MODELING DIETARY SELECTIVITY BY BORNEAN ORANGUTANS - EVIDENCE FOR INTEGRATION OF MULTIPLE CRITERIA IN FRUIT SELECTION

Food patch visitation was compared to the availability of fruit patche s of different species during 2 years in a Bornean lowland forest to e xamine orangutan (Pongo pygmaeus) diet selectivity. Feeding on both th e pulp and the seeds of nonfig fruit varied directly with fruit patch availability, demonstrating preference for these foods over fig fruit or other plant parts (bark or leaves). Factors determining fruit selec tivity rank were examined through multiple regression analysis. Modeli ng selectivity for 52 chemically unprotected ''primate-fruit'' pulp sp ecies revealed strong preferences for species of (i) large crop size ( numbers of fruits ripening in an individual patch), (ii) high pulp wei ght/fruit, and (iii) high pulp mass per swallowed unit of pulp + seed, demonstrating orangutan sensitivity especially to patch size (g of pu lp or total energy/patch) and perhaps to fruit handling time. Modeling selectivity for 18 fig species showed that 4 factors significantly in fluenced fig species rank: crop size, pulp weight/fruit, and 2 chemica l variables, percentage digestible carbohydrate and percentage phenoli c compounds in the fig fruit pulp. The selectivity rank based on the o verall nutrient gain from feeding in the fruit patch (the product of t he first 3 variables) is proportionally depressed by the percentage ta nnin content, demonstrating that orangutans integrate values for these variables in selecting fig patches. The conclusions from these result s and from analysis of selectivity for seeds and for other fruit types are that orangutan foraging decisions are strongly influenced by the meal size expected from a feeding visit (i.e., by patch size), that ta nnins and other toxins deter feeding, and that the energy content, rat her than the protein content, of foods is important in diet selection. The foraging strategy of orangutans is interpreted relative to these results and to Bornean fruiting phenology. By integrating spatial, mor phometric, and chemical variables in analysis, this study is the first to demonstrate the application of foraging theory to separate out the key variables that determine diet selection in a primate. Multivariat e analysis should routinely be applied to such data to distinguish amo ng the many covarying attributes of food items and patches; inferences drawn in previous studies of primate diet selection, which ignore key spatial and morphological variables and rely on univariate correlatio ns, are therefore suspect.