The feeding ecology of Cat Ba Langurs (Trachypithecus poliocephalus) on Cat Ba Island, Vietnam.

Abstract

The Cat Ba langur (Trachypithecus poliocephalus) is a Critically Endangered colobine species, endemic to Cat Ba Island, Vietnam. Intense hunting pressure saw the population decline to just 50 individuals in 2000 and - although strict protection measures have helped control poaching on the island - the long-term survival of the species is still threatened due to the small size of the population, isolation of subgroups and ongoing threats to their habitat. Data on the diet and nutritional ecology were collected between May 2016 and April 2017 on two reproductive groups (Group A and Group B) living in the Cua Dong subpopulation to determine the quality of their habitat and whether food availability is limiting the growth of the population. Knowledge on the feeding ecology of a species is important because it provides information about their ability to survive in lower-quality habitats and the key resources required in their habitat to ensure their ongoing survival and the growth of the population. The two groups of langurs spent most of their time eating leaves (63.36% and 66.57%) and fruit (12.49% and 13.65%), consuming young leaves more often than mature leaves and unripe fruit more often than ripe fruit. Flowers, stems, seeds, bark and petioles made up the rest of their diet. The langurs fed on a total of 103 food items from 56 different plant species. Group A consumed 18 species for more than 1% of the time, while Group B consumed 14 species for more than 1% of the time, accounting for 73.96% and 72.19% of their diets, respectively. Only three species consumed by the langurs were found among the top 20 species in their habitat and only two of the three were consumed for more than 1% of the time. This all suggests the langurs were being selective about their food choices and not just eating foods based on the frequency they were found within their habitat. The average concentrations of nutrients in the langur's food items were roughly similar to those found in the foods of other Asian colobines, except for copper and manganese, which were slightly lower. In addition to this, the average levels of crude protein, copper, zinc, sodium and iron in all plant parts tested were lower than the adequate nutrient concentrations put forward by the National Research Council for non-human primates. Despite this, however, there was no indication the langurs were suffering from any kind of nutrient deficiency, suggesting the current nutrient levels in the diet are sufficient for the species. The strong negative correlation between the concentration of ADF and NDF in young leaves eaten by the langurs and the selectivity index of the food items, suggests the monkeys were selecting young leaves with low fibre concentrations and may indicate a high overall fibre content in the plants at the site. Feeding-related agonistic incidents, along with avoidance behaviours, indicates that contest competition is occurring in this subpopulation. Group A had a higher number of feeding-related agonistic incidents, fewer individuals nearby when feeding and a higher rate of agonism overall, compared to Group B. This may be due to the large area of home range overlap Group A has with a bachelor group, which could lead to faster depletion of food resources. Additionally, the intergroup encounters observed in this study were more aggressive when they involved the adult male from a reproducing group and the bachelor group compared to when they involved the adult males from the two reproducing groups, suggesting that group take-over attempts could be a problem in this subpopulation. These problems will continue to get worse if the size of the subpopulation increases without the ability to expand the area they currently inhabit or move to another subpopulation. Thus, the small area currently inhabited by the Cua Dong subpopulation and the lack of connection between subpopulations could be limiting the growth of the population. Show more