Digestive efficiency, digesta passage, resting metabolism and methane production in captive juvenile nutria (Myocastor coypus)

Abstract

Although it is known that most herbivores produce methane (CH4), CH4 emissions in rodents are generally considered negligible and have rarely been measured in live animals. We measured CH4 emission in four captive juvenile nutria (Myocastor coypus) fed a diet of pelleted lucerne, as well as food intake, digestibility, and digesta mean retention time (MRT) of a solute and a particle marker. Marker excretion patterns revealed secondary peaks indicative of coprophagy, with MRTs of 30.2 ± 4.2 h and 24.2 ± 4.2 h for solutes and particles, respectively, and a resulting MRTsolute/MRTparticle ratio of 1.26 ± 0.07, which is still typical for a ‘mucus-trap’ colonic separation mechanism. At a dry matter intake of 28 ± 6 g kg body mass−0.75 d−1, the nutria digested organic matter and neutral detergent fibre at 59 ± 3% and 46 ± 3%, respectively, similar to what might be expected from horses on a diet with this fibre content. The respiratory quotient (CO2/O2) was 0.95 ± 0.02, the resting metabolic rate 266 ± 31 kJ kg body mass−0.75 day−1 and CH4 emissions averaged at 1.72 ± 0.17 L day−1 and 39.8 ± 11.3 L per kg dry matter intake; this at a CH4/CO2 ratio of 0.08 ± 0.04. Accordingly, methane yield was of a magnitude expected for a hypothetical ruminant of this body mass. While rodents’ CH4 contributions to global budgets might be low due to their low body size, this should not give rise to the assumption that CH4 production is not a relevant part of their digestive physiology.