Ecological stoichiometry of ants in a New World rain forest

Abstract

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.

On the biogeography of salt limitation: A study of ant communities

“…Both genera, Pheidole and Solenopsis, may be buffered from NaCl deprivation by being carnivorous (16). Consistent with our second prediction, carnivorous ant genera and subfamilies are less likely to recruit to salt than those with lower delta 15N signatures (24). Sodium stress may also be tempered by reducing body sodium (17), increasing the efficiency of osmoregulation (30,31), switching sodium with phosphate or potassium (2), or feeding at higher trophic levels (16).…”

Bacterial gut symbionts are tightly linked with the evolution of herbivory in ants

“…Because honeydew and plant exudates are rich in sugar but poor in essential amino acids (33)(34)(35), these ants may require additional nitrogen from alternative sources (30,31,36). Based on the incidence of gut microbes in exudate-feeding ants, Cook and Davidson hypothesized that they rely on the metabolic activities of bacteria to make up for their dietary shortcomings (21).…”

Leucoagaricus gongylophorus uses leaf-cutting ants to vector proteolytic enzymes towards new plant substrate

“…Particularly, the latter transition allowed the ants to evolve much larger and complex societies (Mueller et al, 1998;Villesen et al, 2002;Schultz and Brady, 2008;Fernandez-Marin et al, 2009;Mehdiabadi and Schultz, 2010) after they had managed to overcome a series of challenges that are associated with a herbivorous life-style (Schiøtt et al, 2008De Fine Licht et al, 2010. Although depending on live vegetation rather than dead leaf litter undoubtedly implied better access to the proteins that normally limit insect growth, an almost exclusive fungal diet is generally poorer in protein than the carnivorous diets of many hunter-gatherer species in the Myrmicinae subfamily of ants (Davidson, 2004). Hosts and symbionts can thus be assumed to have been under consistent selection to maximize protein yield as this will determine colony growth rate and the production of winged virgin queens that transmit both their own genes and clonal copies of the fungal symbiont to future generations.…”