Thus, methane energy losses could become a serious constraint in species with large body size. Similarly, allometric relationships were the basis of the investigation of Smith et al. who found that the body mass distribution in a herbivore fauna will impact this fauna’s contribution to the global methane budget. Apparantly, methane production scales differently than metabolic requirements or rates. In order to test the concept of disproportionately increasing methane losses with increasing herbivore M with an original dataset, we chose herbivores of another clade, tortoises. In tortoises, a large range of M is available with minimal differences in digestive anatomy and Guanethidine Sulfate physiology. Scaling of food intake, gut capacity or digesta retention with M is generally similar in herbivorous reptiles and mammals. The results of this study suggest that in herbivores, methane production scales linearly with body mass, and the proportional losses of energy from feed ingested due to methane output increase with increasing body mass. Although the existing data must still be considered scarce, the parallel findings in ruminant and nonruminant mammalian herbivores and herbivorous tortoises strongly suggest a general scaling pattern. Similar scaling patterns in reptiles and mammals have been found for other parameters such as field metabolic rate, feed intake, or ingesta particle size – although on different levels; whilst some other measures appear relatively similar between herbivorous reptiles and mammals, such as the proportion of the gut contents of total body mass or GSK J4 hydrochloride the achieved digestibilities. Generally, it is assumed that energy metabolism in reptiles is roughly a tenth of that observed in mammals. The difference in the intercept a of the regression equation describing dry matter intake in the tortoises of this study compared to the intercept of 0.047 found in herbivorous mammals in general fits this pattern, as does the difference in the intercept describing the absolute methane output. Consequently, when methane production is expressed per unit intake, there is no significant difference in the intercept a between tortoises and nonruminant mammals. This finding indicates a common adaptation of the gastrointes- tinal fauna between ectotherms and endotherms. Other similarities between the microbial faunas of herbivorous reptiles and mammals have been reported, such as the number of gut bacteria and the presence of protozoa, cellulase activity, or the concentration of fermentation products. A relatively similar methane production per unit food intake in reptiles and mammals means that the processes of microbial fermentation must be similar even though the microbial faunas of reptiles and mammals will vary distinctively in their temperature sensitivity. The findings suggest that methane production is a more or less constant, unavoidable by-product of microbial fermentation in herbivores. Because of the well-documented differences in ingesta retention times between herbivorous reptiles and mammals, the similarity in methane scaling between reptiles and mammals also indicates that retention time as such is not the main factor influencing the scope of methane production, even if it may be relevant when comparing data within species. Our results also suggest that the increase in methane production with increasing body size is not only due to an increase in fibre digestibility at higher body sizes; when expressed per unit of digestible fibre intake, the effect of an increasing methane production remains and scales similarly with M as when expressed in relation to other intake measures.