Fluid and particle retention in captive okapi (Okapia johnstoni)

Retention time of food in the digestive tract is among the key variables that describe the digestive strategy of a herbivore. Mean retention time (MRT) was measured on 4 captive specimens of the okapi, a strictly browsing ruminant. Retention time was quantified on different diets, using Co-EDTA (fluid phase) and Cr-mordanted fibres (1–2 mm) (particle phase) as pulse-fed markers. Average food intake was 55–65 g DM/(kg BW^0.75*d). Fecal excretion of the markers was quantified over 10 days. Different models to calculate retention time and passage rate in the gastrointestinal tract (GIT) and the reticulorumen (RR) were applied. Average MRT_particleGIT was quantified to be 47 ± 8 h and MRT_fluidGIT 36 ± 5 h. Concerning estimation of retention times in the reticulorumen, MRT_particleRR was quantified to be 27 ± 7 h, while MRT_fluidRR was 17 ± 4 h. The quotients MRT_particle/MRT_fluid were quantified to be 1.3 ± 0.1 for the GIT and 1.6 ± 0.2 for the RR. Compared to data established with comparable markers, the okapi has low coefficients of MRT_particle/MRT_fluid. A less well developed retention mechanism for fibres compared to species like cattle or sheep can be explained by a comparatively high fermentation rate and low digestibility of the natural food of the okapi—browse—in comparison to grass.     

Fluid and particle retention in the digestive tract of the addax antelope (Addax nasomaculatus)--adaptations of a grazing desert ruminant

Retention time of food in the digestive tract is a major aspect describing the digestive physiology of herbivores. Differences in feed retention times have been described for different ruminant feeding types. In this study, a dominantly grazing desert ruminant, the addax (Addax nasomaculatus), was investigated in this respect. Eight animals with a body weight (BW) of 87+/-5.3 kg on an ad libitum grass hay (Chloris gayana) diet were available. Co-EDTA and Cr-mordanted fibers (<2 mm) were used as pulse-dose markers. Mean retention time (MRT) in the digestive tract was calculated from faecal marker excretion. Average daily intake of the addax was found to be 1.7 kg dry matter (DM) or 60+/-8.3 g DM/kg BW(0.75). The MRT of fluid and particles in the reticulo-rumen (MRT(fluid)RR and MRT(particle)RR) were quantified to be 20+/-5.8 and 42+/-7.0 h respectively. When compared to literature data, MRT(fluid)RR was significantly longer than in cattle species, and MRT(particle)RR was significantly longer than in 11 taxa of all feeding types. The ratio of MRT(particle)RR/MRT(fluid)RR (2.3+/-0.5) was found to be within the range described for grazing ruminants. The long retention times found in the addax can be interpreted as an adaptation to a diet including a high proportion of slow fermenting grasses, while the long retention time of the fluid phase can be interpreted as a consequence of water saving mechanisms of the desert-adapted addax with a potentially low water turnover and capacious water storing rumen.     

Fluid and particle retention in the digestive tract of the addax antelope (Addax nasomaculatus)--adaptations of a grazing desert ruminant.

Retention time of food in the digestive tract is a major aspect describing the digestive physiology of herbivores. Differences in feed retention times have been described for different ruminant feeding types. In this study, a dominantly grazing desert ruminant, the addax (Addax nasomaculatus), was investigated in this respect. Eight animals with a body weight (BW) of 87+/-5.3 kg on an ad libitum grass hay (Chloris gayana) diet were available. Co-EDTA and Cr-mordanted fibers (<2 mm) were used as pulse-dose markers. Mean retention time (MRT) in the digestive tract was calculated from faecal marker excretion. Average daily intake of the addax was found to be 1.7 kg dry matter (DM) or 60+/-8.3 g DM/kg BW(0.75). The MRT of fluid and particles in the reticulo-rumen (MRT(fluid)RR and MRT(particle)RR) were quantified to be 20+/-5.8 and 42+/-7.0 h respectively. When compared to literature data, MRT(fluid)RR was significantly longer than in cattle species, and MRT(particle)RR was significantly longer than in 11 taxa of all feeding types. The ratio of MRT(particle)RR/MRT(fluid)RR (2.3+/-0.5) was found to be within the range described for grazing ruminants. The long retention times found in the addax can be interpreted as an adaptation to a diet including a high proportion of slow fermenting grasses, while the long retention time of the fluid phase can be interpreted as a consequence of water saving mechanisms of the desert-adapted addax with a potentially low water turnover and capacious water storing rumen.     

Differences in selective reticulo-ruminal particle retention as a key factor in ruminant diversification

The measurement of passage rate is important for the concept of ruminant diversification. While supporters of Hofmann's 1989 feeding type classification claim that browsing ruminants have faster passage rates than grazing ruminants, other researchers consider the passage rate to depend on body size alone. To date, no convincing comparison of ruminant passage rates has been put forward. For comparative purposes, we suggest the use of the "selectivity factor", which is an expression of how much longer particles of a defined size (<2 mm) are retained in the ruminant digestive tract than fluids. From the limited data available, it seems that grazing ruminants display selectivity factors between 1.56 and 3.80, whereas browsers have a much narrower range of 1.14-1.80. This suggests that browsers are not able to selectively retain particles as long as grazers. Intake of browsers, on the other hand, may not be limited by physical fill of the forestomach to the same degree as in grazers. This result can explain several observations on the digestive physiology of browsers, some of which have been linked to a rumen bypass mechanism. We propose that the ability for selective particle retention is a key factor for understanding the physiological consequences of ruminant diversification.     

Tooth wear in captive wild ruminant species differs from that of free-ranging conspecifics

The mesowear method evaluates the wear patterns of herbivore cheek teeth by visually evaluating the facet development of the occlusal surfaces. It thus allows classification of most herbivorous ungulates into browsers, grazers or intermediate feeders, due to the fact that in grazers, tooth wear is characterized by a comparatively high degree of abrasion, most probably due to the presence of silicacious phytoliths in grasses, a higher amount of dust and grit adhering to their forage, or both. It has been suggested that excessive tooth wear could be a particularly limiting factor in the husbandry of captive large browsing species, and major tooth wear was demonstrated in captive as compared to free-ranging giraffe. If this increased tooth wear in captivity was an effect of feeding type and diets fed, then it would be expected that other browsing species are affected in a similar manner. In order to test this hypothesis, we investigated the dental mesowear pattern in captive individuals of 19 ruminant species and compared the results to data on free-ranging animals. Compared to free-ranging populations, captive browsers show a significantly more abrasion-dominated tooth wear signal. The reverse applies to captive grazers, which tend to show a less abrasion-dominated wear in captivity. Captive ruminants were generally more homogenous in their wear signature than free-ranging ruminants. If grit contamination in the natural habitat is a major cause of dental wear in grazers, then diets in captivity, although similar in botanical composition, most likely contain less abrasives due to feeding hygiene. If dental wear is one of the major factors limiting longevity, then captive grazers should achieve longer lifespans than both captive browsers and free-ranging grazers. In particular with respect to browsers, the results suggest that captive feeding regimes could be improved.     

Adaptation of ruminants to browse and grass diets: are anatomical-based browser-grazer interpretations valid?

As a result of pioneering work of Hofmann (1973, 1989), nutritional ecologists classify ruminants into three feeding-type categories: browsers (concentrate feeders), grazers, and intermediate or mixed feeders. Hofmann proposed that these feeding types result from evolutionary adaptations in the anatomy of the digestive system and that one consequence is shorter retention of the digesta in the rumen of browsers, and thus a decreased efficiency of fiber digestion relative to that of grazers. We examined the hypotheses that (1) fiber digestion of browsers is lower than that of grazers, (2) salivary gland size is larger in all browsers than in grazers, (3) the browser's larger salivary glands produce larger volumes of thin serous saliva than those of grazers, and (4) thus, browsers have higher liquid passage rates than do grazers. We found that the extent of fiber digestion is not significantly different between browsers and grazers, although fiber digestion is positively related to herbivore size. In general, salivary gland size is approximately 4 times larger in browsers than grazers, but some browsers (e.g., greater kudu) have small, grazer-sized salivary glands. Resting (non-feeding or ruminating) saliva flow rates of mule deer (browser) and domestic sheep and cattle (grazers) were not significantly different from each other. Finally, ruminal liquid flow rates were not different between feeding types. We conclude that many of Hofmann's nutritional and physiological interpretations of anatomical differences amongst ruminants are not supportable.     

Relationships between oral morphology and feeding style in the Ungulata: a phylogenetically controlled evaluation

In ungulates it is argued that specialization in the consumption of a particular type of food (feeding style) is reflected in morphological adaptations of the organs involved in the selection, processing and digestion of food. We analysed the differences in size and morphology of some oral traits that have been functionally related to food-selection ability (muzzle width, incisor-arcade shape, incisor shape), prehension of food (incisor protrusion), food comminution (molar occlusal surface area, hypsodonty (high-crowned molars)) and intake rate (incisor breadth) between ungulate species with different feeding styles (browser, mixed feeder, grazer). Grazers were characterized by large-body-size species. After controlling only for body mass, we found that grazers had wider muzzles and incisors, more-protruding incisors and more-bulky and higher-crowned molars than did mixed feeders and browsers. When the analyses took into account both body mass and phylogeny, only body mass and two out of the three hypsodonty indexes used remained significantly different between feeding styles. Browsers were smaller, on average, than mixed feeders and grazers, whilst grazers and mixed feeders did not differ in size. Also, browsers had shorter and less-bulky molars than did mixed feeders and grazers; the latter two feeding styles did not differ from each other in any of the hypsodonty indexes. We conclude that the adaptation to different dietary types in most of the oral traits studied is subsumed by the effects of body mass and the sharing of common ancestors. We hypothesize that differences in the ability to exploit different food resources primarily result from differences in body mass between species, and also discuss why hypsodonty characterizes feeding styles.     

Correlation of relative muzzle width and relative incisor width with dietary preference in ungulates

Qualitative observations suggest that grazing ungulates have relatively broader muzzles than browsing ones, and that grazers have incisors that are all of a similar size, as opposed to the large central and smaller lateral incisors seen in browsers. These differences may be correlated respectively with the need for grazing ungulates to maintain a large daily intake, or for browsing ungulates to forage selectively in a stand of vegetation. Quantitative examination of relative muzzle width and incisor width ratio in 95 species of living ungulates, correlated with seven different types of dietary preferences, substantiated these observations, although phylogenetic history may exhort a strong influence on morphological proportions. For example, equids have relatively narrower muzzles than grazing ruminants despite their less selective mode of feeding. The narrowest relative muzzle widths are not found in regular browsers, but in high level browsers and in mixed feeders in open habitats. Incisor width ratio can distinguish grazers from browsers, but can not be used to discriminate mixed feeders from other feeding types, and grazers appear to have incisors that are relatively broader overall than those of other dietary types, in correlation with their relatively broader muzzles.     

An in vitro evaluation of browser and grazer fermentation efficiency and microbiota using European moose spring and summer foods

Evolutionary morphological and physiological differences between browsers and grazers contribute to species‐specific digestion efficiency of food resources. Rumen microbial community structure of browsers is supposedly adapted to characteristic nutrient composition of the diet source. If this assumption is correct, domesticated ruminants, or grazers, are poor model animals for assessing the nutritional value of food consumed by browsing game species. In this study, typical spring and summer foods of the European moose (Alces alces) were combined with rumen fluid collected from both dairy cows (Bos taurus) and from moose, with the aim of comparing fermentation efficiency and microbial community composition. The nutritional value of the food resources was characterized by chemical analysis and advanced in vitro measurements. The study also addressed whether or not feed evaluation based on in vitro techniques with cattle rumen fluid as inoculum could be a practical alternative when evaluating the nutritional value of plants consumed by wild browsers. Our results suggest that the fermentation characteristics of moose spring and summer food are partly host‐specific and related to the contribution of the bacterial phyla Firmicutes and Bacteriodetes to the rumen microbial community. Host‐specific adaptations of the ruminal microbial community structure could be explained from the evolutionary adaptations related to feeding habitats and morphophysiological differences between browsers and grazers. However, the observed overall differences in microbial community structure could not be related to ruminal digestion parameters measured in vitro. The in vitro evaluation of digestion efficiency reveals that equal amounts of methane were produced across all feed samples regardless of whether the ruminal fluid was from moose or dairy cow. The results of this study suggested that the nutritional value of browsers' spring and summer food can be predicted using rumen fluid from domesticated grazers as inoculum in in vitro assessments of extent of digestion when excluding samples of the white water lily root, but not of fermentation characteristics as indicated by the proportions of individual fermentation fatty acids to the total of volatile fatty acids.     

Rumination of different-sized particles in muskoxen (Ovibos moschatus) and moose (Alces alces) on grass and browse diets,and implications for rumination in different ruminant feeding types

The obligatory, periodic regurgitation of forestomach material and its subsequent re-mastication is the hallmark of the most diverse extant large herbivore group, the ruminants. Although the process of rumination is well understood in domestic species, differences between free-ranging wild ruminant species, for example of different body size or different feeding type, remain speculative to date. Here we investigate the proportion of plastic particles of varying size (1, 10 and 20 mm) and density (1.03, 1.20 and 1.44 mg/ml) that are recovered intact or ruminated-upon after insertion into the reticulorumen (RR) of domestic cattle (Bos primigenius f. taurus) on grass silage, and of muskoxen (Ovibos moschatus; n = 4) and moose (Alces alces; n = 2) both fed browse and grass diets. In the three species, the proportion of particles leaving the RR intact depended on particle size, with density showing no effect in this study. The major proportion of 1 mm particles was excreted intact, whereas intact 10–20 mm particles were only excreted sporadically, and not in all animals. Intact particles were mostly found in the initial samples after marker application, and mean retention times of intact particles were always shorter than those of ruminated particles. There were no differences between moose and muskoxen, but diet had a significant effect, with a higher proportion of 1 mm particles ruminated upon on the grass diet in both species, indicating a higher ‘filter-bed effect’ with entrapment of small particles in a fibre mat in the RR on a grass diet. Given that less particles were ruminated on the grass diet, one could either assume that free-ranging browsers ruminate less than grazers on similar food intakes (or that they have higher food intakes at similar levels of rumination). The existing data on time-budgets of free-ranging ruminants, however, does not suffice to test these hypotheses. The fact that indication of a ‘filter-bed effect’ was also detectable in moose raises the question whether adaptations described in ‘cattle-type’ ruminants really serve to re-inforce the processes of RR contents stratification and the ‘filter-bed’, or whether they are not rather directed at other aims, such as maximizing microbial yield from the RR.     

Nutrition of captive lowland anoa (Bubalus depressicornis): a study on ingesta passage,intake, digestibility,and a diet survey

Members of the Bovini genus are classified as grazers. Smaller species of ruminants are not expected to be able to digest particularly fibrous diets and are more often classified as intermediate feeders or browsers. Anoas (Bubalus spp.) are interesting in this respect as they are the smallest representatives of the Bovini, being only 10–20% of the body weight of other species of the same genus. A feeding trial was carried out with four lowland anoas (Bubalus depressicornis) at London Zoo, investigating diet digestibility by total fecal collection and passage rates by the simultaneous administration of a fluid (Co‐EDTA) and a particle (Cr‐mordanted fibre <2 mm) marker. The diet consisted of legume hay, dairy cow pellets, browse, fruits, and vegetables. The achieved digestibility coefficients averaged 70±4% for dry matter and 57±7% for cell walls (NDF). Mean retention times for the total gastrointestinal tract were 25±4.1 hr for fluid and 39±6.7 hr for particles, respectively. The ratio of forestomach particle:fluid retention was 2.14±0.40. Additional information regarding anoa diets in captivity was collected through a survey targeting all institutions that have anoas in their collection currently. Suitability of the provided diet was evaluated using the ratio of unstructured:structured feeds (unstructured feeds pellets, grains, produce; structured feeds=roughage, browse) on a dry matter basis and an assumed complete consumption of offered unstructured diet items, with only the remaining intake capacity being met by structured items. The use of this ratio reliably predicted one facility that reported chronic diet‐related problems. As other ruminants, anoas should receive a diet with restricted amounts of concentrates and fruits. The comparatively high fibre digestibility and the high selective particle retention in the forestomach suggest a classification of an intermediate/grazing ruminant. Zoo Biol 24:125–134, 2005. © 2005 Wiley‐Liss, Inc.     

Higher masseter muscle mass in grazing than in browsing ruminants

Using cranioskeletal measurements, several studies have generated evidence that grazing ruminants have a more pronounced mastication apparatus, in terms of muscle insertion areas and protuberances, than browsing ruminants, with the resulting hypothesis that grazers should have larger, heavier chewing muscles than browsers. However, the only investigation of this so far [Axmacher and Hofmann (J Zool 215:463-473, 1988)] did not find differences between ruminant feeding types in the masseter muscle mass of 22 species. Here, we expand the dataset to 48 ruminant species. Regardless of phylogenetic control in the statistical treatment, there was a significant positive correlation of body mass and masseter mass, and also a significant association between percent grass in the natural diet and masseter mass. The results support the concept that ruminant species that ingest more grass have relatively larger masseter muscles, possibly indicating an increased requirement to overcome the resistance of grass forage. The comparative chewing resistance of different forage classes may represent a rewarding field of ecophysiological research.     

Extinctions of herbivorous mammals in the late Pleistocene of Australia in relation to their feeding ecology: No evidence for environmental change as cause of extinction

Abstract There has been debate over the cause of the extinction of ‘megafauna’ species during the late Pleistocene of Australia. One view is that environmental change, either natural or human‐induced, was the main factor in the extinctions. Some support for this comes from the observation that, among herbivores, most of the species that went extinct were apparently browsers rather than grazers. Browsers would presumably have been more dependent on shrubland and woodland habitats than grazers, and it has been argued that such habitats might have contracted in response to aridity or changed fire regimes in the late Pleistocene. Here, we test this idea by comparing extinction rates of browsers and grazers in the late Pleistocene, controlling for body mass in both groups. We show that in both browsers and grazers the probability of extinction was very strongly related to body mass, and the body mass at which extinction became likely was similar in the two groups. It is true that more browsers than grazers went extinct, but this is largely because most very large herbivores in the late Pleistocene were browsers, not because large browsers were more likely to go extinct than similarly sized grazers. This result provides evidence against some forms of environmental change as a cause of the extinctions.     

Quantifying rapid spatial and temporal variations of CO<Subscript>2</Subscript> fluxes from small,lowland freshwater ponds

In Lake Tanganyika, several lineages of cichlids have diversified into 200 species. Tribe Tropheini contains the greatest richness of herbivorous species that are classified into four ecomorphs: grazers, browsers, suckers, and pickers. All of these ecomorphs coexist on littoral rocky shores. To reveal the differences among species within ecomorphs, we analysed fish shape and functional morphology related to feeding habits, using geometric morphometrics, and measured intestine length. Furthermore, we tested the relationship between genetic and morphological distances. As a result, diversities of functional morphology among ecomorphs, and among species within ecomorphs, were revealed. In grazers, morphological diversity was greatest in jaws and the opening direction of mouth, suggesting that these species have adapted to feed on various substrata. In browsers, intestine length varied among species, indicating that this ecomorph consists of species with various specializations in herbivory. Morphological divergence was found only in the mandible and occlusal facet in grazers. No clear relation was found between morphological and phylogenetic distances in browsers, which are a polyphyletic group, indicating that their traits were acquired by convergence in multiple lineages. Our data suggest that the observed morphological variation among species enable sympatric species to separate their feeding niches.     

Retention of solute and particle markers in the digestive tract of captive Somali wild asses (<Emphasis Type="Italic">Equus africanus somaliensis</Emphasis>)

In contrast to the domestic horse, whose digestive physiology has been thoroughly investigated, knowledge on the digestive physiology of wild equids is scarce. Comparisons between the domestic horse and the domestic donkey suggest that wild asses might achieve higher digestibilities. This could derive from longer retention times or a greater difference in the mean retention time (MRT) of particles vs. fluid (the selectivity factor (SF)). Here, we measured MRT of a solute (fluid; MRT_solute) and a particle (<2 mm; MRT_particle) marker in five captive male Somali wild asses (Equus africanus somaliensis) fed a diet of 95% grass hay. At a mean dry matter intake of 94 ± 3 g kg^?0.75 day^?1, MRT_solute was 33.3 ± 5.4 h and MRT_particle 39.6 ± 3.9 h, resulting in a SF of 1.21 ± 0.14. For their food intake, Somali wild asses appeared to have slightly higher MRT_particle than expected based on domestic equid data, in contrast to Grevy zebras (Equus grevyi), potentially indicating higher capacities of the digestive tract. However, considering data on domestic horses, donkeys, and zebra, there was no evident difference in the SF of wild equids compared to domestic ones. Together with an absence of reported anatomical differences in the digestive tract of wild and domestic equids, the data suggest a general similarity in the digestive physiology of equid species that contrasts with the diversity in the digestive physiology of ruminants, and that might be one contributing factor to a lack of sympatric, niche-differentiated equid species.     

Assessing the magnitude of intra- and interspecific competition in two coral reef fishes

Spatial associations of seven herbivore species in the Kruger National Park, South Africa, are analyzed using a new technique, Correlative Coherence Analysis (CoCA). CoCA is a generalization of the concept of correlation to more than two sequences of numbers. Prior information on the feeding ecology and metabolic requirements of these species is used to contrast spatial scales at which hypothesized guild aggregation or competition occurs. These hypotheses are tested using 13 years of aerial census data collected during the dry season. Our results are consistent with the hypothesis that distributions of large and small species of the same feeding type (i.e., grazers and browsers) overlap in potentially resource-rich areas, but have lower similarity values across all areas because the higher tolerance of large species for low quality foods results in a more even spatial distribution of large species compared to small species.     

Use of δ13C values to determine vegetation selectivity in East African herbivores

Summary The quantitative plant species composition of the rumen contents of a large number of individuals from eight East African herbivores was determined by direct visual analysis. All plant species were classified as either C₃ or C₄, and an estimated ₁₃C for the rumen sample was calculated. This estimated value was compared to a measured value determined directly from rumen subsample. The two methods of determining quantitative C₃ and C₄ composition differed by less than 1%, and the isotopic analysis has the advantage of being rapid and totally objective.The isotopic analysis allowed us to differentiate between grazers and browsers and to determine the quantitative dependence of each animal on C₃ and C₄ photosynthetic types. Kongoni, wildebeest, cattle, and sheep were nearly pure grazers on the Athi Kapiti Plains; and the Grant's gazelle were predominantly browsers. Thompson's gazelle, goast and impala were intermediate. The species most dependent upon browse showed a marked and rapid shift to grass within a few days following rain. This isotopic method may have general utility in the study of East African ecology.     

Investigations on the stratification of forestomach contents in ruminants: an ultrasonographic approach

The degree of stratification in the reticulorumen contents is considered a major explanatory factor for other morphological and physiological differences between ruminant feeding types. However, information on reticulorumen (RR) contents is usually gathered from dead animals or indirect such as passage kinetics. We hypothesized that, although the contents of the gastrointestinal tract can usually not be evaluated by ultrasonography, the three typical layers of stratified RR contents (gas dome, fibre mat, fluid) can be demonstrated by this technique. In three domestic cows, the gas dome in the cows’ rumens could be demonstrated sonographically by reverberation lines running in parallel to the line demarcating the rumen mucosa, as is typical for the sonographic image of large, gas-filled spaces. More ventrally, the area behind the rumen mucosa showed typical pattern indicating gaseous inclusions, corresponding to the fibre mat inside the rumen. Further ventrally, in two of the animals, the area behind the demarcation line appeared dark without reverberation lines, as is typical for large fluid-filled spaces. When the technique was applied to a captive, habituated, browse-fed moose, no gas dome could be demonstrated, supporting the hypothesis that the reticulorumen contents of browsers are less stratified. The results of this study indicate that sonography represents a useful tool for the demonstration of RR contents stratification in live animals.     

The evolution of phylogenetic differences in the efficiency of digestion in ruminants

This study investigates, for the first time (to our knowledge) for any animal group, the evolution of phylogenetic differences in fibre digestibility across a wide range of feeds that differ in potential fibre digestibility (fibre to lignin ratio) in ruminants. Data, collated from the literature, were analysed using a linear mixed model that allows for different sources of random variability, covariates and fixed effects, as well as controlling for phylogenetic relatedness. This approach overcomes the problem of defining boundaries to separate different ruminant feeding styles (browsers, mixed feeders and grazers) by using two covariates that describe the browser-grazer continuum (proportion of grass and proportion of browse in the natural diet of a species). The results indicate that closely related species are more likely to have similar values of fibre digestibility than species that are more distant in the phylogenetic tree. Body mass did not have any significant effect on fibre digestibility. Fibre digestibility is estimated to increase with the proportion of grass and to decrease with the proportion of browse in the natural diet that characterizes the species. We applied an evolutionary model to infer rates of evolution and ancestral states of fibre digestibility; the model indicates that the rate of evolution of fibre digestibility accelerated across time. We suggest that this could be caused by a combination of increasing competition among ruminant species and adaptation to diets rich in fibre, both related to climatically driven environmental changes in the past few million years.     

Fluid and particle retention times in the black rhinocerosDiceros bicornis, a large hindgut-fermenting browser

The mean retention time (MRT) of ingesta in the gastrointestinal tract is one of the major determinants of herbivore digestive physiology. We examined MRTs of fluids and particles in the gastrointestinal tract of six adult captive black rhinocerosesDiceros bicornis on conventional zoo diets. Fluid MRT ranged from 25–45 h and averaged 31 h. Particle MRT ranged from 28–59 h and averaged 38 h. In the six animals, both fluid and particle MRT declined as relative dry matter intake (g/kg metabolic body mass) increased. Black rhinoceroses, which are large hindgut-fermenting browsers, retained ingesta for a shorter period relative to their body size than grazing equids or grazing rhinoceros species. Our findings support the hypothesis that browsing hindgut fermenters have relatively shorter MRTs than grazing hindgut fermenters.