Abstract:

The nutritional ecology of Grey-cheeked Mangabey (Lophocebus ugandae Groves) inhabiting Mabira and Lwamunda, two of Lake Victoria Basin (LVB) forest reserves is not well understood. Yet this knowledge would provide relevant information for formulation of an effective management strategy that would guarantee the health of the forests and synchronously conserve their forest fauna. The principal objective of this study therefore was to investigate the nutritional ecology of mangabeys inhabiting three contrasting habitats: fairly undisturbed Mabira (Ml), regenerating Mabira (M2) and severely degraded Lwamunda (LD) with different management histories within the LVB. To achieve this objective, a three-year study was undertaken to generate information on the diet, fruiting phenology and quality of fruits from ten priority trees species commonly utilized as food by 3 groups of habituated mangabeys. In each of the three habitats, phenological data was collected to provide information on fruit availability, quality and timelines for their consumption. The phenology of 10 individual trees of 10 priority fruit trees species utilized by mangabeys in each study area was documented on a monthly basis to establish fruit abundance and availability. A systematic 5 minute-scan sampling was supplemented with ad libitum observations to collect feeding data for 5 consecutive days at the end of the month for a study period of 24 months. On each day, mangabeys were trailed from 7am to 3pm. Fruit remains under trees where mangabeys were feeding and throwing on the forest floor were carefully examined and the exact parts that were being fed on by mangabeys were identified. Mechanisms of fruit processing prior to consumption were also recorded. In order to get an insight of the macronutrients contained in the most utilized fruits, samples for nutritional analyses of different fruit parts and stages of development were collected on a monthly basis and analyzed using standard laboratory methods. One km2 area was randomly selected in each study habitat to collect a single data set on the number and size of forest gaps for assessment of deforestation and its effect on food availability. Results showed that there were several inter-related factors that influenced dietary composition and foraging behaviour of mangabeys in Mabira and Lwamunda forests. The principal factors included fruit availability and quality. Fruit availability varied seasonally and according to type of habitat while quality varied with fruit type/part, stage of fruit development and level of secondary compounds. It was observed that as food availability declined, fruit processing by mangabeys increased in Lwamunda and M2 but it was nonexistent in Ml of Mabira Forest. Of the ten priority fruits commonly consumed by mangabeys in Lwamunda, M2 and Ml, about 60%, 20% and <1% respectively were processed to access endosperm or pulp. The stage of fruit development dictated the specific part(s) of fruit consumed and the mode of ingestion. The skin of ripe fruits was peeled off and discarded while the pulp gnawed off in the mouth and ingested. The intermediate ripening stages of most fruits were avoided except for a few genera. Level of fruit ripeness also determined pattern of fruit processing. Ficus species (sur and exasperata) were consumed whole while unripe fruits of Canarium indicum, Maesopsis eminii, Pseudospondias microcapa were processed. Large-sized fruits were consumed in piece-meal portions; medium-sized fruits were gnawed at to access the desired part(s) and small-sized ones were invariably ingested whole. Immature fruits belonging to C. indicum, P. microcapa and M. eminii and Myrianthus arboreus were processed to access endosperm: while pulp of most fruits was consumed when fruits were ripe. Abundance of fruits commonly eaten by mangabeys did not translate into availability. The number of fruiting trees was significantly higher in M2 than Ml and LD. However, edible fruits in Ml were significantly higher than in LD (P<0.001 using one-way ANOVA). Availability and nutrient content of priority fruits in LVB also influenced Mangabey nutritional ecology. Fall-back fruits (FBFs) that were available for most of the time contributed to Mangabey diet during periods of low food availability. Nutritionally, seasonal fruits had twice as much protein, fat and sugars as FBFs. Protein seemed to be an important nutrient component in Mangabey diet as exemplified by the consumption of seeds and endosperms. It was closely followed by fat as indicated by preference for B.unijugata (Baker) aril and Cellis spp seeds with > 10% fat levels. However, the phenological data indicated that the number of fruiting FBF trees appeared insufficient to sustain Mangabey populations in M2 and LD. It was therefore concluded that the actual FBFs in the two habitats were probably the raided crops and domestic fruits growing in the wild respectively since they sustained mangabeys during fruit scarcity. In a health ecosystem (forest), the proportion of FBFs in comparison with preferred fruits should be small but it was observed that in Lwamunda and M2, the ratio of FBFs to preferred fruits was 2:1 and 1:1 respectively. This implied that Lwamunda was ecologically unhealthier than M2 in terms of Mangabey diet.The level of secondary compounds, particularly tannins, which are known to have anti digestive properties among primates also seemed to have influenced Mangabey diet. During periods of food scarcity, mangabeys selectively consumed parts of unripe fruit (endosperms of P. microcapa, M. aboreus, C. indicum and M. eminif) that were known to contain relatively low levels of condensed tannins below a threshold of 2.6g/100g catechin equivalent. Fruit parts with tannins levels above the threshold were avoided and rarely consumed by mangabeys unless they contained relatively high protein content and low fibre. This type of trade-off was exemplified by consumption of Celtis seeds with relatively high levels of protein (=10%) and tannins (3.08g/100g). Fruit processing behaviour to select specific fruit parts for consumption seems to be related to the level of tannins and an adaptation to cope with food scarcity. In a forest environment, gap size seemed to have influenced food availability. The mean gap size varied from 50.12±24.8 m2 in fairly undisturbed Mabira (Ml) to 96.42±24.9 m in regenerating Mabira (M2) and 644.58±53.98 m2 in severely degraded Lwamunda (LD) forest reserves. Generally, fruits utilized by mangabeys as food declined as the number and sizes of gaps increased. The level of decline was significant (one way ANOVA p < 0.05). During periods of fruit scarcity, mangabeys devised different patterns of fruit processing as a coping strategy. 2 However, it was noted that interpreting interrelated factors influencing Mangabey diet and foraging behaviour was complex. To unravel some of these complexities, the consumption or rejection of specific fruit parts of B. unijugata at different developmental stages were investigated. The nutritional criteria used depended on level of fat and condensed tannins. When seeds or arils had low tannins, mangabeys ate them but rejected them when tannin levels increased beyond 2.6g/100g catechin equivalent. While fat content in seeds and aril increased withincreasing maturity, it only influenced aril consumption but not seed consumption because of its high tannin level. In conclusion, nutritional content of food items appeared to be pivotal to understanding primate foraging decisions including trade-offs between consumption of critical macro nutrients and secondary metabolites. Generally, mangabeys in the three forests selected fruit parts that were low in fibre and tannins, but rich in protein, high in sugars and fats. These included ripe fruit pulp of Rheedia edulis, aril of B. unijugata and seeds of Celtis species. The quality and quantity of fruits appear to be the main factors that influenced mangabey nutrition. Several recommendations were drawn from the study: for instance, development of credible strategies and policies for conservation of forest reserves to sustain Mangabey population in LVB, repetition of the study with increased study sites, mangabey groups, sample size of monitored tree species and inclusion of other secondary metabolites like terpenoids and alkaloids.