Leafcutting Ant Diet Selection: The Role of Nutrients, Water, and Secondary Chemistry

The relationship of plant secondary chemistry, water content, and nutrient content to the palatability of leaves to the leafcutting ant Atta cephalotes was determined in a study of individuals from 50 woody plant species in tropical dry forest of Costa Rica. The study took place during the yearly period of maximum leaf harvest, in the early rainy season. The palatability of plants was determined by presenting leaf disks to ants on active foraging trails of three ant colonies. The distribution of several classes of polar secondary compounds in leaf samples was determined using chemical spot tests, and that of ant—deterrent nonpolar compounds using laboratory bioassays of plant nonpolar extracts. Nitrogen, energy, and water availability in leaves were assessed using quantitative analyses. Plants of high relative palatability generally lacked deterrent nonpolar extracts but did contain hydrolyzable tannins; the opposite was true for plants of low palatability. Little or no relationship was found to exist between plant palatability and measures of leaf nitrogen, energy content, and moisture content, or the presence of alkaloids and condensed tannins in leaves. No statistically significant interactions among leaf characteristics were found. The number of leaf disks harvested by ant colonies decreased during the study period. This decrease was unrelated to nutrient and moisture availability in leaves, but may be related to changes in secondary chemistry over the course of the season. Although plants were not equally palatable to all colonies tested, there was no evidence that colony—to—colony differences in plant preferences were related to the plant characteristic studied. Overall, the results suggest that plant secondary chemistry is the best predictor of plant species utilization by leafcutting ants.