Gut Capacity, Functional Allocation of Gut Volume and Size Distributions of Digesta Particles in 2 Macropodid Marsupials (Macropus-Robustus-Robustus and M-R-Erubescens) and the Feral Goat (Capra-Hircus)

Abstract

Gut capacity (wet mass of all digesta) was measured in the euro (Macropus robustus erubescens), the wallaroo (M. r. robustus) and the feral goat (Capra hircus). Gut capacity, as a proportion of body mass (mean+/-s.e.), was greatest (P<0.01) in the goat at 20.4+/-1.5%, 13.2+/-0.5% in the wallaroo, and 11.6+/-1.2% in the euro. Similarly, the mass of total fermentation contents was greatest (P<0.01) in the goat at 17.1+/-1.2% of body mass, 11.2+/-0.5% in the wallaroo and 10.6+/-0.7% in the euro. However, the allocation of the gut to various functions was similar among the species; the fermentation contents of the stomach were 70%, the small intestines were 9%, the caecum was 5%, and the colon was 9% of the total mass of digesta. There were no mechanisms in the gut of the two macropodids that selectively delayed digesta; the size distribution of digesta particles was similar in all sections of the gut. However, in the goat 25% of the dry matter from the ruminoreticulum was retained on 2.4- and 1.2-mm sieves, but no digesta from the other sections of the gut were retained on the 2.4-mm sieve and only 0.5-2% was retained on the 1.2-mm sieve. It was concluded that the goat has evolved a larger gut to meet higher metabolic energy requirements, but the allocation of the gut to various functions is similar among ruminants and macropodids, and this is a response to the evolution of herbivory in grasslands. The selective retention of large particles in the rumen, rumination and greater capacity account for the more complete digestion of plant cell wall by ruminants than by similar-sized macropodids.