Trachypithecus margarita (Elliot, 1909)

Tran, Van Bang, Hoang, Minh Duc, Luu, Hong Truong, Workman, Catherine & Covert, Herbert, 2019, Diet of the Annamese langur (Trachypithecus margarita) (Elliot, 1909) at Takou Nature Reserve, Binh Thuan Province, Vietnam, Raffles Bulletin of Zoology 67, pp. 352-362 : 360

How does T. margarita on Takou Mountain select food through the day and throughout the year? Even though our analysis failed to demonstrate significant differences in time used for consuming each kind of food item in the morning and afternoon or during the wet and dry seasons, our data indicate that the diet of T. margarita at Takou Mountain is consistently dominated by immature leaves throughout the day, while they fed on more fruits and mature leaves in the morning and more flowers in the afternoon. This dietary pattern was not previously recorded for other Trachypithecus species. There are however some similarities to that observed for Pygathrix nigripes in Nui Chua and Phuoc Binh NP ( Vietnam) and in Mondulkiri Province ( Cambodia). In Nui Chua and Phuoc Binh NP, Pygathrix nigripes spend more time of eating fruits and flowers in the morning than in the afternoon ( Hoang et al., 2009). In Mondulkiri, Rawson (2009) found Pygathrix nigripes consumed significantly more fruits in early mornings compared to early afternoons using a method that divided daytime into four periods.

Chapman & Chapman (1991) argued that most primate species spend more time feeding on fruits in the morning because it is high in nutrients, is easily digested and provides more energy ( Garber, 1987; Milton, 1993) to restore energy that has been used over the night. However, colobine monkeys appear to differ from this generalisation; for example, Colobus guereza , had a peak in fruits consumption in the afternoon as noted by Chapman & Chapman (1991). In addition, the amount of leaves consumed in the morning in diet of T. margarita and Pygathrix nigripes were higher than 50% and more than fruits. Therefore, the diurnal diet pattern of T. margarita at Takou Mountain and Pygathrix nigripes in Vietnam could be considered as characteristic diurnal feeding pattern of Asian colobine monkeys. Further studies of foraging behaviour on Asian colobines are needed to better understand such feeding patterns.

Throughout the year, T. margarita at Takou often preferred one food item per month, consuming one kind of food item predominantly over other in term of feeding time. For example, in February 2010, langurs spent 71.8% (n = 224) of feeding time consuming flowers, from only one species ( Radermachera hainanensis ). Immature leaves accounted for 97.1% (n = 539) in the diet of this species in June 2010 with fruits only 0.7% in the diet. The amount of fruits consumed in June 2011 was 87.4% (n = 263) and immature leaves was 11.3% (n = 34). Most of the fruits consumed in this month came from Beilschmiedia sp. The preferred food item in each month may relate to the preferred species and its abundance in the habitat. In addition, concentrating on eating one kind of plant part of an abundant species can reduce travel distance and hence reduce the cost of travel.

Although seasonal change in weather conditions affect the availability of plant parts in the forest and can therefore influence the availability of food resources, leading to changes in primate feeding behaviour ( Newton, 1992; Hanya, 2004; Zhou et al., 2006), there were no significant correlation of time spent for consuming each type of food item by T. margarita at Takou Mountain with between the wet and dry seasons. The habitat of T. margarita on Takou Mountain is an evergreen forest with few deciduous trees; therefore, seasonal changes in habitat may be less obvious and reflected in the diet of T. margarita found in this study. Immature leaves, mature leaves, and fruits (especially figs) are usually available in this habitat throughout the year. However, we also need additional studies on the phenology of plants to understanding the food selection by the langurs in term of food availability in their habitat. Because of the slight difference in potential food resources between the two seasons, the langurs adjusted their food consumption only to some extent.