Differences in selective reticulo-ruminal particle retention as a key factor in ruminant diversification

The measurement of passage rate is important for the concept of ruminant di versification. While supporters of Hofmann's 1989 feeding type classificati on claim that browsing ruminants have faster passage rates than grazing rum inants, other researchers consider the passage rate to depend on body size alone. To date, no convincing comparison of ruminant passage rates has been put forward. For comparative purposes, we suggest the use of the "selectiv ity factor", which is an expression of how much longer particles of a defin ed size (<2 mm) are retained in the ruminant digestive tract than fluids. F rom the limited data available, it seems that grazing ruminants display sel ectivity factors between 1.56 and 3.80, whereas browsers have a much narrow er range of 1.14-1.80. This suggests that browsers are not able to selectiv ely retain particles as long as grazers. Intake of browsers, on the other h and, may not be limited by physical fill of the forestomach to the same deg ree as in grazers. This result can explain several observations on the dige stive physiology of browsers, some of which have been linked to a rumen byp ass mechanism. We propose that the ability for selective particle retention is a key factor for understanding the physiological consequences of rumina nt diversification.