Review: Comparative methane production in mammalian herbivores

Abstract

Methane (CH4) production is a ubiquitous, apparently unavoidable side effect of fermentative fibre digestion by symbiotic microbiota in mammalian herbivores. Here, a data compilation is presented of in vivo CH4 measurements in individuals of 37 mammalian herbivore species fed forage-only diets, from the literature and from hitherto unpublished measurements. In contrast to previous claims, absolute CH4 emissions scaled linearly to DM intake, and CH4 yields (per DM or gross energy intake) did not vary significantly with body mass. CH4 physiology hence cannot be construed to represent an intrinsic ruminant or herbivore body size limitation. The dataset does not support traditional dichotomies of CH4 emission intensity between ruminants and nonruminants, or between foregut and hindgut fermenters. Several rodent hindgut fermenters and nonruminant foregut fermenters emit CH4 of a magnitude as high as ruminants of similar size, intake level, digesta retention or gut capacity. By contrast, equids, macropods (kangaroos) and rabbits produce few CH4 and have low CH4 : CO2 ratios for their size, intake level, digesta retention or gut capacity, ruling out these factors as explanation for interspecific variation. These findings lead to the conclusion that still unidentified host-specific factors other than digesta retention characteristics, or the presence of rumination or a foregut, influence CH4 production. Measurements of CH4 yield per digested fibre indicate that the amount of CH4 produced during fibre digestion varies not only across but also within species, possibly pointing towards variation in microbiota functionality. Recent findings on the genetic control of microbiome composition, including methanogens, raise the question about the benefits methanogens provide for many (but apparently not to the same extent for all) species, which possibly prevented the evolution of the hosting of low-methanogenic microbiota across mammals.

Anaerobic Fungi: Past, Present, and Future

“…Several rodent hindgut fermenters and non-ruminant foregut fermenters also emit CH 4 of a magnitude as high as ruminants, but in contrast equids, macropods and rabbits produce much less (Clauss et al, 2020). There has been a move in recent years to understand why some domesticated ruminants produce lower CH 4 compared to others (Shi et al, 2014), and a subsequent push to utilize feed-based approaches to decrease ruminal methanogenesis (Jeyanathan et al, 2014).…”

Tooth tales told by dental diet proxies: An alpine community of sympatric ruminants as a model to decipher the ecology of fossil fauna

“…We estimate the fractionation factor ε between the carbon stable isotope composition of food resources ( 13 Cdiet) and enamel carbonate using a body mass-dependent equation calibrated for foregutfermenting mammals (Tejada-Lara et al, 2018; Table 1; see Text-SI and Appendix 1). While there is uncertainty in the optimal parameterization of ε (Clauss et al, 2020;Manthi et al, 2019) in our calculations this offset differs by less than 1.1 ‰ between taxa. In parallel to carbon isotope analysis of tooth enamel, for some individuals food remains stuck between cusps and teeth in the jaw were sampled also for carbon isotope analysis, if present, to directly assess the  13 C values of ingested plants (see Text-SI and Table S2).…”

Strong influence of vertebrate host phylogeny on gut archaeal diversity

“…While previous work has indicated that host phylogeny influences archaeal abundance (12–14, 30, 42), the work has largely been focused on methanogen diversity (especially Methanobrevibacter) and was strongly biased towards mammals. In this work, we not only identified a signal of host-Archaea phylosymbiosis while accounting for host diet, habitat, and other factors (Figure 2A), but we also identified specific archaeal ASVs to be associated with host phylogeny globally and for particular host clades (Figures 2C & S12), and we observed a significant signal of cophylogeny (Figure 2D & 2E).…”

“…Regardless, we provide evidence congruent with the hypothesis that Methanothermobacter abundance is modulated by host body temperatures. We note that among the host species in which methane emission data exists (14,30) , avian species with high abundances of Methanothermobacter have emission rates on the higher end of mammal emission rates ( Figure S20), suggesting that Methanothermobacter is indeed a persistent inhabitant in the gut of some avian species.…”