Salt intake in Amazonian ants: too much of a good thing?

Although herbivory is widespread among insects, plant tissues rarely provide the optimal balance of nutrients for insect growth and reproduction. As a result, many herbivorous insects forage elsewhere for particular amino acids and minerals. Recent studies have shown that both herbivory and recruitment to sodium are commonplace among tropical rainforest ants, but little is known about how ants regulate their sodium intake at the individual and colony levels. In social insects, foragers may respond not only to their own nutritional deficiencies but also to those of their nestmates, who may have different nutritional requirements depending on their developmental stage, sex, or caste. Here, we investigate how salt stress among rainforest ants affects their preferences for salt and subsequent survival. We found that ants recruited more to salt than to any other bait type tested, confirming the strong preference for salt of ants in this region. Initially, we observed similarly high recruitment to salt among workers of the arboreal, herbivorous/omnivorous ant species Camponotus mirabilis. However, when provided with unrestricted access to high concentrations of salt, C. mirabilis workers suffered significantly higher mortality relative to controls, suggesting that C. mirabilis workers forage for sodium to the point of toxicity. Nonetheless, surviving workers showed reduced preference for salt at the end of the experiment, so some but not all individuals were able to regulate their salt intake beneath lethal dosages. We discuss how salt intake regulation may depend on colony members other than workers.