ウサギはなぜ糞を食べる？

　The utilization of dietary energy and protein must depend on microbial activity in the gut in every herbivore. For animals adopting the cecum as a microbial habitat, from the viewpoint of the protein utilization, the position of the cecum in the digestive tract is less advantageous than that of foregut fermenters. As a solution to this problem, cecum fermenters perform cecotrophy in order to utilize microbial protein proliferating in the cecum. Cecotrophy is supported by the mechanism that separates microbes from digesta in the colon, sends them back into the cecum and promotes microbial proliferation in the cecum. The colonic separation mechanism can be classified into two types. One is the mucus-trap type separation of bacteria with mucus from digesta in the proximal colon of rodents. The other is the wash-back type separation of particle phase of the colonic contents and liquid phase containing microbes in the colon of rabbits. For microbes guaranteed to inhabit the cecum under colonic separation mechanism, it is necessary for them to obtain sufficient nutrients for survival and proliferation. The source of nitrogen is easily obtained as urea transfered from the blood flow. On the other hand, the energy source is considered to be the limiting factor for bacterial proliferation due to the limited amount of easily usable energy source. In fact, cecal microbial proliferation of rabbits and guinea pigs increases when adding indigestible but fermentable sugars to the feed. As a result, the amount of cecotrophy increases, and the percentage of nitrogen accumulation in the body tends to increase.

総説 (Review)