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.
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 was 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
The level of secondary compounds, particularly tannins, which are known to have antidigestive
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.
content in seeds and aril increased with increasing 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 macronutrients
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.