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Summary

This study proposed to compare the distribution patterns of the two species Semnopithecus johnii (Nilgiri langur) and S. priam thersites (common langur) in the southern Western Ghats and determine the role of ecological factors in maintaining their distributional boundaries at a specific site where their ranges adjoin. Such comparative information can help in better understanding of the ecology of the langurs, their adaptability to certain environmental factors and habitat suitability, which can contribute to a) our understanding of the evolution of these primates and b) informed conservation initiatives for these threatened primates.

Objectives
  1. Evaluate the distribution of the two langur species with respect to each other and determine the extent to which they are distributed parapatrically within the southern Western Ghats region.
  2. Determine the role of environmental gradients (gradients in altitude and vegetation) in maintaining the pattern of distribution of each species.
  3. At a given site where the range of the two species overlaps, determine the role of ecological factors including inter‐specific competition and intestinal parasite loads in maintaining the pattern of distribution of each species.
Status
  • In the southern Western Ghats, about 6% of the study area for the Nilgiri langur and 1% for the Hanuman langur were found to be areas of high probability of occurrence of which nearly 40% was inside the Protected Area network. The distribution of the Hanuman langur was very fragmented and consequently the area of parapatry was less than 1%, with key areas of distributional overlap found in the Srivilliputtur Wildlife Sanctuary and the Kalakkad-Mundanthurai Tiger Reserve and east of the Anamalai hills.
  • The study showed a clear avoidance between the two langur species, with the Hanuman langur showing a stronger avoidance. The influence of environmental factors and their importance in determining the distribution differed between the langurs – the Hanuman langur was primarily and positively driven by aridity and elevation while the Nilgiri langur was negatively influenced by aridity. The areas of parapatry were characterized by higher mean aridity levels compared to areas of no overlap, elevation between 200m to 1450 m, and at least 10% (to a maximum of 80%) tree cover.
  • Areas where only Nilgiri langur was found, showed greater plant species richness compared to areas of co-occurrence and areas where the Hanuman langur alone was found in. Vegetation in areas of parapatry was found to be more similar to areas where only Nilgiri langur was found in than to areas where only Hanuman langur was found in.
  • Comparing the intestinal parasites of the langurs, overall, the Hanuman langur showed greater prevalence of infection by helminthic parasites, especially nematodes. In areas of co-occurrence, for the Nilgiri langur, an increase in the prevalence of Ascaris sp., Toxocara sp., and Trichostrongyle sp. was seen. For the Hanuman langur this was more pronounced with an increase in prevalence of Ancylostoma/Oesophagastomum sp., Ascaris sp., Toxocara sp., Trichostrongyl sp., Moniezia sp., and Hymenolepis sp.
  • In conclusion, several factors appear to be influencing the distribution patterns of these langurs at different scales. It is important to our understanding of primate evolution and for conservation efforts of these threatened primate species to undertake further studies covering the different aspects of reproductive and social behaviour, population estimation and long term monitoring of key populations and their health.