Solute and particle retention in a small grazing antelope, the blackbuck (Antilope cervicapra)

Digesta retention patterns have been suggested to play a major role in ruminant diversification. Two major digestion types have been proposed, termed 'cattle-type' and 'moose-type', that broadly correspond to the feeding categories of grazers and intermediate feeders on the one, and browsers on the other hand. We measured and calculated the mean retention time (MRT) of a solute and a particle (>2mm) marker in the gastrointestinal tract (GIT) and the reticulorumen (RR) of a small grazer, the Indian blackbuck (Antilope cervicapra, n=5, body mass of 26±4kg) and an intermediate feeder, the nilgai (Boselaphus tragocamelus, n=5, body mass of 168±21kg). MRTsolute and MRTparticle were 29±4.1h and 60±6.6h in blackbuck and 28±2.5h and 54±8.9h in the nilgai for the GIT, and 14±1.7h, 45±5.0h, 19±2.0h and 45±8.4h for the RR, respectively. With a selectivity factor (SF, the ratio of MRTparticle to MRTsolute) in the RR of 3.2±0.28 for blackbuck and 2.3±0.36 for nilgai, both species are clearly in the category of 'cattle-type' ruminants. In particular, the high SFRR of blackbuck, in spite of its small body size, is remarkable, and leads to specific predictions on the RR anatomy of this species (such as a particularly large omasum), which can be tested in further studies. The adaptive value of a high SFRR is mainly considered as an increase in microbial productivity in the RR; exemplary estimations based on the measured passage rates of solutes indicate an 8% higher production of microbial mass in blackbuck compared to nilgai.