan Reserve, Assam, India.

Authors:

Gayatri Thampy[1], Jayanta Das², and W. Scott McGraw¹

Affiliations:

¹Department of Anthropology, The Ohio State University, 114 Lord Hall, 124 West 17^th Avenue, Columbus, Ohio 43210-1364

²Animal Ecology and Wildlife Biology Lab, Department of Zoology, Gauhati University, Guwahati, Assam – 781014, India

Short title:

Hoolock gibbons and habitat degradation

Abstract

This paper presents preliminary feeding and ranging data on a group of hoolock gibbons (Hoolock hoolock) (Mootnick and Groves 2005) inhabiting the Borajan reserve of the Bherjan-Borajan-Podumoni Wildlife Sanctuary in Assam, India. The five km² reserve and its inhabitants are under intense pressure from the surrounding human population. The study was carried out from June 20^th to July 30^th 2003 during which time the group consisted of an adult male and adult female that occupied a home range of approximately 3 ha, had a maximum day range of 200 m, spent 17 % of the observation period feeding, and subsisted on similar proportions of fruit (44.5%) and leaves (42.7%). We hypothesize that the group’s activity levels (71% of the day resting) and strikingly folivorous diet are responses to a paucity of preferred food items in a heavily degraded and extremely small territory.

Keywords: Hylobatids, feeding ecology, folivory, frugivory, aggression.
Introduction

Hoolock or White-Browed gibbons (Hoolock hoolock) (Mootnick and Groves 2005) are large hylobatids with a distribution extending from north-eastern India across Bangladesh and Burma to south-western China (Mukherjee 1986, Geismann 1995, Das and Srivastava 2001). Information on the feeding habits of hoolocks has existed for well over one hundred years and the species, like other gibbons, is characterized by a predominantly frugivorous diet (Blanford 1888-1891, Candler 1904, McCann 1933, Mukherjee et al. 1982, 1988, Choudhury 1991). The large geographic range of hoolocks extends outside the tropics and these apes are thus subject to strong seasonal effects on the availability of preferred food items (Gittins and Tilson 1984). This paper provides preliminary information on activity patterns, range use and gross feeding percentages on one hoolock group observed during the monsoon season in Assam, India. We compare our data with those from other groups in India and Bangladesh.

Methods

The Borajan reserve is part of the 7.22 km² Bherjan-Borajan-Podumoni Wildlife Sanctuary. The Borajan reserve is a 5 km² lowland, tropical, wet, evergreen rainforest block surrounded by villages, tea estates and agricultural fields (Figure 1). Borajan (27°25'N - 95°22'E) is located in the Tinsukia District of Assam and lies at an average elevation of 122 m (Kakati 1997). The forest is heavily logged, dominated by Diptocarpus spp. and contains a high density of secondary-forest shrubs, ferns, and lianas. The canopy in the study group’s region was discontinuous and reached a maximum height of approximately 20 m. Most of the reserve is crossed by several 1 to 2 m deep ravines that cut the terrain into disconnected earthen mounds on which trees grow. Borajan lies in the tropical monsoonal high-humidity zone with a hot wet summer and a cool dry winter. Annual rainfall is between 1300 and 4000 mm with most rain occurring during the months of April - September (Choudhary 1990). In addition to hoolock gibbons, the sanctuary is home to three primate species: the Assamese macaque (Macaca assamensis), pig-tailed macaque (Macaca nemestrina), and capped langur (Trachypithesus pileatus).

The study was carried out between June 20^th and July 30^th 2003 during which there were eleven gibbon individuals distributed in five groups within the sanctuary. Two groups had been subjects of a prior study (Kakati 1997). We studied one group consisting of an adult male and an adult female using a time point sampling scheme (NRC 1981). At each ten-minute interval we recorded data on (1) activity budget (Feeding = feeding and foraging without travelling, Resting = sitting, sleeping, Locomotion = long distance travel, Social = vocalizations, grooming, aggressive encounters) and (2) diet (plant species, plant part consumed). We collected and dried all plants gibbons were observed feeding on and these were identified with the help of local experts and the botanical list of Kakati (1997). Home range was calculated by walking the periphery of the group’s territory and computing the circumference. Maximum day-range was calculated using reference markers such as a nearby village, forest trails or known trees in the vicinity. Dietary proportions were calculated from the proportion of feeding observations out of the total number of observations.

Results

Activity budgets and feeding proportions for both individuals are presented in Table 1. The group’s home range is approximately three hectares and the maximum day range is approximately 200 m. Twenty-four plant species were either consumed directly or harvested for insects. The dietary proportions for the male and female are listed in Table 1. During the study, fruit comprised 45% of feeding observations while leaves and insects comprised 43 % and 6% respectively. Approximately 85% of all leaves eaten were obtained from lianas, epiphytes, ferns or shrubs.

We witnessed four incidents of overt aggressive behaviour by the male towards the female. Each incident occurred while both individuals were feeding on fruit from Artocarpus chama and involved the male charging and displacing the female and then taking the Artocarpus fruit being eaten by her. In each episode, the male waited until the female had opened the fruit before charging. Immediately after each incident, the female groomed the male. On one occasion, the male was observed feeding approximately 60 yards outside his territory and was separated from the female by approximately 100 yards. Although we never observed the neighbouring group during the course of the study, we believe our male was intruding into that group’s territory by approximately 40 yards.

Discussion

Chivers (1984) states that gibbon home ranges average approximately 34 ha with some as large as 50 ha. Our survey of the literature finds that the territories of hoolock groups can be considerably larger including up to 4 km² (400 ha) (Tilson 1979, Gittins and Tilson 1984, Mukherjee 1986, Alfred and Sati 1990 a,b, Islam and Feeroz 1992, Ahsan 2001). The home ranges of three groups at Borajan are 3 ha (this study) 10.5 ha and 5.4 ha (Kakati 1997). Thus, compared to studies of hoolock and other gibbon species elsewhere, the Borajan gibbons have exceptionally small territories. Prior to this study, the smallest home range reported for hoolock gibbons was that of a group in an isolated 3.2 ha forest patch in Bangladesh (Gittins and Tilson 1984) and these authors remark, “it is unclear how this group could survive in such a small area” (Gittins and Tilson 1984: 259).”

Hoolock gibbons have been characterized as highly frugivorous (Chivers 1974, Tilson 1979, Alfred and Sati 1994) with fruit, especially figs (Ficus sp.), contributing up to 70% of their diet (Chivers 1984, Islam and Feeroz 1992, Ahsan 2001, Bartlett 1999). Many gibbon characteristics including size-monomorphism, territoriality, ranging behavior and monogamous pairings have been attributed to gibbons “systematic exploitation of small, scattered fruit patches” (Leighton 1987: 144). Recent studies indicate that some hoolock groups are characterized by significantly reduced amounts of fruit. Mukherjee (1986) found fruit and leaf intake for one hoolock group observed in Tripura India were 40% and 50%, respectively. Two hoolock groups studied at Borajan during the dry season (January – April) consumed 20.7% fruit (including 5% figs) and 60% leaves (Kakati 1997). Of the fruit consumed by one of these groups, 38% consisted of figs from a single Ficus benjamina tree (Kakati 1997). Fruit intake for our study group falls approximately midway between the neighbouring groups of Kakati (1997) and the percent of time devoted to leaf and fruit consumption are roughly similar for the male and female gibbons.

There are at least two explanations for the modest levels of fruit intake in our study group. First, the group’s 3 ha home range contains no mature Ficus trees and neither individual was ever observed feeding on figs. It is likely that overall frugivory would have been substantially higher had there been even a single mature Ficus tree. Second, seasonality has a strong effect on the availability of preferred of foods. Various authors have noted that folivory in hoolock gibbons increases during the wet (monsoon) season when fruit tends to be less abundant (Gittins and Tilson 1984, Alfred and Sati 1994). Because this study was carried out during the height of the monsoon season, we are not surprised at the extent that both individuals relied on leaves. Additional fieldwork during non-monsoon months is needed to assess annual dietary variation.

Even with these considerations, the percent of time our group fed on fruit is most likely inflated due to the idiosyncratic nature of the preferred fruit - Artocarpus chama -within their small home range. Fruits of this tree are large, exceptionally hard and require long processing times to access the pulp surrounding each seed. Our data set is small, however approximately 22% of total food intake consisted of observations from a single feeding bout on the fruits of Artocarpus chama. Thus, a significant proportion of frugivory by our group consisted of trying to access the edible portion of one food item and not fruit ingestion per se.

Fruits are high-energy foods, typically subject to great competition (Oates 1987). With relatively few fruit trees (including no Ficus spp.) in their small home range, we strongly suspect that our group is unable to find preferred quantities of fruit. Primates typically respond to food shortages in at least three ways: (1) by resorting to lower quality foods, (2) by expanding a territory to maintain dietary quality and/or (3) by shifting to a significantly different dietary pattern (e.g., Lambert 2002). We believe our study group is using at least one of these mechanisms by resorting to a feeding pattern dominated by leaves. We also found that compared to other hoolock groups (Mukherjee 1986, Alfred and Sati 1990a, Islam and Feeroz 1992, Ahsan 2001), the Borajan gibbons in this study spent relatively little time feeding and a great deal of time resting (Table 1). The fact that approximately 85% of all leaves eaten were obtained from lianas, epiphytes, ferns and shrubs (rather than trees) might indicate an additional constraining factor, however this avenue of inquiry must await analysis of the chemical properties of available leaves in Borajan (e.g., Glander 1982). Finally, we are not surprised that the few hostile interactions between the male and female were the result of direct competition over the single fruit that was readily available during the study, that of Artocarpus chama. It is not yet possible to discuss patterns of feeding or intra-group aggression, however we look forward to addressing these issues in future studies.

These preliminary data have implications for understanding the adaptive capacity of hoolock gibbon and for their conservation. Mukherjee et al. (1991-1992) remarked that, “(hoolock’s) largely frugivorous diet, and their preference for mature, undisturbed, tall and dense forest, with a continuous canopy, set them apart from other non-human primate species of this area. These requirements make this species particularly susceptible to the effects of habitat destruction and unlikely to survive in the secondary forests in which macaques and langurs can survive in” (1991-1992:33). Borajan’s gibbon population has declined in the last seven years (Kakati and J. Das pers. comm.), and the reserve does have a high percentage of secondary forest shrubs, lianas, ferns and epiphytes. Given that the diet of our group is dominated by leaves from these plants, it is possible that this largely folivorous diet combined with reduced activity levels are mechanisms employed to withstand sub-optimal conditions. The effects of a degraded habitat, an extremely reduced territory and a shortage of preferred food items on the adaptive capacity and the viability of the gibbon population in Borajan can be discerned only through long-term observational studies. If the large proportion of leaves in the diet and the reduced activity levels of this hoolock gibbon group prove to be adaptive by sustaining a viable population, it would provide some encouragement that primates, including other gibbon species (e.g., Zhou et al. 2005), may survive similarly stressful conditions. Unlike the western black crested gibbon in Laos, the gibbons in Borajan do not suffer from hunting pressure (Johnson 2005). Therefore, conservation programs must be oriented towards community-based management of the forests and public education.

Acknowledgements

We are grateful for help extended by the Forest Department, Assam, India. In particular, we would like to thank the Chief Conservator of Forests, Mr. Mohan Chandra Malakar, the District Forest Officer of Tinsukia district, Mr. S. Rao, the Range Officer, Mr. Ranjit Datta and other forest department officials and guards. This paper would not have been possible without the help of Dr. Kashmira Kakati who provided advice and important information on plant identifications. We also thank our research assistant Mr. Munna Dev Karmokar, the family of Dr. Jayanta Das and the people of Borajan village for their hospitality and generosity. The support of The Ohio State University’s Department of Anthropology is greatly appreciated. We confirm that clearance was received from the Institutional Animal Care and Use Committee (IACUC) of The Ohio State University prior to the beginning of the study.

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Table 1: Activity budget and feeding proportions. All feeding proportions are percentages off the total number of feeding observations.

	Male	Female	Mean (M &F)
Number of days of data (N)	24	22	23
Total number of Observations (N)	589	537	563
Time Resting/Social	71.60 %	71.00 %	71.3 %
Time Locomoting	07.00 %	03.70 %	5.35 %
Time Other	06.50 %	05.20 %	5.85 %
Time Feeding	14.70 %	20.10 %	17.4 %
Total feeding observations (N)	87	122	105
% Fruit Intake	45.98 %	43.04 %	44.5 %
% Leaf Intake	43.68 %	41.80 %	42.7 %
% Insect intake	5.75 %	5.74 %	5.75 %
% Other	4.59 %	9.42 %	7.05 %

 
Note: Total time spent feeding on fruits of Artocarpus chama (34.2%) comprises 76.8% of all frugivory. 22% of the total feeding observations consisted of a single bout of feeding on the fruits of Artocarpus chama on one day. Total time spent eating leaves of non-trees such as lianas, shrubs, epiphytes (36.3%) comprises 85% of all folivory.




 
  
  
  
  
 
 
  
  
  
 
 
  
 
 
  
  
  
   
    
    
    Figure 1: Schematic representation of Borajan Wild Life Sanctuary in Tinsukia district of Assam, India (not to scale). The diagram shows the extent to which the sanctuary is circumscribed and fragmented by villages, fields and tea estates. It also shows the approximate territories of four groups. Group 4 was the subject for this pilot study.
    
    
   
  
   
 

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[1] Corresponding Author: Gayatri Thampy, 243 Calderdale ct., Blacklick, OH- 43004; Phone: 001-614-501-1978; Email: thampy.2@osu.edu