Dietary composition and feeding preference of Mantled guereza Colobus guereza (Rüppell, 1835), in Maze National Park, Ethiopia

Study area

We conducted this study at Maze National Park (MzNP) along the Maze River, a critical habitat for our target species. This park is located in a bio-diverse region of southern Ethiopia, situated between the Gamo and Gofa Zones. It provides a unique and varied landscape that supports a rich array of flora and fauna. The park is situated between 6°18′30″ to 6°29′00″ N latitude and 37°7′30″ to 37°22′30″ E longitude, positioning it within a specific ecological and climatic zone of Ethiopia. The elevation ranges from 900 to 1,200 m above sea level, which influences the climate, vegetation, and biodiversity (Befekadu & Afework, 2006). The annual rainfall varies between 843 mm and 1,321 mm, which reflect variability in precipitation. The Maze area’s climate exhibits a distinct seasonal pattern characterized by a rainy season from March to October and a dry season from November to February. The lowest temperature during the rainy season is 15.3 °C in June, suggesting relatively mild temperatures, which may be beneficial for plant growth and biodiversity during this time, and the highest temperature during the dry season is 33.5 °C in February (Mamo, 2012), indicating hotter conditions that could lead to water stress for plants and increased evaporation rates.

The park is home to a remarkable variety of mammalian fauna (e.g., orbi Ourebi aourebi, bohor red buck Redunca redunca, buffalo Syncerus caffer, warthog Phacochoerusafricanus, bush buck Tragelaphus scriptus, greater kudu (Tragelaphus strepsiceros), lesser kudu (Tragelaphus imberbis), Water buck Kobus ellipsiprymnus, bush pig Potamocherus larvatus, anubus baboon Papio anubis, vervet monkey Chlorocebus pygerythrus, Mantled guereza Colobus guereza, lion Panthera leo leopard Panthera pardus, wildcat Felis silvestris, and serval cat Leptailurus serval, in addition to varied floral composition. It also comprises varieties of bird species, reptiles, amphibians and insects.

Maze National Park (MzNP) is predominantly covered by savannah grassland interspersed with scattered deciduous broad-leaved trees, creating a diverse landscape that supports a variety of wildlife. The majority of the park consists of plains enriched by open vegetation dominated by Combretum and Terminalia species, which play a critical role in providing habitat and food sources for the local fauna. The Maze River, originating from the surrounding highlands, flows along with its numerous tributaries, traverses the park from the northern to the southern end, creating vital riparian zones that serve as corridors for wildlife movement and habitat for aquatic and semi-aquatic species. This unique combination of savannah grasslands and riverine habitats makes MzNP an important area for our target species.

The Maze River is essential to the park’s biological well-being because it forms verdant riparian areas that provide a variety of species with places to reproduce and feed. Given an important habitat to primates, particularly for guerezas, in which no feeding activity has been detected rather than the riverine habitat over the study period (Fig. 1).

Study groups

Three groups of C. guereza were targeted for this study. One group with three individuals at Maze camp site (group-1); another group with two individuals at Domba site (group-2); and the third group with three individuals at Lemasse site (group-3); were randomly selected along the River Maze. We monitored those groups in their home ranges for the duration of the study, with a research team assigned to each group to look at dietary ecology and potential differences in feeding activities.

Data collection

Data analysis

We combined feeding data from the three groups into one dataset before the computations of proportions of each diet component. The analyses were executed in XLSTAT 2023.1.3 (1,407) and SPSS software version 22. Of the total of 11,520 scans, we recorded 5,168 (44.86%) feeding activities over the study period, (Wet: 2,382, 46.09%; Dry: 2,786, 53.91%). We computed the proportion of the diet components for seven food classes (young leaf, mature leaf, fruit, bark, shoot, flower, unidentified invertebrates) recorded through the study period by dividing the number of records of a particular diet component by the total number records from all diet components. The proportion of each diet component was then converted into percentages. The chi-square test was employed to test for the seasonal and monthly variations in proportions of the diet components. To assess the feeding preferences of various food plant species in a given habitat, we computed the selection ratio for each species. Stem density measurement for each food plant species present in the study area, we calculated the total stem density, which represents the number of stems per unit area. We also determined the percentage of each food plant species relative to the total number of food plants in the study area.

Computation of selection ratio: The selection ratio (SR) for each species was calculated using the formula: SR = Percentage of Species/Stem Density Percentage.

Where: The percentage of a certain food plant species among all food plants is referred to as the percentage of species. The stem density percentage describes the ratio of the stem density of a particular food plant species to the total stem density of all species.

Thus, a species is preferred over others in the habitat if the selection ratio is greater than 1. A species may be avoided if its selection ratio is smaller than 1, which indicates that it is less desirable. A selection ratio equal to 1 indicates neutral selection, meaning the species is utilized in proportion to its availability.

Field permit

The Office of Executive Research Directorate and the Biodiversity Research and Conservation Center, Arba Minch University were approved the fieldwork under research permit (AMU/TH2/BRCC/09/2014). Hereby, we can guarantee that no animal capture and tissue or blood sample was taken from the subject species, as data were recorded through direct observation without animal capture.

Food plant species consumption and preferences

We found that six tree plants (Acacia polyacantha, Millettia ferruginea, Moringa stenopetala, Syzygium guineense, Trichilia emetica and Ficus sycomorus) and two shrub species (Carissa spinarum, Grewia villosa) that were grouped under seven families (Apocynaceae, Fabaceae, Myrtaceae, Moringaceae, Meliaceae, Moraceae and Malvaceae) were consumed by C. guereza in the study site. Except for the Fabaceae, all are represented by a single species. Overall, Trichilia emitica was the most top plant species preferred to the rest (Table 1).

During the wet season, Trichilia emetica contributed the largest proportion to the total amount of young leaf consumption, accounting for 52.85% (n = 948). This was followed by Grewia villosa at 20.68% (n = 371), while Moringa stenopetala was rarely reported, with only 0.5% contributions (n = 9) (Table 2). The second most popular food item this season was mature leaves, with Trichilia emetica and Grewia villosa making up the largest portions making up 53.12% and 46.88% respectively (Table 2). During the dry season, the Moringa stenopetala contributed the largest portion (386, 49.87%) of all young leaf consumption and Millettia ferruginea contributed the least (0.13%) (Table 2).

Diet composition of guerezas

The annual diet of C. guereza comprised of young leaf, matured leaf, fruit, bark, shoot and flower (Table 3). Young leaf was the most consumed plant part in the overall annual diet. Based on seasons, there was seasonal variation in number of feeding records of all food plant items (Young leaf: χ² = 405. 140, df = 1, p < 0.05; Mature leaf: χ² = 651. 563, df = 1, p < 0.05; Fruit: χ² = 105. 593, df = 1, p < 0.05; Shoot: χ² = 125. 063, df = 1, p < 0.05). Young leaf as the major food item was more frequently consumed in the wet season (n = 1,794, 75.31%) while mature leaves were consumed more over the other food components during the dry (n = 1,215, 43.61%) than wet season (Table 3). Interestingly, consumption of unidentified invertebrates was also recorded to increase by about 1.63% in the dry season (Table 3). The results also demonstrate some monthly variations in consumption of different plant parts or diet components by guerezas (Table 4).

Food plant species consumption and preferences

Comparable to other studies on feeding ecology of Colobus guereza across its geographical range, our study reports very few plant species consumed by this primate. In this study, we recorded eight plant species in the Maze National Park which are fewer than that observed in Kalinzu Forest (39 plant species) by Matsuda et al. (2020). Of these eight species, only two plant species Trichilia emitica (39.6%) and Grewia villosa (21.01%) had the highest feeding records and thus dominate monkeys’ diet. The observation of few plant species eaten by guerezas suggests that the dietary plant richness of this primate is very low in the park. This provides an urgent need to conserve the park to ensure the long-term presence of important food plant species. It appears that the guerezas in MzNP consumes food plant species as expected from its availability across its home range. Most of the plant species preferred (having high selection ratios) are those which are quite abundant in the groups’s home ranges (Tables 1 and 5). However, this does not necessarily indicate that they are the most nutritious or preferred food sources, but rather they are fed because they are quite abundant in the habitat and not because they are most nutritious. Future studies should analyze nutrient content and other phytochemical composition of plants eaten in order to draw decisive conclusion on plant food preferences.

Our study demonstrates a seasonal variation in frequency with which certain plant species were eaten. For example, Trichilia emitica was most frequently eaten during the dry season while Syzigium guinense and Grewia villosa were eaten in the dry season. Seasonal variation in plant phenological patterns can in part explain the observed variation in feeding plant species between seasons. We observed that monthly dietary diversity increased as the number of available plants with young leaves reduced during the dry season (Kibaja et al., 2023) (Table 2). For example, the plant species such as Ficus sycomorus, Millettia ferruginea and Moringa stenopetala were not used as food source for the study species during the wet season, because of sufficient young leaves, with exceptions thus Moringa stenopetala only contributed small amount in May (Table S2). On the other hand, Moringa stenopetala significantly contributed to the study species bearing more young leaves during the dry season (Table 2). This way, eight plant species from seven families and one non-plant source, i.e., unidentified invertebrates offered food items to the C. guereza in the study area. This was particularly due to the effects of the declining availability of young leaves from Trichilia emetica and Grewia villosa. Much of the dietary diversity in the study group is seemingly attributable to the availability of young leaf portion of their diet.

Many colobine species, have increased dietary extent during times and areas with low availability or quality of resources (Hu, 2011; Clink et al., 2017). The present study depicted, the dietary extent increased with decreasing in young leaf availability during the dry season.

Food plant parts/item consumption

We found that the guerezas exploited different plant parts, leaves being mostly eaten in the MzNP. However, it is not surprising for them to consume mostly young leaves because these monkeys like other colobines are anatomically adapted to feed on leaves that facilitate their leaf-eating habits, including their preference for young leaves due to their nutritional benefits (e.g., Gonzalez & McGraw, 2021; Mola et al., 2022). In line with this, studies show that leaves accounted for high proportion (42–49%) by folivorous-frugivorous monkeys (Lima & Bicca-Marques, 2024). Another study has shown that Bale monkey, a folivore specialist, spend more time munching on new bamboo tree leaves in Southern Ethiopia (Mekonnen et al., 2018). Similarly, the leaves accounted for highest proportion of Colobus guereza’s food items (71.6%) in Borena-Sayint National Park, Northern Ethiopia (Hussein, Afework & Dereje, 2017) and 82% in Bale Mountains National Park, Ethiopia (Petros et al., 2018).

Furthermore, young leaves were highly eaten compared to mature leaves. This observation is in line with a study on feeding ecology of guerezas at Saja Forest, Kaffa Zone, Southwest Ethiopia, that reported the monkeys to eat young leaves over mature leaves (Mola et al., 2022). Similarly, Matsuda et al. (2020) reported the C. guereza to consume up to 87% young leaves in the Kalinzu Forest in Uganda. Young leaves are preferred because they have low fiber content, high nutrients and are easier to digest (Leighton, 1993; Matsuda et al., 2020). Thus, by preferentially consuming these food items, the guerezas are able to maximize their nutrient intake while minimizing the ingestion of toxic compounds. Interestingly, the guerezas were observed to increase the consumption of invertebrates during the dry season by 1.63% (Table 2). The high consumption of invertebrates during the dry season could be strategy to increase intake of proteins from invertebrates rather than getting it from young leaves which were slowly eaten in this season.

The results of this study have demonstrated some seasonal dietary flexibility for the guerezas in the study site. We observed the study species use young leaves and matured leaves interchangeably during the wet and dry seasons. They consume a lot of young leaves during the wet season and mature leaves during the dry season and vice versa. Throughout the study months, there were considerable changes in availability and consumption rate of diet items (Table 5). This is attributed to seasonal variations in phenological patterns that affect the availability of food items which eventually influence seasonal dietary composition for the guerezas. For instance, in the field, we observed that when young leaves were insufficient during the dry season, hence the monkeys change their diet use by increasing consumption of mature leaves. This is consistent with the previous study where resource availability is highly variable; folivorous monkeys eat more leaves during periods of low fruit availability (Hanya & Bernard, 2012). Research findings found, proboscis monkeys varied in response to monthly changes in food availability, but did not vary among forest types (Feilen & Marshall, 2020). In addition, the influence of seasonality on the diet reported at Tanjung Putting National Park, thus fruits comprised high proportion of the diet from January to May, while young leaves consumed the highest proportion of the diets from June to December (Yeager, 1989). This might be attributed the fact, that the season contribute to the availability and even the quality of diet components and this drives the flexibility for feeding of the species. However, Colobus guereza consumed high amount of young leaf during the study period, in riverine habitat of the park.

The results of this study demonstrate that the guerezas exhibit seasonal and monthly dietary variability in response to availability of food components across months or seasons (Tables 4 and 5). Dietary flexibility is a strategy that enables primates to survive during periods of food shortage (Feilen & Marshall, 2020) or exploit different parts having different food resources across their home ranges or habitats (Sha & Hanya, 2013; Seiler & Robbins, 2016; Mekonnen et al., 2018; Tesfaye et al., 2021).