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An increase in body mass (M) is traditionally considered advantageous for herbivores in terms of digestive efficiency. However, recently increasing methane losses with increasing M were described in mammals. To test this pattern in non-mammal herbivores, we conducted feeding trails with 24 tortoises of various species (M range 0.52–180 kg) fed a diet of grass hay ad libitum and salad. Mean daily dry matter and gross energy intake measured over 30 consecutive days scaled to M 0.75 ( 95%CI  0.64–0.87)  and M 0.77 ( 95%CI  0.66–0.88) , respectively. Methane production was measured over two consecutive days in respiration chambers and scaled to M 1.03 ( 95%CI  0.84–1.22) . When expressed as energy loss per gross energy intake, methane losses scaled to 0.70 ( 95%CI  0.47–1.05) M 0.29 ( 95%CI  0.14–0.45) . This scaling overlaps in its confidence intervals to that calculated for nonruminant mammals 0.79 ( 95%CI  0.63–0.99) M 0.15 ( 95%CI  0.09–0.20) , but is lower than that for ruminants. The similarity between nonruminant mammals and tortoises suggest a common evolution of the gut fauna in ectotherms and endotherms, and that the increase in energetic losses due to methane production with increasing body mass is a general allometric principle in herbivores. These findings add evidence to the view that large body size itself does not necessarily convey a digestive advantage.

Methane Output of Tortoises: Its Contribution to Energy Loss Related to Herbivore Body Mass