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 M0.75 (95%CI 0.64–0.87) and M0.77 (95%CI 0.66–0.88), respectively. Methane production was measured over two consecutive days in respiration chambers and scaled to M1.03 (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) M0.29 (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) M0.15 (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.
Citation: Franz R, Soliva CR, Kreuzer M, Hatt J-M, Furrer S, Hummel J, et al. (2011) Methane Output of Tortoises: Its Contribution to Energy Loss Related to Herbivore Body Mass. PLoS ONE 6(3): e17628. https://doi.org/10.1371/journal.pone.0017628
Editor: Brock Fenton, University of Western Ontario, Canada
Received: December 13, 2010; Accepted: February 3, 2011; Published: March 9, 2011
Copyright: © 2011 Franz et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study was funded by DFG (German Research Foundation) grant CL 182/5-1. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Among the different advantages commonly linked to an increase in body size , a widespread concept is that of an increasing digestive efficiency in larger herbivores. Based on the observation that energetic requirements of animals scale to metabolic body mass (i.e., M0.75) but gut capacity scales linearly with body mass (M1.0) in mammalian herbivores, Bell  and Jarman  deducted that at larger M, more gut capacity was available per unit energy requirement/food intake. This so-called ‘Jarman-Bell principle’  was further refined subsequently – and has found widespread application in ecology –.
This attractive concept provides an intuitive reason for the observation that larger-bodied herbivores usually ingest food of lower nutritional quality , . However, recent findings do not support the notion that digestibility ,  or ingesta retention  increase systematically with body mass in mammals, and also not in herbivorous reptiles . Among potential disadvantages, ingesta particle size – one of the factors influencing digestive efficiency – increases with body mass , , and it has been suggested that energetic losses due to methane production are also higher in larger animals .
Methane production has been mainly measured in domestic herbivores to address the issue of feed energy use or, more recently, methane mitigation to reduce greenhouse gas emissions . Studies on methane production of non-domestic species have mainly been to complete national or global methane budgets . In contrast, comparative investigations on methane production with respect to herbivore physiology are rare. Methane production has been demonstrated in faeces of captive specimens of nearly all herbivorous terrestrial herbivores, including reptiles , and methanogenes have been demonstrated by fluorescence microscopy in land and marine iguanas . In vivo methane production has not been investigated in reptiles to our knowledge. Recently, Franz et al. ,  presented data collections that suggest that methane production scales linearly with M in ruminant and nonruminant mammalian herbivores. The implication of this finding is that because food intake scales to M0.75, energetic losses due to methane increase per unit ingested food with increasing body size. 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 physiology. Scaling of food intake, gut capacity or digesta retention with M is generally similar in herbivorous reptiles and mammals , . The aim of our study was to test whether, in tortoises, voluntary food intake scales to M0.75, and methane production scales linearly with M.
Materials and Methods
This study was performed in accordance with Swiss animal welfare legislation (approved by the Cantonal Veterinary Office Zurich under experimental licence number 192/2006). We performed intake and respiration chamber measurements in 24 individual tortoises of the species Testudo graeca (n = 5, 1.16±0.95 kg, range 0.52–2.83 kg), T. hermanni (n = 6, 1.28±0.36 kg, range 0.91–1.72 kg), G. nigra (n = 2, 5.50±0.28 kg, range 5.30–5.70 kg), Geochelone sulcata (n = 8, 27.8±18.0 kg, range 7.2–50.0 kg), Dipsochelys dussumieri (n = 3, 141±38 kg, range 104–180 kg). Animals were kept individually for 30 days at 27–30°C for intake measurements after an adaptation period of one week. The diet consisted of grass hay and salad in varying proportions; details on intake and digestibility measurements were described previously . Water was available ad libitum at all times. Feed offered and left over was quantified, and faeces were collected completely. Representative subsamples were used to determine dry matter (DM), crude protein, gross energy (GE) and neutral detergent fibre (NDF) concentrations using standard methods ; these data allowed the calculation of the apparent digestibility of DM, GE and NDF . Experimental conditions or sample size did not always allow all analyses to be performed for all individuals (cf. Table 1). The ingested diets contained crude protein at 130±18 g kg DM−1 (range 95–170) and NDF at 488±107 g kg DM−1 (296–662).
After 30-day intake measurements, tortoises were transferred to open circuit respiration chambers constructed and operated as described in Soliva and Hess  for two consecutive 22.5 h periods (temperature 29±1°C, constant humidity 60%, pressure 987±8 hPa; chambers for M from 0.5–10 kg: volume 0.85 m3, air flow 1.09±0.08 m3 h−1; chambers for M from 20–180 kg: volume 4.55 m3, air flow 6.08±2.77 m3 h−1). Animals were measured individually except for the tortoises <5 kg; after pilot measurements, two groups of five individuals between 0.5–2 kg and one group of three individuals between 2–3 kg were measured together, and results divided by the number of animals. Animals had access to feed and water in the respiration chambers. All gas volumes were corrected for standard conditions (1013 hPa, 0°C, 0% relative humidity). Methane concentrations were measured by Binos 1001 (infra-red; Fisher-Rosemount, Baar-Walterswil, Switzerland). Following various conventions in the scientific literature, daily methane production was not only expressed in absolute terms, but also in relation to DM, GE, digestible energy (DE) and digestible NDF (dNDF) intake. Data were analysed after ln-transformation using regression analysis with PSAW 18.0 (SPSS Inc., Chicago, IL), indicating 95% confidence intervals (95%CI) according to y = a Mb or lny = lna+b lnM.
Mean dry matter intake (in kg d−1) of the tortoises scaled to 0.005 (95%CI 0.004–0.007) M0.75 (95%CI 0.64–0.87) (n = 22, r2 = 0.90, p<0.001) and mean daily gross energy intake (in kJ d−1) to 86.1 (95%CI 64.5–114.7) M0.77 (95%CI 0.66–0.88) (n = 21, r2 = 0.92, p<0.001). In contrast, mean daily methane production scaled linearly to M (Table 1, Fig. 1). During measurements in the respiration chamber, it was noted that methane production was not constant throughout the day but occurred in distinct bursts (Fig. 2).
When expressed in relation to intake of digestible energy and fibre, methane losses scaled to M0.32 and M0.30, respectively (Table 1, Fig. 3 and 4). The 95%CI of scaling exponent b overlapped between tortoises, nonruminant mammals, and ruminants where data had been obtained in previous assessments , , except for the scaling exponent when methane was related to digestible energy (not significant in ruminants). The 95%CI of factor a was invariably higher in ruminants than in the other two groups (Table 1).
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 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 (0.005) 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 (0.014 in tortoises vs. 0.181 in nonruminant mammals, Table 1). Consequently, when methane production is expressed per unit intake, there is no significant difference in the intercept a between tortoises and nonruminant mammals (Table 1).
This finding indicates a common adaptation of the gastrointestinal 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 (230±140 h , ) and mammals (40±25 h ), 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 (Table 1).
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, 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 (14%) 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 (0.17), 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 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 increasing 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 . Further studies combining in vivo measurements and microbiological analyses should unravel the fundamental principles behind the link between microbial fibre fermentation in vertebrate herbivores and methane production.
We thank the Zoological Garden of Zurich, Peter Sandmeier, Ruth Huber and Otti Steck for providing the experimental animals. This is contribution no. 99 of the DFG Research Unit 533 “The Biology of Sauropod Dinosaurs”.
Conceived and designed the experiments: MC JH CRS MK J-MH. Performed the experiments: RF SF MC. Analyzed the data: RF JH MC. Contributed reagents/materials/analysis tools: CRS MK SF J-MH. Wrote the paper: RF MK MC.
- 1. Sander PM, Christian A, Clauss M, Fechner R, Gee CT, et al. (2011) Biology of the sauropod dinosaurs: the evolution of gigantism. Biol Rev 86: 117–155.PM SanderA. ChristianM. ClaussR. FechnerCT Gee2011Biology of the sauropod dinosaurs: the evolution of gigantism.Biol Rev86117155
- 2. Bell RHV (1971) A grazing ecosystem in the Serengeti. Sci Am 225: 86–93.RHV Bell1971A grazing ecosystem in the Serengeti.Sci Am2258693
- 3. Jarman PJ (1974) The social organization of antelope in relation to their ecology. Behaviour 48: 215–266.PJ Jarman1974The social organization of antelope in relation to their ecology.Behaviour48215266
- 4. Geist V (1974) On the relationship of social evolution and ecology in ungulates. Am Zool 14: 205–220.V. Geist1974On the relationship of social evolution and ecology in ungulates.Am Zool14205220
- 5. Parra R (1978) Comparison of foregut and hindgut fermentation in herbivores. In: Montgomery GG, editor. The ecology of arboreal folivores. Washington DC: Smithsonian Institution Press. pp. 205–229.R. Parra1978Comparison of foregut and hindgut fermentation in herbivores.GG MontgomeryThe ecology of arboreal folivoresWashington DCSmithsonian Institution Press205229
- 6. Demment MW, Van Soest PJ (1985) A nutritional explanation for body size patterns of ruminant and nonruminant herbivores. Am Nat 125: 641–672.MW DemmentPJ Van Soest1985A nutritional explanation for body size patterns of ruminant and nonruminant herbivores.Am Nat125641672
- 7. Illius AW, Gordon IJ (1992) Modelling the nutritional ecology of ungulate herbivores: evolution of body size and competitive interactions. Oecologia 89: 428–434.AW IlliusIJ Gordon1992Modelling the nutritional ecology of ungulate herbivores: evolution of body size and competitive interactions.Oecologia89428434
- 8. Fleming TH (1991) The relationship between body size, diet, and habitat use in frugivorous bats, Genus Carollia (Phyllostomidae). J Mammal 72: 493–501.TH Fleming1991The relationship between body size, diet, and habitat use in frugivorous bats, Genus Carollia (Phyllostomidae).J Mammal72493501
- 9. Barboza PS, Bowyer RT (2000) Sexual segregation in dimorphic deer: a new gastrocentric hypothesis. J Mammal 81: 473–489.PS BarbozaRT Bowyer2000Sexual segregation in dimorphic deer: a new gastrocentric hypothesis.J Mammal81473489
- 10. Gaulin SJC (1979) A Jarman-Bell model of primate feeding niches. Hum Ecol 7: 1–20.SJC Gaulin1979A Jarman-Bell model of primate feeding niches.Hum Ecol7120
- 11. McNab BK (2002) The physiological ecology of vertebrates. A view from energetics. Ithaca & London: Cornell University Press. BK McNab2002The physiological ecology of vertebrates. A view from energeticsIthaca & LondonCornell University Press
- 12. Codron D, Lee-Thorp JA, Sponheimer M, Codron J, de Ruiter D, et al. (2007) Significance of diet type and diet quality for ecological diversity of African ungulates. J Anim Ecol 76: 526–537.D. CodronJA Lee-ThorpM. SponheimerJ. CodronD. de Ruiter2007Significance of diet type and diet quality for ecological diversity of African ungulates.J Anim Ecol76526537
- 13. Owen-Smith N (1988) Megaherbivores - the influence of very large body size on ecology. Cambridge: Cambridge University Press. N. Owen-Smith1988Megaherbivores - the influence of very large body size on ecologyCambridgeCambridge University Press
- 14. Pérez-Barberìa FJ, Elston DA, Gordon IJ, Illius AW (2004) The evolution of phylogenetic differences in the efficiency of digestion in ruminants. Proc R Soc B 271: 1081–1090.FJ Pérez-BarberìaDA ElstonIJ GordonAW Illius2004The evolution of phylogenetic differences in the efficiency of digestion in ruminants.Proc R Soc B27110811090
- 15. Clauss M, Nunn C, Fritz J, Hummel J (2009) Evidence for a tradeoff between retention time and chewing efficiency in large mammalian herbivores. Comp Biochem Physiol A 154: 376–382.M. ClaussC. NunnJ. FritzJ. Hummel2009Evidence for a tradeoff between retention time and chewing efficiency in large mammalian herbivores.Comp Biochem Physiol A154376382
- 16. Clauss M, Schwarm A, Ortmann S, Streich WJ, Hummel J (2007) A case of non-scaling in mammalian physiology? Body size, digestive capacity, food intake, and ingesta passage in mammalian herbivores. Comp Biochem Physiol A 148: 249–265.M. ClaussA. SchwarmS. OrtmannWJ StreichJ. Hummel2007A case of non-scaling in mammalian physiology? Body size, digestive capacity, food intake, and ingesta passage in mammalian herbivores.Comp Biochem Physiol A148249265
- 17. Franz R, Hummel J, Müller DWH, Bauert M, Hatt J-M, et al. (2011) Herbivorous reptiles and body mass: effects on food intake, digesta retention, digestibility and gut capacity, and a comparison with mammals. Comp Biochem Physiol A 158: 94–101.R. FranzJ. HummelDWH MüllerM. BauertJ-M Hatt2011Herbivorous reptiles and body mass: effects on food intake, digesta retention, digestibility and gut capacity, and a comparison with mammals.Comp Biochem Physiol A15894101
- 18. Fritz J, Hummel J, Kienzle E, Arnold C, Nunn C, et al. (2009) Comparative chewing efficiency in mammalian herbivores. Oikos 118: 1623–1632.J. FritzJ. HummelE. KienzleC. ArnoldC. Nunn2009Comparative chewing efficiency in mammalian herbivores.Oikos11816231632
- 19. Fritz J, Hummel J, Kienzle E, Streich WJ, Clauss M (2010) To chew or not to chew: faecal particle size in herbivorous reptiles and mammals. J Exp Zool A 313: 579–586.J. FritzJ. HummelE. KienzleWJ StreichM. Clauss2010To chew or not to chew: faecal particle size in herbivorous reptiles and mammals.J Exp Zool A313579586
- 20. Clauss M, Hummel J (2005) The digestive performance of mammalian herbivores: why big may not be that much better. Mammal Rev 35: 174–187.M. ClaussJ. Hummel2005The digestive performance of mammalian herbivores: why big may not be that much better.Mammal Rev35174187
- 21. Martin C, Morgavi DP, Doreau M (2010) Methane mitigation in ruminants: from microbe to the farm scale. Animal 4: 351–365.C. MartinDP MorgaviM. Doreau2010Methane mitigation in ruminants: from microbe to the farm scale.Animal4351365
- 22. Crutzen PJ, Aselmann I, Seiler W (1986) Methane production by domestic animals, wild ruminants, other herbivorous fauna, and humans. Tellus 38B: 271–284.PJ CrutzenI. AselmannW. Seiler1986Methane production by domestic animals, wild ruminants, other herbivorous fauna, and humans.Tellus38B271284
- 23. Hackstein JHP, Van Alen TA (1996) Fecal methanogenes and vertebrate evolution. Evolution 50: 559–572.JHP HacksteinTA Van Alen1996Fecal methanogenes and vertebrate evolution.Evolution50559572
- 24. Mackie RI, Rycyk M, Ruemmler RL, Aminov RI, Wikelski M (2004) Biochemical and microbiological evidence for fermentative digestion in free-living land iguanas (Conolophus pallidus) and marine iguanas (Amblyrhynchus cristatus) on the Galápagos Archipelago. Physiol Biochem Zool 77: 127–138.RI MackieM. RycykRL RuemmlerRI AminovM. Wikelski2004Biochemical and microbiological evidence for fermentative digestion in free-living land iguanas (Conolophus pallidus) and marine iguanas (Amblyrhynchus cristatus) on the Galápagos Archipelago.Physiol Biochem Zool77127138
- 25. Franz R, Soliva CR, Kreuzer M, Steuer P, Hummel J, et al. (2010) Methane production and body mass in ruminants and equids. Evol Ecol Res 12: 727–738.R. FranzCR SolivaM. KreuzerP. SteuerJ. Hummel2010Methane production and body mass in ruminants and equids.Evol Ecol Res12727738
- 26. Franz R, Soliva CR, Kreuzer M, Hummel J, Clauss M (2011) Methane in rabbits (Oryctolagus cuniculus) and guinea pigs (Cavia porcellus) on a hay-only diet: implications for the scaling of methane procution with body mass in nonruminant mammalian herbivores. Comp Biochem Physiol A 158: 177–181.R. FranzCR SolivaM. KreuzerJ. HummelM. Clauss2011Methane in rabbits (Oryctolagus cuniculus) and guinea pigs (Cavia porcellus) on a hay-only diet: implications for the scaling of methane procution with body mass in nonruminant mammalian herbivores.Comp Biochem Physiol A158177181
- 27. Smith FA, Elliott SM, Lyons SK (2010) Methane emissions from extinct megafauna. Nature Geosci 3: 374–375.FA SmithSM ElliottSK Lyons2010Methane emissions from extinct megafauna.Nature Geosci3374375
- 28. Franz R, Hummel J, Kienzle E, Kölle P, Gunga HC, et al. (2009) Allometry of visceral organs in living amniotes and its implications for sauropod dinosaurs. Proc R Soc B 276: 1731–1736.R. FranzJ. HummelE. KienzleP. KölleHC Gunga2009Allometry of visceral organs in living amniotes and its implications for sauropod dinosaurs.Proc R Soc B27617311736
- 29. AOAC (1997) Official methods of analysis. Arlington, VA, USA: Association of Official Analytical Chemists. AOAC1997Official methods of analysisArlington, VA, USAAssociation of Official Analytical Chemists
- 30. Robbins C (1993) Wildlife feeding and nutrition. San Diego, CA: Academic Press. C. Robbins1993Wildlife feeding and nutritionSan Diego, CAAcademic Press
- 31. Soliva CR, Hess HD (2007) Measuring methane emission of ruminants by in vitro and in vivo techniques. In: Makkar HPS, Vercoe PE, editors. Measuring methane production from ruminants. Dordrecht: Springer. pp. 15–31.CR SolivaHD Hess2007Measuring methane emission of ruminants by in vitro and in vivo techniques.HPS MakkarPE VercoeMeasuring methane production from ruminantsDordrechtSpringer1531
- 32. Nagy KA, Girard IA, Brown TK (1999) Energetics of free-ranging mammals, reptiles, and birds. Ann Rev Nutr 19: 247–277.KA NagyIA GirardTK Brown1999Energetics of free-ranging mammals, reptiles, and birds.Ann Rev Nutr19247277
- 33. Bennett AF, Dawson WR (1976) Metabolism. In: Gans C, Dawson WR, editors. Biology of the Reptilia, Vol 5 (Physiology A). New York: Academic Press. pp. 127–223.AF BennettWR Dawson1976Metabolism.C. GansWR DawsonBiology of the Reptilia, Vol 5 (Physiology A)New YorkAcademic Press127223
- 34. Meienberger C, Wallis IR, Nagy KA (1993) Food intake rate and body mass influence transit time and digestibility in the desert tortoise (Xerobates agassizii). Physiol Zool 66: 847–862.C. MeienbergerIR WallisKA Nagy1993Food intake rate and body mass influence transit time and digestibility in the desert tortoise (Xerobates agassizii).Physiol Zool66847862
- 35. Hatt JM, Clauss M, Gisler R, Liesegang A, Wanner M (2005) Fiber digestibility in juvenile captive Galapagos tortoises (Geochelone nigra). Zoo Biol 24: 185–191.JM HattM. ClaussR. GislerA. LiesegangM. Wanner2005Fiber digestibility in juvenile captive Galapagos tortoises (Geochelone nigra).Zoo Biol24185191
- 36. Karasov WH, Petrossian E, Rosenberg L, Diamond JM (1986) How do food passage rate and assimilation differ between herbivorous lizards and nonruminants mammals? J Comp Physiol B 156: 599–609.WH KarasovE. PetrossianL. RosenbergJM Diamond1986How do food passage rate and assimilation differ between herbivorous lizards and nonruminants mammals?J Comp Physiol B156599609
- 37. Kirkwood JK (1996) Nutrition of captive and free-living wild animals. In: Kelly N, Wills J, editors. BSAVA manual of companion animal nutrition and feeding. Cheltenham, UK: British Small Animal Veterinary Association. pp. 235–243.JK Kirkwood1996Nutrition of captive and free-living wild animals.N. KellyJ. WillsBSAVA manual of companion animal nutrition and feedingCheltenham, UKBritish Small Animal Veterinary Association235243
- 38. McBee RH, McBee VH (1982) The hindgut fermentation in the green iguana, Iguana iguana. In: Burghardt GM, Rand AS, editors. Iguanas of the world: behaviour, ecology, and conservation. New Jersey: Noyes Publishers. pp. 77–83.RH McBeeVH McBee1982The hindgut fermentation in the green iguana, Iguana iguana.GM BurghardtAS RandIguanas of the world: behaviour, ecology, and conservationNew JerseyNoyes Publishers7783
- 39. Troyer K (1984) Behavioural acquisition of the hindgut fermentation system by hatchling Iguana iguana. Behav Ecol Sociobiol 14: 189–193.K. Troyer1984Behavioural acquisition of the hindgut fermentation system by hatchling Iguana iguana.Behav Ecol Sociobiol14189193
- 40. Fenchel TM, McRoy CP, Ogden JC, Parker P, Rainey WE (1979) Symbiotic cellulose degradation in green turtles Chelonia mydas. Appl Environ Microbiol 37: 348–350.TM FenchelCP McRoyJC OgdenP. ParkerWE Rainey1979Symbiotic cellulose degradation in green turtles Chelonia mydas.Appl Environ Microbiol37348350
- 41. Nagy KA (1977) Cellulose digestion and nutrient assimilation in Sauromalus obesus, a plant-eating lizard. Copeia 1977: 355–362.KA Nagy1977Cellulose digestion and nutrient assimilation in Sauromalus obesus, a plant-eating lizard.Copeia1977355362
- 42. Foley WJ, Bouskila A, Shkolnik A, Chosniak I (1992) Microbial digestion in the herbivorous lizard Uromastyx aegypticus (Agamidae). J Zool 226: 387–398.WJ FoleyA. BouskilaA. ShkolnikI. Chosniak1992Microbial digestion in the herbivorous lizard Uromastyx aegypticus (Agamidae).J Zool226387398
- 43. Troyer K (1984) Structure and function of the digestive tract of a herbivorous lizard Iguana iguana. Physiol Zool 57: 1–8.K. Troyer1984Structure and function of the digestive tract of a herbivorous lizard Iguana iguana.Physiol Zool5718
- 44. Barboza PS (1995) Digesta passage and functional anatomy of the digestive tract in the desert tortoise (Xerobates agassizii). J Comp Physiol B 165: 193–202.PS Barboza1995Digesta passage and functional anatomy of the digestive tract in the desert tortoise (Xerobates agassizii).J Comp Physiol B165193202
- 45. Bjorndal KA (1979) Cellulose digestion and volatile fatty acid production in the green turtle (Chelonia mydas). Comp Biochem Physiol A 63: 127–133.KA Bjorndal1979Cellulose digestion and volatile fatty acid production in the green turtle (Chelonia mydas).Comp Biochem Physiol A63127133
- 46. Hailey A (1997) Digestive efficiency and gut morphology of omnivorous and herbivorous African tortoises. Can J Zool 75: 787–794.A. Hailey1997Digestive efficiency and gut morphology of omnivorous and herbivorous African tortoises.Can J Zool75787794
- 47. Pinares-Patiño CS, Ulyatt MJ, Lassey KR, Barry TN, Holmes CW (2003) Rumen function and digestion parameters associated with differences between sheep in methane emissions when fed chaffed lucerne hay. J Agric Sci 140: 205–214.CS Pinares-PatiñoMJ UlyattKR LasseyTN BarryCW Holmes2003Rumen function and digestion parameters associated with differences between sheep in methane emissions when fed chaffed lucerne hay.J Agric Sci140205214
- 48. Okine EK, Mathison GW, Hardin RT (1989) Effects of changes in frequency of reticular contractions on fluid and particulate passage rates in cattle. J Anim Sci 67: 3388–3396.EK OkineGW MathisonRT Hardin1989Effects of changes in frequency of reticular contractions on fluid and particulate passage rates in cattle.J Anim Sci6733883396
- 49. Prins RA, Kreulen DA (1991) Comparative aspects of plant cell wall digestion in mammals. In: Hoshino S, Onodera R, Minoto H, Itabashi H, editors. The rumen ecosystem. Tokyo: Japan Scientific Society Press. pp. 109–120.RA PrinsDA Kreulen1991Comparative aspects of plant cell wall digestion in mammals.S. HoshinoR. OnoderaH. MinotoH. ItabashiThe rumen ecosystemTokyoJapan Scientific Society Press109120
- 50. Van Soest PJ (1994) Nutritional ecology of the ruminant. Ithaca: Cornell University Press. 476 p.PJ Van Soest1994Nutritional ecology of the ruminantIthacaCornell University Press476
- 51. Hatt JM, Gisler R, Mayes R, Lechner-Doll M, Clauss M, et al. (2002) The use of dosed and herbage n-alkanes as markers for the determination of intake, digestibility, mean retention time and diet selection in Galapagos tortoises. Herpetol J 12: 45–54.JM HattR. GislerR. MayesM. Lechner-DollM. Clauss2002The use of dosed and herbage n-alkanes as markers for the determination of intake, digestibility, mean retention time and diet selection in Galapagos tortoises.Herpetol J124554
- 52. Anonymous (1999) Anonymous1999http://www.uhaul.com/supergraphics/states/south_dakota/turtle/archelon.html. http://www.uhaul.com/supergraphics/states/south_dakota/turtle/archelon.html.
- 53. McCrabb GJ, Berger KT, Magner T, May C, Hunter RA (1997) Inhibiting methane production in Brahman cattle by dietary supplementation with a novel compound and the effects on growth. Austr J Agric Res 48: 323–329.GJ McCrabbKT BergerT. MagnerC. MayRA Hunter1997Inhibiting methane production in Brahman cattle by dietary supplementation with a novel compound and the effects on growth.Austr J Agric Res48323329
- 54. Goel G, Makkar HPS, Becker K (2009) Inhibition of methanogens by bromochloromethane: effects on microbial communities and rumen fermentation using batch and continuous fermentations. Br J Nutr 101: 1484–1492.G. GoelHPS MakkarK. Becker2009Inhibition of methanogens by bromochloromethane: effects on microbial communities and rumen fermentation using batch and continuous fermentations.Br J Nutr10114841492
- 55. Tomkins NW, Colegate SM, Hunter RA (2009) A bromochloromethane formulation reduces enteric methanogenesis in cattle fed grain-based diets. Anim Prod Sci 49: 1053–1058.NW TomkinsSM ColegateRA Hunter2009A bromochloromethane formulation reduces enteric methanogenesis in cattle fed grain-based diets.Anim Prod Sci4910531058
- 56. Sawyer MS, Hoover WH, Sniffen CJ (1974) Effects of a ruminal methane inhibitor on growth and energy metabolism in the ovine. J Anim Sci 38: 908–914.MS SawyerWH HooverCJ Sniffen1974Effects of a ruminal methane inhibitor on growth and energy metabolism in the ovine.J Anim Sci38908914
- 57. Pimentel M, Lin HC, Enayati P, van den Burg B, Lee H-R, et al. (2006) Methane, a gas produced by enteric bacteria, slows intestinal transit and augments small intestinal contractile activity. Am J Physiol 290: G1089–G1095.M. PimentelHC LinP. EnayatiB. van den BurgH-R Lee2006Methane, a gas produced by enteric bacteria, slows intestinal transit and augments small intestinal contractile activity.Am J Physiol290G1089G1095
- 58. Soares ACF, Lederman HM, Fagundes-Neto U, de Morais MB (2005) Breath methane associated with slow colonic transit time in children with chronic constipation. J Clin Gastroenterol 39: 512–515.ACF SoaresHM LedermanU. Fagundes-NetoMB de Morais2005Breath methane associated with slow colonic transit time in children with chronic constipation.J Clin Gastroenterol39512515
- 59. El Oufir L, Flourié B, Bruley des Varannes S, Barry JL, Cloarec D, et al. (1996) Relations between transit time, fermentation products, and hydrogen consuming flora in healthy humans. Gut 38: 870–877.L. El OufirB. FlouriéS. Bruley des VarannesJL BarryD. Cloarec1996Relations between transit time, fermentation products, and hydrogen consuming flora in healthy humans.Gut38870877
- 60. Cloarec D, Bornet F, Gouilloud S, Barry JL, Salim B, et al. (1990) Breath hydrogen response to lactulose in healthy subjects: relationship to methane producing status. Gut 31: 300–304.D. CloarecF. BornetS. GouilloudJL BarryB. Salim1990Breath hydrogen response to lactulose in healthy subjects: relationship to methane producing status.Gut31300304
- 61. Stephen A, Wiggins HS, Englyst HN, Cole TJ, Wayman BJ, et al. (1986) The effect of age, sex and level of intake of dietary fibre from wheat on large-bowel function in thirty healthy subjects. Br J Nutr 56: 349–361.A. StephenHS WigginsHN EnglystTJ ColeBJ Wayman1986The effect of age, sex and level of intake of dietary fibre from wheat on large-bowel function in thirty healthy subjects.Br J Nutr56349361
- 62. Chatterjee S, Park S, Low K, Kong Y, Pimentel M (2007) The degree of breath methane production in IBS correlates with the severity of constipation. Am J Gastroenterol 102: 837–841.S. ChatterjeeS. ParkK. LowY. KongM. Pimentel2007The degree of breath methane production in IBS correlates with the severity of constipation.Am J Gastroenterol102837841
- 63. Hwang L, Low K, Khoshini R, Melmed G, Sahakian A, et al. (2010) Evaluating breath methane as a diagnostic test for constipation-predominant IBS. Dig Dis Sci 55: 398–403.L. HwangK. LowR. KhoshiniG. MelmedA. Sahakian2010Evaluating breath methane as a diagnostic test for constipation-predominant IBS.Dig Dis Sci55398403
- 64. Pimentel M, Chatterjee S, Chow EJ, Park S, Kong Y (2006) Neomycin improves constipation-predominant irritable bowel syndrome in a fashion that is dependent on the presence of methane gas: subanalysis of a double-blind randomized controlled study. Dig Dis Sci 51: 1297–1301.M. PimentelS. ChatterjeeEJ ChowS. ParkY. Kong2006Neomycin improves constipation-predominant irritable bowel syndrome in a fashion that is dependent on the presence of methane gas: subanalysis of a double-blind randomized controlled study.Dig Dis Sci5112971301
- 65. Low K, Hwang L, Hua J, Zhu A, Morales W, et al. (2010) A combination of rifaximin and neomycin is most effective in treating irritable bowel syndrome patients with methane on lactulose breath test. J Clin Gastroenterol 44: 547–550.K. LowL. HwangJ. HuaA. ZhuW. Morales2010A combination of rifaximin and neomycin is most effective in treating irritable bowel syndrome patients with methane on lactulose breath test.J Clin Gastroenterol44547550