Ecological stoichiometry of ants in a New World rain forest.

C:N stoichiometry was investigated in relation to diet (delta(15)N), N-deprivation, and worker body size for a diverse assemblage of tropical Amazonian ants. Relative nitrogen (N) deprivation was assayed for 54 species as an exchange ratio (ER), defined as SUCmin/AAmin, or the minimum sucrose concentration, divided by the minimum amino acid concentration, accepted as food by >/=50% of tested workers. On average, N-deprivation (ER) was almost fivefold greater for N-omnivorous and N-herbivorous (N-OH) taxa than for N-carnivores. In two-way ANOVAs at three taxonomic levels (species and species groups, genera, and tribes), higher ER was associated with small body size and (marginally) with less carnivorous diets. ERs did not differ systematically between trophobiont-tending and "leaf-foraging" functional groups, but specialized wound-feeders and coccid-tenders were prominent among high ER taxa. Paradoxically, some high ER taxa were among the most predatory members of their genera or subfamilies. Biomass % N was lower in N-OH taxa than in carnivores and varied inversely with N-deprivation (log ER) in the former taxa only. In an expanded data set, N-content increased allometrically in N-OHs, N-carnivores, and all ants combined, and with carnivory in large-bodied ants only. Exceptional taxa included small-bodied and predaceous Wasmannia, with high % N despite high ER, and Linepithema, with the lowest % N despite high delta(15)N. Patterns in C:N stoichiometry are explained largely at the genus level and above by elemental composition of alarm/defensive/offensive chemical weaponry and, perhaps in some cases, by reduced N investment in cuticle in taxa with high surface:volume ratios. Several consequences of C:N stoichiometry identify Azteca, and possibly Crematogaster, as taxa preadapted for their roles as prominent associates of myrmecophytes. C:N stoichiometry of ants should be incorporated into models of strategic colony design and examined in a phylogenetic context as opportunities permit.