show Abstracthide AbstractIron overload disorder is a prevalent condition in captive black rhinoceroses (Diceros bicornis), while this condition has not been reported in the wild. The objective of this study was to gain a deeper understanding of the relationship between vitamin D, iron metabolism, inflammatory status, metabolic status, the gut microbiome, diet components, and season in black rhinoceroses, to elucidate potential causes of iron overload disease (IOD). Part of this investigation concentrated on the description of the gut microbiome, gut mycobiome, and the presence of plant residues based on biomarker analysis of fecal samples collected bi-monthly during the period April 2021 to April 2022 in five zoological institutes in Europe with in total 12 black rhinoceroses.