Regional variability in faecal bile acids and its implications for species identification in archaeology

Abstract Faecal bile acids (BAs) are increasingly used in archaeological studies to detect domesticated species and reconstruct herding practices. However, it is unclear how environmental factors influence BA profiles and how much these profiles vary across regions. This uncertainty raises questions about their reliability for species-specific identifications across diverse settings. To address this, we analysed faecal BA profiles from free-grazing sheep ( Ovis aries ) across a wide range of climatic and elevational zones in Iceland, Germany, Tanzania, Iran, Mongolia, and Tibet. To test whether closely related species living under similar environmental conditions can be distinguished based on their BA profiles, we also analysed goats ( Capra aegagrus ssp. hircus ) from Tanzania and blue sheep ( Pseudois nayaur ) from Tibet, each compared to their respective co-occurring domestic sheep. We find significant regional differences in sheep primary, secondary, and oxo-BA profiles, with BA abundances influenced by altitude and precipitation. For instance, cholic acid varies with precipitation, while deoxycholic acid and lithocholic acid are significantly influenced by elevation. Oxo-BAs consistently account for 50–80% of total BAs in these animal groups, comparable to cattle (~ 60%) and much higher than in monogastric species (9–25%), suggesting their potential for separating ruminants from non-ruminants. While no individual BA or ratio serves as a universal marker for sheep, intra-regional comparisons suggest that goats and sheep in Tanzania, and blue sheep and domesticated sheep in Tibet, can be distinguished based on their faecal BA profiles. These findings show that caution is needed when applying modern BA reference data across regions. Species identification based on one region’s samples may lead to misinterpretation elsewhere. Because BA composition is shaped by both species and environment, robust interpretation requires a regionally grounded, multi-proxy approach.