Food selection by the northern yellow-cheeked crested gibbons (Nomascus annamensis), northern Cambodia
Authors:   , Hon Naven 
Published: November 2021

<p>Tropical regions have extremely high plant diversity, which in turn supports a high diversity of animals. However, not all plant species are selected by animals as food sources, with some herbivores selecting only specific plants as food as not all plants have the same nutrient make up. Animals must select which food items to include in their diets, as the amount and type of nutrients in their diet can affect lifespan, health, fitness, and reproduction.  Gibbon populations have declined significantly in recent years due to habitat destruction and hunting. Northern yellow-cheeked crested gibbon (Nomascus annamensis) is a newly described species, and has a limited distribution restricted to Cambodia, Laos and Vietnam. The northern yellow-cheeked crested gibbons play an important role in seed dispersal, yet little is currently known about this species, including its food selection and nutritional needs. However, data on food selection and nutritional composition of selected food items would greatly inform the conservation of both wild and captive populations of this species.  This study aims to quantify food selection by the northern yellow-cheeked crested gibbons by investigating the main plant species consumed and the influence of the availability of food items on their selection. The study also explores the nutritional composition of food items consumed by this gibbon species and identifying key plant species that provide these significant nutrients.  A habituated group of the northern yellow-cheeked crested gibbons with five members located in northern Cambodia was studied for 12 weeks during the dry season, and focal animal sampling was used to observe individual feeding behaviours. Four main activity categories were recorded including resting, feeding, travelling and socializing. Phenological data was recorded from transect lines, and plant densities from 20 vegetation plots inside the home range of this group of gibbons were also measured. Seventy-four plant samples from 20 tree and liana species that were consumed by this group of gibbons were collected for nutritional analyses. These samples were initially dried in sunlight, and then oven dried before levels of protein, total non-structural carbohydrates, lipids, fibres and condensed tannins were measured at the Nutritional Ecology Lab at Hunter College of the City University of New York, New York.  The northern yellow-cheeked crested gibbon individuals spent most of the time resting, followed by feeding, travelling and socializing. Their main diet was fruit, supplemented with young leaves, flowers, mature leaves, and occasionally insects. Individuals selected food from 37 plant species, but predominately fed on just 16 of these species. The three most-consumed species were fruit from Ilex umbellulata (tree), Ficus. sp (liana), and young leaves from Lithocarpus elegans (tree). There was a significant relationship between feeding time and the availability of flowers, indicating that flowers were actively selected for when present. However, there was no significant relationship between feeding time and the availability of fruit or young leaves. Only a small number of plants bore fruit, with very low densities in the home range of this gibbon study group, but these plants produced a large abundance of fruit. These findings clearly indicate that fruit is the main diet for northern yellow-cheeked crested gibbons in the dry season, with young leaves, flowers, mature leaves and insects acting as secondary food sources. The northern yellow-cheeked crested gibbons selected only a small number of specific plants in their territory for food sources, indicating that any selective logging targeting these plant species would reduce food availability, and restrict the diet of these animals.  Fruits consumed by the northern yellow-cheeked crested gibbons were rich in carbohydrates and lipids, while young leaves were richest concentration of protein. Mature leaves had a high moisture content, whereas flowers contained condensed tannins more often than other plant tissues. All plant tissues consumed had similar amount of fibres. Generally, the food items consumed had higher concentrations of carbohydrate than protein or lipids. The overall diet of this group of gibbons was low in lipids. These results indicate that northern yellow-cheeked crested gibbon consumed food items with high concentrations of carbohydrate, and only selected a few food sources with high level of protein.  Overall, these findings have contributed important knowledge that can be used for long-term conservation of this gibbon species. There are a small number of key food species in the home range which need special protection. The northern yellow-cheeked crested gibbons consumed food items from different plant species, and as a consequence, all plant species selected play an important role for nutritional requirements by this gibbon species. When considering feeding requirement by the northern yellow-cheeked crested gibbons for captive breeding programs, diets should include fruit, which is rich in carbohydrates, and young leaves, which have high levels of protein.</p>