Retention of solutes and particles in the gastrointestinal tract of a grazing cervid: Pere David's deer (Elaphurus davidianus)

Ruminants are classified into three groups, according to their feeding behaviour: browsers, intermediate feeders and grazers. Corresponding to their dietary preferences, multiple morphological and physiological adaptations have been described, resulting in another classification: moose-type' and cattle-type' ruminants. Digesta retention patterns in the gastrointestinal tract (GIT) and reticulorumen (RR) are considered major criteria to distinguish these types, as cattle-type ruminants show shorter retention of fluids (measured by a solute marker) than of particles, while in moose-type ruminants, both are retained for more similar periods. To what extent these digestive types are specific to phylogenetic lineages is still unclear. We measured mean retention times (MRTs) of solutes and particles (2 and 20mm) in the strictest grazing cervid: the Pere David's deer (Elaphurus davidianus; n=5; body mass=155.0 +/- 14.5kg). The MRTs of solutes, small and large particles in the GIT were 34 +/- 4, 60 +/- 7 and 69 +/- 9h, respectively. The ratio of the MRT of small particles versus solutes in the RR was 2.0 +/- 0.1, similar to other cattle-type ruminants. The results confirm the hypothesis that Pere David's deer can be classified as cattle-type ruminants, corresponding to both dietary preferences and previously described morphological traits. The results complement previous findings, showing that both cattle-type and moose-type physiologies are found among bovids as well as cervids, indicating that these digestion types can be considered convergent adaptations.