Figure 4. Schematic explanation of circular reasoning in the traditional approach of explaining a positive effect of body mass on digestibility.

a) The difference in the scaling of gut capacity (measured as wet or dry gut contents; BM^1.0) and daily dry matter intake (BM^0.75), or actual dry matter gut fill rate, results in more gut available per unit digesta at higher BM, and should hence lead to increased mean retention times at higher BM (BM^0.25). If these increased retention times are used to postulate a higher digestibility at higher BM, the situation in b) occurs: The increasing digestibility reduces the actual gut fill rate, hence increases the difference in the scaling of gut capacity and gut fill rate even more, which should translate into even longer retention times.