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A consequence of the poaching crisis is that managed rhinoceros populations are increasingly important for species conservation. However, black rhinoceroses (BR; Diceros bicornis) and Sumatran rhinoceroses (SR; Dicerorhinus Sumatrensis) in human care often store excessive iron in organ tissues, a condition termed iron overload disorder (IOD). IOD research is impeded by the challenge of accurately monitoring body iron load in living rhinoceroses. The goals of this study were to (i) determine if labile plasma iron (LPI) is an accurate IOD biomarker and (ii) identify factors associated with iron-independent serum oxidative reduction potential (ORP). Serum (106 samples) from SRs (n  = 8), BRs (n  = 28), white rhinoceros (n  = 24) and greater one-horned rhinoceros (GOH; n  = 16) was analysed for LPI. Samples from all four species tested positive for LPI, and a higher proportion of GOH rhinoceros samples were LPI positive compared with those of the other three species (P  < 0.05). In SRs, the only LPI-positive samples were those from individuals clinically ill with IOD, but samples from outwardly healthy individuals of the other three species were LPI positive. Serum ORP was lower in SRs compared with that in the other three species (P  < 0.001), and iron chelation only reduced ORP in the GOH species (P  < 0.01; ~5%). Serum ORP sex bias was revealed in three species with males exhibiting higher ORP than females (P  < 0.001), the exception being the SR in which ORP was low for both sexes. ORP was not associated with age or serum iron concentrations (P  ≥ 0.05), but was positively correlated with ferritin (P  < 0.01). The disconnect between LPI and IOD was unanticipated, and LPI cannot be recommended as a biomarker of advanced rhino IOD. However, data provide valuable insight into the complex puzzle of rhinoceros IOD. [ABSTRACT FROM AUTHOR]