Dietary impact on circulating glucose profiles in the white rhinoceros

Excess dietary glucose may be a factor in several captive wildlife diseases and reproductive abnormalities. The first step in understanding the health consequences of diets high in glucose is to characterize how dietary glucose concentrations change circulating glucose profiles. We adapted the glycemic index approach to detect differences in blood glucose concentrations in white rhinos in response to different meals. Six white rhinos were fasted overnight then randomly assigned to be fed 5 kg of grass hay and one of five meals varying in digestible energy (DE) availability and source (10% DE glucose, 5% DE glucose, 10% DE pelleted horse feed, 10% DE lucerne hay, 10% DE grass hay). After eating, the blood glucose response peaked 45–90 min later and remained elevated up to 180 min. Area under the curve results demonstrated that the blood glucose response was not different between diets. However, at 90 min, serum glucose levels in rhinos eating the 10% lucerne hay diet were significantly lower than the 5% glucose and 10% glucose diets but not the 10% pellet nor 10% grass hay diets. The changes in blood glucose responses to different diets were similar in magnitude to reported domestic horse profiles but are higher than predicted by allometric scaling. We conclude that the grass hay, lucerne hay and low glycemic index horse pellets fed in this study resulted in similar blood glucose responses in white rhinos. The validation of the methodology used in this study is a first step towards elucidating the relationship between glucose, obesity, health and reproduction in rhinos.     

The first report of urine overmarking of pro-oestrus female dung by a male white rhino

Although observed in other Perrisodactyla, urine overmarking in white rhinos has not been described. Using a single opportunistic sighting, we were able to collect two dung samples from one oestrous female white rhino; one unmarked and one overmarked with male urine. Because of the behaviour of both the male and the female, we suggest that the female was in pro-oestrus. We hypothesised that the function of overmarking by the male was for oestrus concealment (i.e. odour masking), as observed in horses. Because dung from oestrous female white rhinos emits a higher proportion of alkanes than non-oestrous dung, we expected the proportion of alkanes emitted from oestrous dung to decrease after overmarking. In contrast, we found that after overmarking the proportion of alkanes emitted increased. We suggest that the function of urine overmarking in white rhinos could be to conceal all signals of reproductive condition, so that neither oestrous nor non-oestrous signals are recognisable, or that a signal is added to indicate that the female has been mated.