Background: Mammalian herbivores produce methane during digestion.
Questions: Do ruminants (such as bovids) produce more methane during digestion than do equids (non-ruminants)? What are the effects of allometric scaling of methane production when different species feed on similar diets?
Methods: From the literature, we collected a set of data on methane production (litres ·day−1) of ruminants (body mass 26–610 kg) and equids (208–850 kg) fed only roughage. To these data we added our own experimental results from three sheep (Ovis orientalis aries, 94 ± 4 kg) and three mini Shetland ponies (Equus ferus caballus, 97 ± 6 kg). We gave these six animals ad libitum access to the same batch of grass hay. We measured their food intake and methane production (using respiratory chambers), and calculated gut fill and food digestibility.
Results: Daily dry matter intake and dry matter digestibility were 39 ± 10 g ·kg−0.75 day−1 and 48 ± 2% in sheep and 72 ± 16 g ·kg−0.75 day−1 and 41 ± 3% in ponies, respectively; the calculated dry matter gut fill was 2.0 ± 0.5% of body mass in sheep and 1.9 ± 0.4% in ponies. Methane production was higher in sheep (30.3 ± 3.0 litres ·day−1) than in ponies (13.4 ± 4.6 litres ·day−1), representing 6.7 ± 1.7% and 1.5 ± 0.2% of gross energy intake, respectively. The data set revealed a linear increase of methane production with body mass (i.e. M1.0) in equids and ruminants. However, a ruminant produces 3.6 times as much methane as does an equid of comparable body mass.
Conclusions: Because energy and food intake scale allometrically with body mass (M0.75), our results mean that energetic losses due to methane production (as a proportion of overall energy intake) increase with increasing body mass. The magnitude of the losses is enough to limit the maximum size of a ruminant’s body, but not that of an equid.