Abstract:
Unlike their wild counterparts, many white rhinoceros females in captivity fail to
reproduce successfully such that current captive populations are not self-sustaining. The
causes of the problem are poorly understood. Variation in cycle length and long periods
of acyclicity are characteristics of the majority of these non-reproducing females in
captivity but it is unknown whether these characteristics are a feature of reproductively
successful free-ranging females. This study therefore aimed to monitor cyclic activity in a
wild population of southern white rhinoceros at Lapalala Wilderness, South Africa, by measuring the concentrations of immunoreactive fecal progestagen metabolites (fPM).
Five adult females were tracked twice per week for 20 months and if located a fresh fecal
sample was collected. Reproductive events and group structural dynamics were also
recorded and subsequently correlated with the fPM data. The baseline concentration of
fPM was 0.69 ± 0.20 μg/g DW while concentrations during pregnancy were 30- to 400-
fold higher. The females exhibited estrous cycle lengths of 30.6 ± 7.7 days and, based on
fPM data, gestation length in one female was 502 ± 3 days. Year-round monitoring
showed no clear evidence of seasonality in ovarian activity. During cyclic luteal activity
females were often seen in the presence of a dominant bull. One female stopped cycling
after removal of the local dominant bull and luteal activity only returned after a new bull
was introduced. This suggests that white rhinoceros females in the wild might need
external stimuli from a male to ovulate. These findings indicate that the irregular cyclicity
reported for white rhinoceros housed in zoos and animal parks may result from conditions
in captivity and account for reduced fertility.
