The majority of methane produced by ruminants exits via the mouth. This is because methane is produced by bacteria -- animal cells cannot perform methanogenesis

The majority of methane produced by ruminants exits via the mouth. This is because methane is produced by bacteria -- animal cells cannot perform methanogenesis. The big difference between ruminants and humans digestively is: - in ruminants, bacteria-assisted "fiber digestion" happens before protease-mediated "protein digestion": the rumen precedes the abomasum. (The other two stomachs, the reticulum and omasum, have an unclear function, but appear to essentially be large filters.) - in other mammals, including most herbivores, bacteria-assisted "fiber digestion" occurs after protease-mediated "protein digestion": the stomach precedes the colon. So while a human or other primate -- including leaf-eating monkeys -- expels most fermented gas via the anus, a ruminant will expel most of it from its mouth (eructation): https://en.wikipedia.org/wiki/Rumen#Stratification_and_mixing_of_digesta https://en.wikipedia.org/wiki/Rumen#Stratification_and_mixin...