Coexisting ant species differ in ability to meet their intrinsic nutritional needs

Coexisting species are often assumed to reduce competition by foraging for foods that differ in nutrient composition, but few studies test whether field nutritional differences correspond to intrinsic differences in macronutrient selectivity. We compared field estimates of worker trophic position (tissue isotopes: δ¹⁵N) and worker tissue stoichiometry (elemental ratios: C:N) among 17 co-occurring Australian ant species with laboratory protein:carbohydrate intake targets (P:C ITs), measured in brood-free worker groups under competition-free, standardised conditions. Field-collected ants differed strongly in trophic and stoichiometric signatures – worker δ¹⁵N spanned 7.1‰ among species and C:N varied 1.33-fold. In contrast, laboratory P:C ITs were comparatively overlapping (mean 1.00 ± 0.06 SE; range 0.60–1.42). Lab P:C ITs did not predict species-mean δ¹⁵N or C:N (H1); pairwise coupling between lab P:C IT and field metrics was weak at best (~ 5% variance; H2), and residual/compensatory divergence was unsupported (H3). Synthesis – these results show that co-occurring ant species can differ markedly in field trophic position and worker stoichiometry despite overlapping lab-measured macronutrient intake selectivity. This lab–field decoupling is consistent with extrinsic ecological constraints on realised nutritional outcomes, but the mechanisms generating these differences, including any role in coexistence, were not directly tested here.