Methanogens in the Gastro-Intestinal Tract of Animals

Abstract

Nearly all vertebrates host methanogens in their gastrointestinal tracts. However, a great fraction of vertebrates emits only traces of methane from their faeces (approx. 1 nmol/g faeces/h) and has no significant amounts of methane in their breath. In contrast, many animals host some 100 times more methanogens in their gastrointestinal tract and emit methane with their breath. These substantial differences are not caused by different feeding habits; rather a genetic factor controls the presence of large amounts of methanogens. The attribute “methane production” is evolutionary stable, and the loss of this character obeys Dollo’s law: once lost in the course of evolution, this character cannot be acquired another time. Also invertebrates can host methanogens in their gastrointestinal tract. In contrast to the vertebrates, only a few taxa of arthropods emit methane: millipedes, termites, cockroaches, and scarab beetles. All other arthropods in our study did not emit methane and did not host even traces of methanogens. As in vertebrates, the diet of the animals is not crucial for the presence of methanogens. Again, a genetic factor seems to control the presence or absence of methanogens. Methanogenesis is also a prerequisite for the presence of intestinal anaerobic protozoa with endosymbiotic methanogens, but not for the presence of impressive structural differentiations of the hindgut epithelium, which—in methanogenic taxa—host enormous amounts of methanogens.