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indicating specific gravities below that of the medium, whereas the smaller
particles were found mainly in the bottom layer. In sheep, particles move in such a
way that the lower layer of the RR does hardly contain any larger particles (Kaske
et al. 1992). Only particles of high density sink into the ventral layers of the
stratified RR contents, from where they can be expelled out of the RR with the next
reticular contraction. As the density of particles is closely correlated to their size,
larger particles being less dense, this mechanism ensures that only small particles
can leave the RR. In the roe deer, however, no such correlation between particle
size and particle density could be demonstrated.
If there is a higher proportion of larger particles in the bottom layer of the roe
deer’s forestomach, one would expect that these larger particles are just as likely to
leave the RR with the next contraction as the small particles are. Indeed, it has
been observed that the faeces of browsers contain larger particles than the faeces of
grazing ruminants of comparable size (Nygren and Hofmann 1990, Renecker and
Hudson 1990, Van Wieren 1996). For our data, the particle distribution of roe deer
faeces corresponds well with the values Ulyatt et al. (1986) give for the abomasal
content of sheep. Given the fact that the threshold particle size for RR escape is
1.5–2 times greater in cattle than in sheep (Ulyatt et al. 1986), one might interpret
the correspondance of roe deer and sheep thresholds, in regard of their different
body size, in such a way that the smaller browser roe deer must have a relatively
greater threshold particle size for RR escape than the bigger grazer sheep.
The data gained in this investigation leaves us in an explanatory dilemma. Given
the results on the stratification behaviour of roe deer RR contents, it would be easy
to explain the absence of selective retention. But selective retention, though less
prominent than in grazers, does occur in browsing ruminants, as can be deducted
from the comparison of the particle size distributions in rumen and faecal contents.
As chewing is the main factor responsible for particle size reduction in domestic
ruminants (Ulyatt et al. 1986), which we assume to be true for browsing ruminants
as well, the larger particles, once in the rumen, have to be selectively retained in
order to be re-subjected to repeated chewing via rumination. However, this
selective retention cannot be explained by the mechanisms of stratification,
filter-bed effect and particle density, as in the grazing ruminants.
Perhaps in browsers/concentrate selectors, particle size itself rather than
particle density is the decisive factor, and larger particles are physically prevented
from passing the reticulo-omasal orifice. Or maybe there is no thorough mixing of
RR ingesta during RR contractions, leaving the newly eaten, coarser particles on
top of the older ingesta, from where they will be regurgitated again for rumination
– a kind of stratification that does not depend on particle density, just on feeding
chronology. This type of stratification would not be reproducible in vitro, and would
not be observed in killed, dissected specimens. Further studies on the physiology of
the reticulorumen of browsing ruminants are warranted.
Acknowledgements: We thank B. Peters and C. Witte for their help in literature acquisition, and
Dr S. Van Wieren and one anonymous referee for their comments on the manuscript.