| Abstract: | Detailed studies of energy/nutrient acquisition and food selection have been completed
for both African apes and orang-utans in mast-fruiting dipterocarp forests, but, to date, no
studies of this type have been completed on orang-utans in non-masting forests, which
experience more muted fluctuations in fruit availability. Such studies may be instructive
in understanding whether the observations on orang-utans in masting habitats are specific
to orang-utans, or specific only to orang-utans in masting habitats. To investigate this,
orang-utan diet composition, energy intake, food selection and behaviour were studied in
relation to orang-utan fruit/flower availability, and chemical and non-chemical food
properties, in the Sabangau peat-swamp forest, Central Kalimantan from July 2005-June
2007. All data were collected using standard methods, and comparisons made with
published data on orang-utans in mast-fruiting habitats and on African apes.
Orang-utan fruit availability in Sabangau was lower and less variable than in the masting
forests of Gunung Palung National Park, West Kalimantan. Daily energy intake was
below requirements in most months, and similar to periods of very low food availability
between mast-fruiting events in Gunung Palung. Mechanisms through which orang-utans
may have survived this prolonged energetic shortfall are suggested. In addition, unlike in
Gunung Palung, where energy intake and fruit availability are tightly linked in both
sexes, the only age-sex class in which energy intake in Sabangau was related to fruit
availability was flanged males. Although selection of preferred vs. fall-back food types in
Sabangau appears to be governed at least partially by the expected rate of energy returns,
fruit and flower selectivity rank appears better explained by food quality: the best multiiii
variate model of adult female fruit selectivity rank contained only protein/fibre ratio, and
bivariate correlations between fruit-pulp selectivity rank and protein/fibre ratio were also
significant for flanged males. Furthermore, adult female and flanged male fruit selectivity
rank was negatively affected by fibre, weight or energy intake in at least one bivariate
analysis. This is suggested to represent selection against ingestion of large amounts of
relatively indigestible fibre, and implies that maximising dietary quality, rather than
energy intake, as has been suggested in masting forests in Borneo, may be the optimal
feeding strategy for orang-utans in habitats where fruit quality is relatively poor and/or
fruit availability is relatively consistent. Finally, compared to orang-utans in Bornean
masting forests, Sabangau orang-utans exhibit a relative lack of modifications in nonfeeding
behaviours in response to reductions in preferred food availability.
These results suggest that, compared to masting forests in Borneo, lower mean quality
and quantity of fruit, and lower variability in these parameters, in the Sabangau peat
swamps leads to important differences in feeding behaviour between these habitat types.
When compared to data on orang-utans in other sites and African great apes, these
observations are compatible with a graded-response hypothesis, in which the less
predictable the availability of high-energy fruit in a site: (1) the stronger the relationship
between fruit availability, fruit consumption and energy intake, (2) the more food is
selected based on energy content, and (3) the stronger the influence of fluctuations in
fruit availability on behaviour and, ultimately, probably also female reproduction. The
implications of these findings are discussed. |
