It is generally assumed that animal species differ in physiological characteristics of their digestive tract. Because investigating digestive processes is often labour-intensive, requiring lengthy adaptation and collection periods, comparisons are often made on the basis of data collated from different studies. We added a new data set on dietary crude fibre (CF) and macromineral (Na, K, Ca, P, Mg) composition and apparent digestibility (aD) of organic matter (OM) and macrominerals in rabbits (Oryctolagus cuniculus; a total of 180 measurements with 12 individuals and 10 different diets) to a literature data collection for rodents and domestic horses (Equus caballus). Significant negative relationships between dietary CF and aD OM were obtained, but the significant difference in digestive efficiency between rabbits and guinea pigs (Cavia porcellus) reported from studies where both species had been investigated under identical conditions were not detectable in the larger data collection. However, the 95% confidence interval of the negative slope of the CF-aD OM relationship did not overlap between domestic horses on the one hand, and rabbits and guinea pigs on the other hand, indicating a less depressing effect of CF in horses. Akaike's information criterion indicated that aD of a macromineral did not only depend on its concentration in the diet, but also on the individual and various other diet characteristics and the presence of other macrominerals, indicating complex interactions. The data indicate similar mechanisms in the absorption of macrominerals amongst rabbits, rodents and horses, with the exception of Na and K in guinea pigs and Ca in chinchillas (Chinchilla laniger) that should be further corroborated. In particular, the occurrence of high aD Ca in hindgut fermenters, regardless of whether they employ a digestive strategy of coprophagy or whether they have fossorial habits, suggests that this peculiarity might represent an adaptation to hindgut fermentation in general.

Methane (CH4) production is a ubiquitous, apparently unavoidable side effect of fermentative fibre digestion by symbiotic microbiota in mammalian herbivores. Here, a data compilation is presented of in vivo CH4 measurements in individuals of 37 mammalian herbivore species fed forage-only diets, from the literature and from hitherto unpublished measurements. In contrast to previous claims, absolute CH4 emissions scaled linearly to DM intake, and CH4 yields (per DM or gross energy intake) did not vary significantly with body mass. CH4 physiology hence cannot be construed to represent an intrinsic ruminant or herbivore body size limitation. The dataset does not support traditional dichotomies of CH4 emission intensity between ruminants and nonruminants, or between foregut and hindgut fermenters. Several rodent hindgut fermenters and nonruminant foregut fermenters emit CH4 of a magnitude as high as ruminants of similar size, intake level, digesta retention or gut capacity. By contrast, equids, macropods (kangaroos) and rabbits produce few CH4 and have low CH4 : CO2 ratios for their size, intake level, digesta retention or gut capacity, ruling out these factors as explanation for interspecific variation. These findings lead to the conclusion that still unidentified host-specific factors other than digesta retention characteristics, or the presence of rumination or a foregut, influence CH4 production. Measurements of CH4 yield per digested fibre indicate that the amount of CH4 produced during fibre digestion varies not only across but also within species, possibly pointing towards variation in microbiota functionality. Recent findings on the genetic control of microbiome composition, including methanogens, raise the question about the benefits methanogens provide for many (but apparently not to the same extent for all) species, which possibly prevented the evolution of the hosting of low-methanogenic microbiota across mammals.

Behavioral observations and small fecal particles compared to other primates indicate that freeranging proboscis monkeys (Nasalis larvatus) have a strategy of facultative merycism (rumination). In functional ruminants (ruminant and camelids), rumination is facilitated by a particle sorting mechanism in the forestomach that selectively retains larger particles and subjects them to repeated mastication. Using a set of a solute and three particle markers of different sizes (b 2, 5 and 8 mm), we displayed digesta passage kinetics and measured mean retention times (MRTs) in four captive proboscis monkeys (6-18 kg) and compared the marker excretion patterns to those in domestic cattle. In addition, we evaluated various methods of calculating and displaying passage characteristics. The mean ± SD dry matter intake was 98 ± 22 g kg−0.75 d−1, 68 ± 7% of which was browse. Accounting for sampling intervals in MRT calculation yielded results that were not affected by the sampling frequency. Displaying marker excretion patterns using fecal marker concentrations (rather than amounts) facilitated com-parisons with reactor theory outputs and indicated that both proboscis and cattle digestive tracts represent a se-ries of very few tank reactors. However, the separation of the solute and particle marker and the different-sized particle markers, evident in cattle, did not occur in proboscis monkeys, in which all markers moved together, at MRTs of approximately 40 h. The results indicate that the digestive physiology of proboscis monkeys does not show typical characteristics of ruminants, which may explain why merycism is only a facultative strategy in this species. Abstract 24Behavioral observations and small fecal particles compared to other primates indicate that 25 free-ranging proboscis monkeys (Nasalis larvatus) have a strategy of facultative merycism 26 (rumination). In functional ruminants (ruminant and camelids), rumination is facilitated by a 27 particle sorting mechanism in the forestomach that selectively retains larger particles and 28 subjects them to repeated mastication. Using a set of a solute and three particle markers of 29 different sizes (<2, 5 and 8 mm), we displayed digesta passage kinetics and measured mean 30 retention times (MRTs) in four captive proboscis monkeys (6-18 kg) and compared the 31 marker excretion patterns to those in domestic cattle. In addition, we evaluated various 32 methods of calculating and displaying passage characteristics. The mean ± SD dry matter 33 intake was 98 ± 22 g kg −0.75 d −1 , 68 ± 7% of which was browse. Accounting for sampling 34 intervals in MRT calculation yielded results that were not affected by the sampling frequency. 35Displaying marker excretion patterns using fecal marker concentrations (rather than amounts) 36 facilitated comparisons with reactor theory outputs and indicated that both proboscis and 37cattle digestive tracts represent a series of very few tank reactors. However, the separation of 38 the solute and particle marker and the different-sized ...

Plains viscachas (Lagostomus maximus) are large South American, fossorial rodents susceptible to diabetic cataracts. Various aspects of their digestive physiology were studied in three different experiments with 9 male and 7 female adult animals and 6 different diets (total n of feeding trials = 35). Viscachas achieved mean retention times of 23-31h, which is of a magnitude also recorded in horses; these did not differ for solute or small particle (<2mm) markers. Secondary marker excretion peaks indicated coprophagy, and were rarer on high-protein as compared to grass hay-only diets. Mean resting metabolic rate was, at 229 kJ/kg0.75/d, lower than expected for a mammal of this size. Digestible energy requirement for maintenance was 445 kJ/kg0.75/d. At 1.6-2.7 L/d, viscachas produced more methane than expected for a hindgut fermenter of their size. On diets that included concentrate feeds, viscachas excreted glucose in their urine, corroborating reports on the susceptibility of this species for diabetes when kept on energy-dense food. Viscachas had a similar apparent digestibility of protein, lipids, and macrominerals as other rodents, rabbits, or domestic horses. This suggests that whether or not a species practices coprophagy does not have a major influence on these measures. Viscachas resemble other hindgut fermenters in their high apparent calcium digestibility. With respect to a digestibility-reducing effect of dietary fiber, viscachas differed from rabbits and guinea pigs but were similar to horses, suggesting that small body size needs not necessarily be linked to lower digestive efficiency on high-fiber diets. three different experiments with 9 male and 7 female adult animals and 6 different diets (total 28 n of feeding trials = 35). Viscachas achieved mean retention times of 23-31h, which is of a 29 magnitude also recorded in horses; these did not differ for solute or small particle (<2mm) viscachas excreted glucose in their urine, corroborating reports on the susceptibility of this 36 species for diabetes when kept on energy-dense food. Viscachas had a similar apparent 37 digestibility of protein, lipids, and macrominerals as other rodents, rabbits, or domestic 38horses. This suggests that whether or not a species practices coprophagy does not have a 39 major influence on these measures. Viscachas resemble other hindgut fermenters in their high 40 apparent calcium digestibility. With respect to a digestibility-reducing effect of dietary fiber, 41 viscachas differed from rabbits and guinea pigs but were similar to horses, suggesting that 42 small body size needs not necessarily be linked to lower digestive efficiency on high-fiber 43 diets. 44 45

Chinchilla (Chinchilla laniger), degus (Octodon degus) and guinea pigs (Cavia porcellus) are South American rodents living in a semi-arid habitat with varying, species-specific adaptations to water deprivation. Nonetheless, several diseases have been linked to insufficient water intake when these species are kept as pets, such as urolithiasis or obstipation. This study evaluated preferences for drinking systems. Six animals of each species were given a choice between an open dish and a nipple drinker. Food intake and water intake were measured daily for 13 days. Chinchillas in this study had significantly lower water intakes than the other two species, indicating particular species-specific adaptations to aridity. All chinchillas favoured open dishes, whereas the degus and guinea pigs had variable individual preferences. Water intake of chinchillas was similar or higher in this study than in previous studies where nipple drinkers were used. The results indicate that degus and guinea pigs can meet their drinking water needs with nipple drinkers; for chinchillas, other drinking systems may be more adequate.