Stable isotope enrichment in laboratory ant colonies: effects of colony age, metamorphosis, diet, and fat storage

Ecologists use stable isotopes to infer diets and trophic levels of animals in food webs, yet some assumptions underlying these inferences have not been thoroughly tested. We used laboratory-reared colonies of Solenopsis invicta Buren (Hymenoptera: Formicidae: Solenopsidini) to test the effects of metamorphosis, diet, and lipid storage on carbon and nitrogen stable isotope ratios. Effects of metamorphosis were examined in ant colonies maintained on a control diet of domestic crickets and sucrose solution. Effects of a diet shift were evaluated by adding a tuna supplement to select colonies. Effects of lipid content on stable isotopes were tested by treating worker ants with polar and non-polar solvents. C-13 and N-15 values of larvae, pupae, and workers were measured by mass spectrometry on whole-animal preparations. We found a significant effect of colony age on C-13, but not N-15; larvae, pupae, and workers collected at 75days were slightly depleted in C-13 relative to collections at 15days (C-13=-0.27 parts per thousand). Metamorphosis had a significant effect on N-15, but not C-13; tissues of each successive developmental stage were increasingly enriched in N-15 (pupae, +0.5 parts per thousand; workers, +1.4 parts per thousand). Availability of tuna resulted in further shifts of about +0.6 parts per thousand in isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on C-13, but treatment with a non-polar solvent resulted in enriched N-15 values of +0.37 parts per thousand. Identifying regular patterns of isotopic enrichment as described here should improve the utility of stable isotopes in diet studies of insects. Our study suggests that researchers using N-15 enrichment to assess trophic levels of an organism at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing with holometabolous insects at one site and spiders or hemimetabolous insects at another site.