Prins and Kreulen and Van Soest suggested that a different group of methanogenes – slower-growing archeae with a generation time of about 4 days that produce methane from acetate in sewers, for example – may actually limit body size in herbivores. They considered ingesta retention a function of body mass and hypothesized that when retention times surpass 4 days, energetic losses due to acetate-based methanogenesis would become prohibitve for the host. In herbivorous reptiles retention times beyond 96 h are common which indicates that other factors than retention time must limit the occurence of slow-growing archeae in herbivores. An interesting question is could methane production by the fast-growing archeae be a constraint on the evolution of body size? This has been suggested for ruminants, GSK J1 due to the high proportion of energetic methane losses in this group ; for nonruminant mammals, these losses might become limiting at extrapolated body masses of 100 metric tonnes – a putative constraint that might apply conceptually for the largest dinosaurs. Reptiles never reached such proportions. When the regression equation from tortoises is directly applied to the largest known chelonian, Archelon ischyros, a marine turtle with an estimated maximum M of 5000 kg, extrapolated methane energy losses per unit of digestible energy intake approach those found in large ruminants. Note that this similarity to ruminants, in spite of the general similarity in scaling between tortoises and nonruminant mammals, is due to the determined exponent b of 0.32, which is numerically higher than the one calculated for nonruminant mammals, though overlapping in its confidence interval. Differences in exponent should be considered with caution when extrapolations beyond the M range are performed that served to generate the regression equation. Why herbivores apparently did not evolve to avoid methane losses is a fundamental question. Intervention studies in domestic ruminants have shown that functional digestion can be maintained in the absence or near-absence of Archeae and without methane production. An alternative view of methanogenes could be that they are among the prerequisites for herbivory. Pimentel et al. showed that, in a models with dogs and guinea pigs, methane slowed intestinal passage by decreasing intestinal contractile activity. In humans, methane production is associated with increased digesta retention times, and is positively correlated with constipation and negatively with diarrhoea. Reduction of methane production by oral antibiotic treatment leads to a reduction of constipation. While offering new insights into potential Griseofulvin therapeutical interventions against human irritable bowel syndrome, these results also give rise to the speculation that the presence of methane, and its passage- delaying effect, was an important component of the evolution of physiological adaptations to herbivory, which requires long passage times. However, confirmation of this hypothesis requires much further research. Our study shows that methane losses not only occur in mammalian but also in reptilian herbivores, and that they scale linearly with body mass, thus representing proportionally increas- ing losses at increasing body size. Therefore, differences in the proportion of ingested energy lost to methane, according to the body size composition of any mammal or reptile herbivore fauna should be considered when reconstructing trophic energy fluxes in ecosystems, or contributions of these ecosystems to changes in the composition of the atmosphere.