Tracking individual dietary choices in complex past food webs: Integrated bulk collagen and compound-specific stable isotope analysis of urban cats

Reconstructing dietary strategies in past urban environments is challenging because of the complexity of anthropogenic impacts on food webs, in which diverse local and imported resources with variable isotopic compositions limit the interpretive power of conventional bulk-collagen stable isotope analysis. We address this problem using domestic cats as dietary proxies, exploiting their opportunistic foraging behavior to track resource partitioning within a medieval urban landscape. We present an integrated multi-isotope study of cat bone collagen from medieval Gdańsk (northern Poland) and six regional comparative sites, combining bulk collagen δ13C, δ15N, and δ34S analysis with compound-specific δ13C and δ15N analysis of amino acids. Despite apparent isotopic homogeneity in the Gdańsk assemblage within the conventional bulk collagen dual-isotope space, the expanded isotopic framework reveals substantial individual-level dietary variability. Sulfur isotopes indicate diverse food provenances, including baseline values of the marine ecosystem and grain imported from inland regions. Amino acid-based fingerprinting identifies diets spanning terrestrial C3 and C4, freshwater, and marine ecosystem baselines, while amino acid-based trophic position proxies reveal highly variable individual trophic levels uncorrelated with bulk collagen δ15N values. These findings demonstrate that isotopic homogeneity in the conventional dual-isotope space can mask substantial dietary heterogeneity. Our study highlights the potential of integrating amino acid analysis with sulfur isotope analysis to reconstruct individual-level food choices in complex past environments.