Comparative investigations on digestion in grazing (Ceratotherium simum) and browsing (Diceros bicornis) rhinoceroses.

Rhinoceroses represent the largest extant herbivores with extensive dietary specialization for plant groups like browse (black rhino Diceros bicornis) or grass (white rhino Ceratotherium simum). However, it is not clear to what extent such diet selection patterns are reflected in adaptations of digestive physiology of the respective feeding types. In this study, feeding trials with four black and five white rhinos were conducted in four zoos. The animals had ad libitum access to the same batch of grass hay (second cut; neutral detergent fiber (NDF) 63% dry matter (DM), crude protein 10.2% DM). Total intake, fecal N content, in vitro digestibility of NDF residues of feces, fecal particle size and mean retention time (MRT) of particles (Cr-mordanted fiber; 1-2mm) and fluid (Co-EDTA) were quantified. The average daily DM intake was 70+/-12 g/kg BW(0.75) for white and 73+/-10 g/kg BW(0.75) for black rhinos. In the in vitro fermentation test fecal NDF residues of black rhinos resulted in higher gas productions at fermentation times of 12 to 24h, indicating that white rhinos have a superior capacity to digest NDF. Average MRT for fluids and particles was 28+/-4h and 43+/-5h in white and 34+/-4h and 39+/-4h in black rhinos. The selectivity factor (SF=MRT(particle)/MRT(fluid)) was higher for white (1.5+/-0.2) than for black rhinos (1.2+/-0.1) (p=0.016). In a comparison of 12 ruminant and 3 rhino species, SF was correlated to percentage of grass in diet (R=0.75). Mean fecal particle size was higher in white (9.1+/-1.94 mm) than in black rhinos (6.1+/-0.79 mm) (p=0.016). The results demonstrate differences between white and black rhinos in terms of retention times and fiber digestibility. The more selective retention of particles by the white rhino corresponds with the higher digestion of fiber measured indirectly. Furthermore there is indication for a general pattern of high SF in grazing ruminants and rhinos. The difference in fecal particle size between both rhino species might be due to the considerable difference in body weight.