Serum testosterone in captive male rhinoceros: Effects of season, age, stress, and sociosexual environment
Publication Type: |
Thesis |
Year of Publication: |
2007 |
Authors: |
Bruce W. Christensen |
Publication/Journal: |
Reproductive Physiology: College of Veterinary Medicine |
Publisher: |
University of Florida |
Keywords: |
breeding, captive, cortisol, eia, elisa, endangered, enzyme immunoassay, free-ranging, management, reproductive physiology, rhinoceros, testosterone, thesis |
Abstract:
All rhinoceros species are in some degree of endangerment. Understanding the reproductive physiology of rhinoceros species will help in management decisions with both free-ranging and captive populations. In mammalian species studied, testosterone plays a key role in spermatogenesis and libido. Testosterone concentrations, depending on the species, may vary according to age, season, social status, stress factors, nutrition, and disease. Using an enzyme-linked immunoassay, we measured serum testosterone concentrations in 47 male black rhinoceros (Diceros bicornis), 38 male white rhinoceros (Ceratotherium simum), and 14 Asian greater one-horned rhinoceros (Rhinoceros unicornis) housed at 43 different institutions in the United States. We analyzed the relationships between testosterone concentrations and age, time of year, serum cortisol concentrations, and presence of conspecific males and females. In male black rhinoceros, serum testosterone concentrations rose until 7.9 years of age and then leveled off, probably indicating completion of sexual maturity, until a decline starting at 21 years of age, likely indicating senescence. No seasonal changes were noted in male black rhinoceros (P > 0.05). Testosterone concentrations rose with increasing numbers of females present (P < 0.05). Average testosterone concentrations also rose with an increased number of conspecific males (P < 0.05). Serum cortisol concentrations were undetectable with our assay in 9 black rhinoceros. In white rhinoceros, testosterone rose during the first 14.5 years of life before declining at a constant rate from that point onward, likely indicating completion of sexual maturity at 14.5 years and then progressive senescence (P < 0.05). No seasonal changes in testosterone or correlation with cortisol concentrations were observed for white rhinoceros (P > 0.05), and testosterone concentrations also rose in white rhinoceros males with increasing numbers of females present (P < 0.05). Data from the Asian greater one-horned (GOH) rhinoceros showed increasing testosterone for the first 3.7 years of life. Data from Asian GOH rhinoceros did not reflect seasonal change, correlation with cortisol concentrations, nor variation with numbers of conspecifics (P > 0.05). The results from this research can be useful in management decisions regarding when and where to move certain males considered valuable to a breeding program.