Methane is the main greenhouse gas contributor to global warming in the livestock sector; it is generated by anaerobic fermentation in the different sections of the gut, and differs significantly among species. Methane is only produced by a certain type of microorganisms called methanogens. The species composition of methanogenic archaea population is largely affected by the diet, geographical location, host and the section of the gut. Consequently, methane production, either measured as total grams emitted per day or per body weight mass, differs greatly between animal species. The main difference between methanogenic activity in different gut sections and animal species is the substrate fermented and the metabolic pathway to complete anaerobic fermentation of plant material. The three main substrates used by methanogens are CO2, acetate and compounds containing methyl groups. The three dominant orders of methanogens in gut environments are Methanomicrobiales, Methanobacteriales and Methanosarcinales. They normally are present in low numbers (below 3 % of total microbiome). This review will describe the main metabolic pathways and methanogens involved in CH4 production in the gut of different host animal, species, as well as discuss general trends that influence such emissions, such as geographical distribution, feed composition, section of the gut, host age and diurnal/season variation. Finally, the review will describe animal species (large and small domestic ruminants, wild ruminants, camelids, pigs, rabbits, horses, macropods, termites and humans) specificities in the methanogens diversity and their effects on methane emission.This work was supported by FEDER/Ministerio de Ciencia, Innovación yUniversidades - Agencia Estatal de Investigación (grant number AGL2017-89289) and European Union's H2020 program under National Institutes ofHealth (Feed-a-Gene, grant number 633531)
