Faecal particle size: digestive physiology meets herbivore diversity.

In herbivore ecophysiology, comparative chewing efficiency has only recently received increased attention. This measure is best assessed on un-processed forage-only diets; corresponding comparative datasets are missing. We measured a faecal mean particle size (MPS [mm]) in 14 large herbivore species (body mass (M) range 60­4000 kg; 8 ruminants and 6 hindgut fermenters) fed a consistent grass hay diet, in which intake, digesta mean retention times (MRT [h]) and digestive efficiency (as digestibility of faecal fibre measured by 96 h cumulative in vitro gas production GP96h [ml per 200 mg faecal fibre], and metabolic faecal nitrogen MFN [% organic faecal matter]) had been quantified simultaneously. MPS was generally lower in ruminants than in hindgut fermenters and increased with M in the total dataset, but was nearly constant among closely related taxa (e.g. within ruminants, within equids) irrespective of M. MPS (but not MRT) was significantly correlated to GP96h, whereas MRT (but not MPS) was significantly correlated to MFN, suggesting different effects of these factors on different aspects of digestibility. Combinations of measures including MPS mostly explained digestibility better than other combinations. The phylogenetic signal λ, which was mostly 1 when linking any single measure to digestibility, was estimated 0 in models that linked digestive efficiency to combinations of measures. These results support the intuitive concept that species diversification in large herbivores is tightly related to digestive physiology, and that chewing efficiency as measured by faecal particle size is an integral aspect of this scenario.

Fibre digestibility in large herbivores as related to digestion type and body mass--an in vitro approach.

The coexistence of different ungulate species in a given ecosystem has been the focus of many studies. Differences between ruminant foregut fermenters and hindgut fermenters were remarkable for example in the way they ingest and digest high fibre diets. Digestion trials based on total collections are difficult to conduct or are sometimes even not possible for wild animals in the field or in zoos. To gain information on the fibre digestion achieved by these animals and the influence of body mass (BM) thereon, a method using spot sampling is desirable. In this study, in vitro fermentation of faecal neutral detergent fibre (NDF) was used as a measure of fibre digestion in large ungulates. Food and faecal samples of 10 ruminant foregut fermenting and 7 hindgut fermenting species/breeds were collected. All animals received 100% grass hay with ad libitum access. The NDF of food and faeces was fermented in vitro in a Hohenheim gas test (HGT) for 96 h. The digestion type generally had an effect on the gas production (GP) of faecal NDF in the HGT with hindgut fermenters showing higher values than ruminant foregut fermenters. At any time interval of incubation, BM had no influence on GP. The results are in accordance with both findings that ruminant foregut fermenters have longer mean retention times and more comprehensive particle reduction and findings of a lack of influence of BM on digesta mean retention time. It can be stated that the HGT (96 h) is a useful and quick method to show also small differences within groups in fibre digestion.

Is there an influence of body mass on digesta mean retention time in herbivores? A comparative study on ungulates.

The relation between body mass (BM) and digesta mean retention time (MRT) in herbivores was the focus of several studies in recent years. It was assumed that MRT scaled with BM(0.25) based on the isometric scaling of gut capacity (BM(1.0)) and allometric scaling of energy intake (BM(0.75)). Literature studies that tested this hypothesis produced conflicting results, arriving sometimes at higher or lower exponents than the postulated 0.25. This study was conducted with 8 ruminants (n=2-6 per species) and 6 hindgut fermenting species/breeds (n=2-6, warthog n=1) with a BM range of 60-4000 kg. All animals received a ration of 100% grass hay with ad libitum access. Dry matter intake was measured and the MRT was estimated by the use of a solute and a particle (1-2 mm) marker. No significant scaling of MRT(particle) with BM was observed for all herbivores (32 BM(0.04), p=0.518) and hindgut fermenters (32 BM(0.00), p=1.00). The scaling exponent for ruminants only showed a tendency towards significance (29 BM(0.12), p=0.071). Ruminants on average had an MRT(particle) 1.61-fold longer than hindgut fermenters. Whereas an exponent of 0.25 is reasonable from theoretical considerations, much lower exponents were found in this and other studies. The energetic benefit of increasing MRT is by no means continuous, since the energy released from a given food unit via digestion decreases over time. The low and non-significant scaling factors for both digestion types suggest that in ungulates, MRT is less influenced by BM (maximal allometric exponent ≤0.1) than often reported.