The trophic ecology of castes in harvester ant colonies

Summary 1. The castes of social insects can vary dramatically in physiology, morphology and behaviour, and are a model of phenotypic plasticity. Dietary variation has been implicated in the evolution of castes and is involved in developmental differentiation. This study uses the elemental and isotopic composition of castes to infer the diets they consumed during larval development, when caste was determined. 2. We analysed the elemental (C : N) and isotopic (δ13C and δ15N) composition of individuals of all castes in colonies of Pogonomyrmex badius, the Florida seed harvester ant, at each of two sites. Sampled herbivores and insectivores were used to calibrate measures of colony trophic position. The proportions of insects and seeds foraged by colonies correlated with inferred trophic position, strengthening the inference of diet from isotopic composition. 3. Pupae of each of the four castes (male, reproductive female or gyne, major worker and minor worker) are distinguishable based on their composition and inferred diet during the larval phase. 4. Within colonies, larval diet inferred from δ15N suggests that the largest individuals assimilated more insect/protein relative to seeds. This result was consistent across colonies despite high variation in a colony’s relative trophic position. 5. Carbon to nitrogen ratio (C : N) of each caste was also different, and differences were consistent across colonies and sites. While females increase in N content with increasing size, the pattern is reversed in males. This result may reflect basic life history differences (e.g. life span) between males and females. Since female reproductive potential correlates with female size in this species, the decrease in N content with increasing female caste size supports that reproductive development is N-limited, and that workers are trophically castrated due to N deprivation during larval development.