Hindle, J.E.; Moestl, E.; Hodges, J.K. 1992. Measurement of urinary oestrogens and 20a-dihydroprogesterone during ovarian cycles of black (Diceros bicornis) and white (Ceratotherium simum) rhinoceroses. Journal of Reproduction and Fertility 94: 237-249, figs. 1-5, table 1.