Monthly plasma oxytocin samples detect lactational but not social management or behavioral variation in ex situ black and white rhinoceroses

The neuropeptide hormone oxytocin is essential for uterine contractions and lactation, facilitates reproductive and social behaviors, and may also regulate stress and psychological state. Plasma samples were collected monthly from black ( Diceros bicornis ; n = 19 males, 17 females) and white ( Ceratotherium simum ; n = 20 males, 40 females) rhinoceroses housed across 33 North American facilities. Focal animal behavioral observations were conducted twice, ~6 months apart, for three to five days each time. Plasma oxytocin was extracted via proteomic reduction, alkylation, and precipitation methods. Successful analytical validation of the enzyme immunoassay consisted of a parallel displacement curve between serially diluted standards and pooled extracts, linearity across dilutions, and 96% recovery of spiked synthetic hormone. Monthly peripheral plasma oxytocin concentrations did not differ between black and white rhinoceroses but were positively associated with age in the former ( p = 0.049). In lactating white rhinoceroses, oxytocin increased from 0-365 days postpartum ( p 0.10). It is possible that more frequent sampling might better detect associations between oxytocin and transient social dynamics. This study provides the first data on oxytocin concentrations in black and white rhinoceroses and establishes a foundation for further investigations. • The enzyme immunoassay was analytically validated for detecting peripheral plasma oxytocin in rhinoceroses; oxytocin concentrations in lactating white rhinoceroses increased from 0-365 days postpartum, serving as a biological validation of the assay. • Plasma oxytocin concentrations were positively associated with age in black but not white rhinoceroses. • There were no differences in monthly oxytocin concentrations based on species, sex, pregnancy status, median group size, degree of social access, or rates of affiliative and agonistic behaviors.