The contents of nutrients and/or toxins is commonly agreed to be a major factor that affects non-human primates’ preference for specific types of plant food^1. However, so far only a few studies have been able to show a positive correlation between the displayed food preferences and the content of any particular nutrient. This lack of positive correlations could possibly be explained by the complex chemical composition of plant foods and the trade-offs that non-human primates have to make in order to gain sufficient amounts of required nutrients, while they at the same time try to minimize the intake of potentially harmful secondary plant metabolites¹.

A possible approach to study the relationship between nutritional content and the food preferences of non-human primates is to present them with pairwise choices of food items that contain very small and therefore presumably negligible amounts of secondary plant compounds². Laska et al. (2000) proposed that cultivated fruits could be used for this purpose. By repeatedly presenting captive spider monkeys with pairwise combinations of 10 cultivated foods, Laska et al. (2000) were able to show a positive correlation between the food preferences displayed by this species and the content of total energy. Additional studies that employed the same approach found that captive squirrel monkeys³, capuchin monkeys⁴ and pacas⁵ , like the spider monkeys, display food preferences that are highly significantly correlated with the total energy content, while pigtail macaques³, on the other hand, were found to prefer foods with high contents of total carbohydrates.

In this study I employed the same method, to assess the occurrence of spontaneous food preferences of three captive white-handed gibbons and analyzed whether these preferences correlated with nutrient composition. All animals were repeatedly presented with all possible pairwise combinations of 10 types of food that are part of their diet in captivity. The animals were trained to voluntarily approach a shelf mounted on a wall in a test room, choose one of the two simultaneously presented food items and then retreat so that the rejected food item could be removed. All pairs were presented to each animal for a total of 12 times and the fruit chosen in every presentation was recorded.

The results showed that the animals displayed a statistically significant preference for one of the two alternatives in 36 out of 45 possible pairwise presentations. The food preference ranking that the gibbons displayed were: Grape > banana > fig > apple > pear > cantaloupe melon > carrot > tomato > cucumber > avocado. By employing spearman rank-order correlational analysis, I found a highly significant possitive correlation between this food preference ranking and the total carbohydrate, fructose and glucose content of the foods (Spearman, P < 0.01), i.e., the animals preferred to consume foods with high contents over fruits with low contents of these nutrients. These results suggest that white-handed gibbons are selective feeders with regard to the source of metabolic energy as neither the content of total energy nor the contents of protein or lipids significantly correlated with the displayed food preference ranking. This supposition is supported by findings that wild gibbons include a considerable proportion of sugar rich fruits in their diet and that they demonstrate clear preferences towards ripe fruits⁶, which typically have the highest content of sugars⁷.

In line with the findings for the white-handed gibbons presented here, the pigtail macaques’ (Macaca nemestrina) displayed preference for food items was significantly correlated with total carbohydrate content of the food items, but not with the contents of total energy, lipids or proteins³. In contrast, the food preferences of several other species have been shown to significantly correlate with the total energy content of food items, independent of the source of metabolic energy^2,4,5. Laska and co-workers (2000, 2001, 2003) have speculated that differences in the degree of frugivory may account for the fact that some primate species seem to be opportunistic with regard to their preferred source of metabolic energy whereas others are not, but so far the data are inconclusive and further research with other species is therefore needed in order to prove or disprove this hypothesis.

References

1. Glander KE (1982) The impact of plant secondary compounds on primate feeding behavior. Yearbook of physical anthropology 25, 1-18.

2. Laska M, Hermandez Salazar LT & Rodriguez Luna E (2000) Food preferences and nutrient composition in captive spider monkeys, Ateles geoffroyi. International Journal of Primatology 21, 671-683.

3. Laska M (2001) A comparison of food preferences and nutrient composition in captive squirrel monkeys, Saimiri sciureus, and pigtail macaques, Macaca nemestrina. Physiology & Behavior 73, 111-120.

4. Visalberghi E, Sabbatini G, Stammati M & Addessi E (2003) Preferences towards novel foods in Cebus apella: the role of nutrients and social influences. Physiology & Behavior 80, 341-349.

5. Laska M, Luna Baltazar JM & Rodriguez Luna E (2003) Food preferences and nutrient composition in captive pacas, Agouti paca (Rodentia, Dasyproctidae). Mammalian Biology 68, 31-41.

6. Ungar PS (1995) Fruit preferences of four sympatric primate species at Ketambe, Northern Sumatra, Indonesia. International Journal of Primatology 16, 221-245.

7. Bollard EG (1970) The physiology and nutrition of developing fruits. pp 387-425 in: Hulme AC (ed.) The biochemistry of fruits and their products vol 1. Academic Press, London.