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

Abstract 1. Coexisting species are often assumed to reduce competition by foraging for different foods with different nutrients, but few studies test whether nutritional partitioning reflects innate differences in nutritional needs. 2. We tested for overlap in field estimates of realised nutritional intake (tissue isotopes: δ¹⁵N; elemental ratios: C:N) among 17 co-occurring Australian ant species, then measured innate nutritional selectivity (protein:carbohydrate intake targets; P:C IT) for the same species using competition-free laboratory feeding experiments to infer whether extrinsic constraints prevent nutritionally optimized diets. 3. Free-ranging ants showed dietary and nutritional partitioning, with worker δ¹⁵N spanning 7.1‰ among species and C:N varying 1.33-fold, whereas lab-maintained ants exhibited overlapping P:C ITs (mean 1.00 ± 0.06 SE; range 0.60–1.42). Lab P:C ITs did not predict species-mean δ¹⁵N or C:N in the field (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), consistent with natural constraints (e.g., competitive monopolisation of carbohydrates, brood-mediated demand, risk-sensitive foraging) decoupling innate targets from realised intake. 4. Synthesis: By integrating field-based proxies of realised intake with controlled lab measures of innate macronutrient selectivity across a co-occurring ant assemblage, we show that strong divergence in realised diets can arise despite narrow, overlapping nutritional selectivity, advancing ecological understanding of how extrinsic constraints can generate nutritional niche differences in communities.