Faecal immunoglobulin A as a non-invasive biomarker of mucosal immunity and health in zoo and wild mammals.

Non-invasive biomarkers of immune function are increasingly important for assessing health, welfare and disease risk in zoo and wild mammals, particularly because they support repeated monitoring while minimizing handling-related disturbance. Secretory immunoglobulin A (IgA), a key component of mucosal immunity, can be quantified from faecal samples and provides a practical measure of gut-associated immune activity without invasive sampling. We conducted a systematic literature review of 21 peer-reviewed studies that quantified faecal IgA across diverse mammalian taxa and ecological contexts. Across species, faecal IgA was technically measurable and biologically responsive, but its interpretation was strongly context dependent. Reported patterns reflected interactions among pathogen exposure, physiological stress, nutritional state, life-history stage and management conditions. In captive settings, faecal IgA frequently varied with individual heterogeneity and management factors and showed inconsistent alignment with endocrine stress markers. In free-ranging populations, faecal IgA more commonly tracked parasite burden, reproductive investment, seasonal variation and host-microbiome dynamics. However, most ecological and welfare-associated patterns were derived from observational designs, which limit causal inference. Additionally, methodological heterogeneity in assay validation, sample processing and preservation limited direct quantitative comparison among studies. Overall, faecal IgA does not function as a unidimensional indicator of stress or welfare, but rather as a context-sensitive marker of mucosal immune allocation. We integrate these findings into a conceptual framework linking external pressures, mucosal immune dynamics, complementary biomarkers and health-related outcomes to guide interpretation across zoo and wild settings. When embedded within longitudinal and multi-marker approaches supported by species-specific validation, faecal IgA has potential to contribute meaningfully to non-invasive health assessment in conservation physiology.