Particle size reduction along the digestive tract of fat sand rats (Psammomys obesus) fed four chenopods

It is generally accepted that microbial digestion contributes little to digesta particle size reduction in herbivores, and that faecal particle size reflects mainly chewing efficiency, and may vary with diet. Nevertheless, a decrease in mean particle size (MPS) along the gastrointestinal tract (GIT) has been reported, especially in hindgut fermenters. However, to what degree the very fine particle fraction (non-food origin, especially microbes) affects MPS is unclear. Fat sand rats (Psammomys obesus, diurnal herbivores, n = 23, 175 ± sd 24 g) consumed one of four chenopods (natural dietary items in the wild) for 30 days. Digestibility was related negatively to dietary fibre content. We determined digesta MPS in the forestomach, glandular stomach, small intestine, caecum, colon and faeces by wet sieving, including (MPS_fines) or excluding (MPS_nofines) particles < 0.25 mm. The proportions of fines were higher and of MPS_fines were correspondingly lower in GIT sections that harbour microbes (forestomach, hindgut), whereas MPS_nofines did not differ between forestomach and glandular stomach. However, MPS_nofines decreased along the GIT, indicating MPS reduction due to digestive (enzymatic and microbial) processes. The four different diets led to different MPS, but the magnitude of MPS reduction in the GIT was not correlated with dietary fibre fractions or dry matter digestibility. These results indicate that within a species, MPS cannot be used as a proxy for diet quality or digestibility, and raise the hypothesis that MPS reduction along the GIT may be more pronounced in smaller than in larger mammalian terrestrial herbivores, possibly due to the fine initial particles produced by chewing in small species.