Howling Monkeys and Leaf-Cutting Ants: Comparative Foraging in a Tropical Deciduous Forest

Abstract

Foraging by mantled howling monkeys (Alouatta palliata) and leaf-cutting ants (Atta colombica) is compared with regard to plant species harvested, plant parts harvested, statistical patterns in foraging effort, seasonal variation in foraging effort, daily foraging patterns, and foliage height attacked. This comparison is possible because intensive, year-long studies of foraging by three colonies of A. colombica and by a group of A. palliata were conducted in the same study area (Hacienda La Pacifica) in the Guanacaste Province of Costa Rica. The results show that howling monkeys and leaf-cutters differ in each of the categories mentioned above, yet share many foraging patterns. Both organisms are generalist herbivores, but are also selective. Both leaf-cutters and howlers harvest a variety of plant parts including mature leaves, new leaves, fruit, and flowers, and both cut more mature leaves and fruit in the rainy season and more new leaves and flower parts in the dry season. Both organisms prefer new leaves to mature leaves if given a choice. Nevertheless, howling monkeys and leafcutters prefer different plant species. Of seven rank correlations between monkey and ant preferences, three were negative, and the best positive correlation was 0.100, which was not significant. In addition, based on yearly averages, howlers harvest more new leaves and fruit and fewer mature leaves than do leaf-cutters. These results are compared with recent theoretical predictions concerning feeding behavior in generalist herbivores. THE PURPOSE OF THIS PAPER iS to compare foraging in mantled howling monkeys (Alouatta palliata) and leaf-cutting ants (Atta colombica). This comparison is possible because intensive, year-long studies of foraging by three colonies of A. colombica and by a group of A. palliata were conducted in the same study area in the Guanacaste Province of Costa Rica by Rockwood (1972, 1973a, 1975, 1976) and by Glander (1975a, 1975b, 1977). While it might appear unusual to compare foraging in two such different animals, there are compelling reasons to do so. Howlers and leaf-cutters are both generalist herbivores that cooccur throughout most of the-neotropics (Moynihan 1976; Weber 1966, 1972). They are generalist herbivores in the sense that they consume quantities of material from many plant species. For example, Glander (1975a) found that howlers sampled 61 of 96 (63.5%) tree species present in their home range, while Rockwood (1972, 1976) found that a colony of A. colombica gathered material from 47 of 70 (67.1 %) woody species present in their foraging area. At the same time, both howlers and leaf-cuters are selective. Glander (1975a, 1977) found that howlers spent 75 percent of their feeding time in only 88 of 1699 trees present in the habitat. Rockwood (1976) has demonstrated similar selectivity in both A. colombica and A. cephalotes. In three colonies of A. colombica studied, 80-90 percent of the mature leaves cut by each colony were selected from 10 plant species out of the 57-62 species available in a foraging area. Similarly, in three A. cephalotes colonies, the top six plant species of 36-44 available to each colony accounted for 82-95 percent of mature leaves cut. Howlers and leaf-cutters both harvest flower and fruit pieces and show decided preferences for new versus mature leaves. These preferences show seasonal patterns. Finally, both species depend on mutualistic relationships for digestion of the vegetation they consume. Howling monkeys have the usual mammalian intestinal flora and depend to an unknown degree on fermentation of their food in an enlarged caecum. Colonies of Atta feed on a fungus which is cultured on vegetation brought into the nest by the ants. Thus, while howling monkeys and leaf-cutting ants are not closely related in an evolutionary sense, they appear to have converged ecologically as generalist herbivores in neotropical forests. Since both howlers and Atta colombica forage mainly in the forest canopy they may be important interspecific compe-