A pilot study on the ontogeny of digestive physiology in Japanese macaques (Macaca fuscata)

An assumption based on the Jarman–Bell principle suggests a positive relationship between body size and the digestive efficiency in animals, where smaller animals are less effective at digesting fibrous food due to shorter digesta passage. To examine the effect of body size within a species and explore a potential physiological background of ontogenetic diet shifts, we measured food intake, digestibility, digesta passage and gut fill in nine Japanese macaques, including three juveniles/subadult animals. Although these three showed a comparable digestive efficiency as the older animals on a low-fiber diet, they did not achieve the long retention times of adults in spite of similar levels of indigestible food intake and gut capacity. While the limited sample size would not allow generalized conclusions on ontogenetic digestive development in primates, this study suggests additional, yet unexplored effects other than food intake, digestion and gut capacity on digesta retention during ontogeny.     

Assessing the Jarman–Bell Principle: Scaling of intake,digestibility, retention time and gut fill with body mass in mammalian herbivores

Differences in allometric scaling of physiological characters have the appeal to explain species diversification and niche differentiation along a body mass (BM) gradient — because they lead to different combinations of physiological properties, and thus may facilitate different adaptive strategies. An important argument in physiological ecology is built on the allometries of gut fill (assumed to scale to BM^1.0) and energy requirements/intake (assumed to scale to BM^0.75) in mammalian herbivores. From the difference in exponents, it has been postulated that the mean retention time (MRT) of digesta should scale to BM^1.0–0.75 = BM^0.25. This has been used to argue that larger animals have an advantage in digestive efficiency and hence can tolerate lower-quality diets. However, empirical data does not support the BM^0.25 scaling of MRT, and the deduction of MRT scaling implies, according to physical principles, no scaling of digestibility; basing assumptions on digestive efficiency on the thus-derived MRT scaling amounts to circular reasoning. An alternative explanation considers a higher scaling exponent for food intake than for metabolism, allowing larger animals to eat more of a lower quality food without having to increase digestive efficiency; to date, this concept has only been explored in ruminants. Here, using data for 77 species in which intake, digestibility and MRT were measured (allowing the calculation of the dry matter gut contents (DMC)), we show that the unexpected shallow scaling of MRT is common in herbivores and may result from deviations of other scaling exponents from expectations. Notably, DMC have a lower scaling exponent than 1.0, and the 95% confidence intervals of the scaling exponents for intake and DMC generally overlap. Differences in the scaling of wet gut contents and dry matter gut contents confirm a previous finding that the dry matter concentration of gut contents decreases with body mass, possibly compensating for the less favorable volume–surface ratio in the guts of larger organisms. These findings suggest that traditional explanations for herbivore niche differentiation along a BM gradient should not be based on allometries of digestive physiology. In contrast, they support the recent interpretation that larger species can tolerate lower-quality diets because their intake has a higher allometric scaling than their basal metabolism, allowing them to eat relatively more of a lower quality food without having to increase digestive efficiency.     

Multiple pathways from three types of sugar receptor sites to metabotropic transduction pathways of the blowfly: study by the whole cell-clamp experiments

Differences in the allometric scaling between gut capacity (with body mass, BM^1.00) and food intake (with BM^0.75) should theoretically result in a scaling of digesta retention time with BM^0.25 and therefore a higher digestive efficiency in larger herbivores. This concept is an important part of the so-called ‘Jarman–Bell principle’ (JBP) that explains niche differentiation along a body size gradient in terms of digestive physiology. Empirical data in herbivorous mammals, however, do not confirm the scaling of retention time, or of digestive efficiency, with body mass. Here, we test these concepts in herbivorous reptiles, adding data of an experiment that measured food intake, digesta retention, digestibility and gut capacity in 23 tortoises (Testudo graeca, T. hermanni , Geochelone nigra, G. sulcata, Dipsochelys dussumieri) across a large BM range (0.5–180 kg) to a literature data collection. While dry matter gut fill scaled to BM^1.07 and dry matter intake to BM^0.76, digesta mean retention time (MRT) scaled to BM^0.17; the scaling exponent was not significantly different from zero for species > 1 kg. Food intake level was a major determinant of MRT across reptiles and mammals. In contrast to dietary fibre level, BM was not a significant contributor to dry matter digestibility in a General Linear Model. Digestibility coefficients in reptiles depended on diet nutrient composition in a similar way as described in mammals. Although food intake is generally lower and digesta retention longer in reptiles than in mammals, digestive functions scale in a similar way in both clades, indicating universal principles in herbivore digestive physiology. The reasons why the theoretically derived JBP has little empirical support remain to be investigated. Until then, the JBP should not be evoked to explain niche differentiation along a body size axis in terms of digestive physiology.     

Rheological properties of digesta suggest little radial or axial mixing in the forestomach of the tammar (Macropus eugenii) and the parma (Macropus parma) wallaby

We examined the physical properties of digesta from the proximal (sacciform) and distal (tubiform) regions of the forestomach of tammar and parma wallabies maintained on a ryegrass sward. The digesta exhibited high viscosity, which, in conjunction with low flow rates calculated from published retention times of particulate marker in the stomach, results in a low potential for macrofluid mixing during onflow of food by displacement. The pseudoplastic nature of the viscosity profile and very low "flow behavior index" also indicate that macrofluid mixing around haustrae would be very localized. These findings indicate that the uniform mixing of whole digesta required for continuously stirred tank reactors (CSTRs) is unlikely. Voidage, the proportion of digesta not occupied by solid material, and "consistency index" were greater in the digesta from the proximal than in the distal regions of the forestomach. We conclude that the mixing conditions in wallaby stomachs are better described as a partially emptying batch reactor (PEBR) than a CSTR series and that lower permeabilities in conjunction with higher viscosities decrease axial mixing and thus increase efficiency in PEBRs.     

Digesta Passage Time,Digestibility, and Total Gut Fill in Captive Japanese Macaques (<Emphasis Type="BoldItalic">Macaca fuscata</Emphasis>): Effects Food Type and Food Intake Level

Digestion is an important process in understanding the feeding ecology of animals. We examined digesta passage time, digestibility, and total gut fill in Japanese macaques (Macaca fuscata; n = 4) under 4 dietary conditions representing the seasonal and regional variations in the diets of wild populations to determine the effects of food type and food intake on these digestive features. Food type is associated with mean retention time (MRT), digestibility, and total gut fill. Dry matter intake (DMI) of food correlates positively with total gut fill but not with MRT or digestibility. Conversely, indigestible DMI affected MRT negatively. Thus, when Japanese macaques consume high-fiber foods, MRT becomes shorter and digestibility is lower than when eating low-fiber foods. Moreover, macaques experience increases in total gut fill when they consume high-fiber diets or a large amount of food. Japanese macaques may excrete difficult-to-digest food components quickly; they nevertheless buffer an increase in food intake by an increase in gut fill. Our study offers new insights into the relationship between feeding ecology and nutritional physiology in primates by simultaneously examining the effects of food type and intake level on MRT and digestibility.     

More than one way to be an herbivore: convergent evolution of herbivory using different digestive strategies in prickleback fishes (Stichaeidae)

In fishes, the evolution of herbivory has occured within a spectrum of digestive strategies, with two extremes on opposite ends: (i) a rate-maximization strategy characterized by high intake, rapid throughput of food through the gut, and little reliance on microbial digestion or (ii) a yield-maximization strategy characterized by measured intake, slower transit of food through the gut, and more of a reliance on microbial digestion in the hindgut. One of these strategies tends to be favored within a given clade of fishes. Here, we tested the hypothesis that rate or yield digestive strategies can arise in convergently evolved herbivores within a given lineage. In the family Stichaeidae, convergent evolution of herbivory occured in Cebidichthys violaceus and Xiphister mucosus, and despite nearly identical diets, these two species have different digestive physiologies. We found that C. violaceus has more digesta in its distal intestine than other gut regions, has comparatively high concentrations (>11 mM) of short-chain fatty acids (SCFA, the endpoints of microbial fermentation) in its distal intestine, and a spike in β-glucosidase activity in this gut region, findings that, when coupled to long retention times (>20 h) of food in the guts of C. violaceus, suggest a yield-maximizing strategy in this species. X. mucosus showed none of these features and was more similar to its sister taxon, the omnivorous Xiphister atropurpureus, in terms of digestive enzyme activities, gut content partitioning, and concentrations of SCFA in their distal intestines. We also contrasted these herbivores and omnivores with other sympatric stichaeid fishes, Phytichthys chirus (omnivore) and Anoplarchus purpurescens (carnivore), each of which had digestive physiologies consistent with the consumption of animal material. This study shows that rate- and yield-maximizing strategies can evolve in closely related fishes and suggests that resource partitioning can play out on the level of digestive physiology in sympatric, closely related herbivores.     

Digestive constraints on an aquatic carnivore: effects of feeding frequency and prey composition on harbor seals

We hypothesized that increased feeding frequency in captive harbor seals would increase nutrient loads and thus reduce retention time and the digestive efficiency of natural prey. We measured daily feed intake and excretion during 6 feeding trials and fed herring (49% lipid), pollock (22% lipid) or an equal mix of each diet over 24 months. Animals were accustomed to feeding at either high or low frequency. Body mass and intake did not vary with season. Although mean retention times were similar between diets and feeding frequencies, solute and particulate digesta markers separated at high feeding frequency. Consistent dry matter digestibility resulted in greater gut fill from pollock than from herring. Digestible energy intakes from pollock were approximately 25% greater than from either herring or the mixed diet. Lipid digestibility of herring declined from 90% to 50% when lipid intake exceeded 60 g kg^–0.75 day^–1. Our hypothesis of a trade-off between intake and digestion was not supported for protein but was supported for lipid. Results of this study imply that a flexible digestive system for harbor seals can compensate for ingesting prey of lower energy density by increasing gut fill and enhancing protein and lipid assimilation, to sustain digestible energy intake.Abbreviations DM dry matter - DEI digestible energy intake - DIT diet-induced thermogenesis - FF feeding frequency - MRT mean retention time Communicated by: G. Heldmaier     

A comparative study of digestion in North Atlantic seabirds

We present data on digestive efficiencies and gut retention times of eight North Atlantic seabird species, fed on two fish species – lesser sandeel Ammodytes marinus and whiting Merlangius merlangus – which commonly occur in the diet of wild seabirds. In an interspecific comparison, there was a positive relationship between retention time and digestive efficiency, which we suggest represents a trade-off between conflicting benefits of efficient digestion and rapid digestion. Analysis of excretion curves revealed that retention time of digesta in the stomach was more important than passage time of digesta through the intestine in determining whole gut retention time. Differences in stomach retention time of lesser sandeel and whiting explained the longer overall retention time of the latter diet. Stomach retention time and whole gut retention time were greater in species with relatively large stomachs, while intestine passage time was correlated with relative intestine length. Species which typically eat a wide range of food types, including low quality items, tended to have slow and efficient digestion and heavy stomachs, whereas species which specialise on readily digestible and energy dense food types had the opposite digestion strategy.     

Retention of solutes and different-sized particles in the digestive tract of the ostrich (Struthio camelus massaicus), and a comparison with mammals and reptiles

Ostriches (Struthio camelus) achieve digesta retention times, digesta particle size reduction and digestibilities equal to similar-sized herbivorous mammals, in contrast to some other avian herbivores. The sequence of digestive processes in their gastrointestinal tract, however, is still unexplored. Using two groups of four ostriches (mean body mass 75.1 ± 17.3 kg) kept on fresh alfalfa, we tested the effect of two intake levels (17 and 42 g dry matter kg(-0.75)d(-1)) on the mean retention time (MRT) of a solute and three different-sized (2, 10, 20 mm) particle markers, mean faecal particle size (MPS), and digestibility. Intake level did not affect MRT, but MPS (0.74 vs. 1.52 mm) and dry matter digestibility (81 vs. 78%). The solute marker (MRT 22-26 h) was excreted faster than the particle markers; there was no difference in the MRT of 10 and 20 mm particles (MRT 28-32 h), but 2mm particles were retained longer (MRT 39-40 h). Because the solute marker was not selectively retained, and wet-sieving of gut contents of slaughtered animals did not indicate smaller particles in the caeca, the long MRT of small particles is interpreted as intermittent excretion from the gizzard, potentially due to entrapment in small grit. The marker excretion pattern also showed intermittent peaks for all markers in five of the animals, which indicates non-continuous outflow from the gizzard. When adding our data to literature data on avian herbivores, a dichotomy is evident, with ostrich and hoatzin (Opisthocomus hoazin) displaying long MRTs, high digestibilities, and gut capacities similar to mammalian herbivores, and other avian herbivores such as grouse, geese or emus with shorter MRTs, lower fibre digestibilities and lower gut capacities. In the available data for all avian herbivores where food intake and MRTs were measured, this dichotomy and food intake level, but not body mass, was related to MRT, adding to the evidence that body mass itself may not be sole major determinant of digestive physiology. The most striking difference between mammalian and avian herbivores from the literature is the fundamentally lower methane production measured in the very few studies in birds including ostriches, which appears to be at the level of reptiles, in spite of general food intake levels of a magnitude as in mammals. Further studies in ostriches and other avian herbivores are required to understand the differences in digestive mechanisms between avian and mammalian herbivores.     

Effects of acclimation and acute temperature change on specific dynamic action and gastric processing in the green shore crab, Carcinus maenas

The effects of temperature acclimation and acute temperature change were investigated in postprandial green shore crabs, Carcinus maenas. Oxygen uptake, gut contractions and transit rates and digestive efficiencies were measured for crabs acclimated to either 10 °C or 20 °C and subsequently exposed to treatment temperatures of 5, 15, or 25 °C. Temperature acclimation resulted in a partial metabolic compensation in unfed crabs, with higher oxygen uptake rates measured for the 10 °C acclimated group exposed to acute test temperatures. The Q₁₀ values were higher than normal, probably because the acute temperature change prevented crabs from fully adjusting to the new temperature. Both the acclimation and treatment temperature altered the characteristics of the specific dynamic action (SDA). The duration of the response was longer for 20 °C acclimated crabs and was inversely related to the treatment temperature. The scope (peak oxygen consumption) was also higher for 20 °C acclimated crabs with a trend towards an inverse relationship with treatment temperature. Since the overall SDA (energy expenditure) is a function of both duration and scope, it was also higher for 20 °C acclimated crabs, with the highest value measured at the treatment temperature of 15 °C. The decline in total SDA after acute exposure to 5 and 25 °C suggests that both cold stress and limitations to oxygen supply at the temperature extremes could be affecting the SDA response. The contractions of the pyloric sac of the foregut region function to propel digesta through the gut, and contraction rates increased with increasing treatment temperature. This translated into faster transit rates with increasing treatment temperatures. Although pyloric sac contractions were higher for 20 °C acclimated crabs, temperature acclimation had no effect on transit rates. This suggests that a threshold level in pyloric sac contraction rates needs to be reached before it manifests itself on transit rates. Although there was a correlation between faster transit times and the shorter duration of the SDA response with increasing treatment temperature, transit rates do not make a good proxy for calculating the SDA characteristics. The digestive efficiency showed a trend towards a decreasing efficiency with increasing treatment temperature; the slower transit rates at the lower treatment temperatures allowing for more efficient nutrient absorption. Even though metabolic rates of 10 °C acclimated crabs were higher, there was no effect of acclimation temperature on digestive efficiency. This probably occurred because intracellular enzymes and digestive enzymes are modulated through different control pathways. These results give an insight into the metabolic and digestive physiology of Carcinus maenas as it makes feeding excursions between the subtidal and intertidal zones.     

No distinct stratification of ingesta particles and no distinct moisture gradient in the fore-stomach of non-ruminants: The wallaby,peccary, hippopotamus,and sloth

Herbivores that digest plant material in the fore-stomach can be divided in ruminants and non-ruminants. This study describes the distribution of feed particles (and inorganic material) and dry matter (DM) in the digestive tract of non-ruminant foregut fermenters. Results from passage trials led us to hypothesize that specific particle-sorting mechanisms, as observed in ruminants, are unlikely in non-ruminants. Therefore, no systematic particle size distribution effects (indicative of a sorting mechanism) should be evident in the fore-stomachs of these animals, but differences in fluid and particle retention suggest that differences in fluid concentration (measured as DM) could occur in the foregut of macropods and hippos. The gut content of eleven Bennett's wallabies (Macropus rufogriseus), six collared peccaries (Pecari tajacu), three pygmy hippos (Hexaprotodon liberiensis), two common hippos (Hippopotamus amphibius) and one two-toed sloth (Choloepus didactylus) were analyzed with an emphasis on the fore-stomach. The ventral and dorsal regions in sacciform compartments, and peripheral and central regions in tubular compartments, were examined. Results were not uniform across the species studied. A potential sedimentation mechanism was observed firstly by the accumulation of sand in the fore-stomach of the peccary and sloth, and secondly by the lower DM content in peripheral versus central and ventral versus dorsal regions of the fore-stomach of the wallabies and common hippos, respectively. However, pair-comparisons for different gut regions of wallabies and peccaries yielded no differences in mean particle size between fore-stomach regions. To conclude, some digesta fractionation does occur in the fore-stomach of the studied groups of non-ruminants, but not in a uniform manner, which in turn is in accordance with morphological dissimilarities of their respective foregut structures. The absence of systematic fractionation effects in non-ruminant foregut fermenters emphasizes the innovative character of the sorting mechanism in ruminants.     

Longer guts and higher food quality increase energy intake in migratory swans

1. Within the broad field of optimal foraging, it is increasingly acknowledged that animals often face digestive constraints rather than constraints on rates of food collection. This therefore calls for a formalization of how animals could optimize food absorption rates. 2. Here we generate predictions from a simple graphical optimal digestion model for foragers that aim to maximize their (true) metabolizable food intake over total time (i.e. including nonforaging bouts) under a digestive constraint. 3. The model predicts that such foragers should maintain a constant food retention time, even if gut length or food quality changes. For phenotypically flexible foragers, which are able to change the size of their digestive machinery, this means that an increase in gut length should go hand in hand with an increase in gross intake rate. It also means that better quality food should be digested more efficiently. 4. These latter two predictions are tested in a large avian long-distance migrant, the Bewick's swan (Cygnus columbianus bewickii), feeding on grasslands in its Dutch wintering quarters. 5. Throughout winter, free-ranging Bewick's swans, growing a longer gut and experiencing improved food quality, increased their gross intake rate (i.e. bite rate) and showed a higher digestive efficiency. These responses were in accordance with the model and suggest maintenance of a constant food retention time. 6. These changes doubled the birds' absorption rate. Had only food quality changed (and not gut length), then absorption rate would have increased by only 67%; absorption rate would have increased by only 17% had only gut length changed (and not food quality). 7. The prediction that gross intake rate should go up with gut length parallels the mechanism included in some proximate models of foraging that feeding motivation scales inversely to gut fullness. We plea for a tighter integration between ultimate and proximate foraging models.     

Digestive performance and selective digesta retention in the long-nosed bandicoot,Perameles nasuta,a small omnivorous marsupial

Bandicoots are opportunistic omnivores that feed on invertebrates, fungi and both epigeal and hypogeal plant parts. We examined the performance of the digestive tract of the long-nosed bandicoot (Perameles nasuta) in terms of intake and total digestibility, patterns of excretion of inert digesta markers, and likely sites of digesta retention, on two diets designed to mimic part of their natural plant and insect diets. On the insect diet (mealworm larvae), bandicoots virtually maintained body mass at a digestible energy intake of 511 kJ · kg^-0.75 · day^-1 and were in strongly positive nitrogen balance. In contrast, on the plant diet (shredded sweet potato), bandicoots ate only one-third as much digestible energy, lost 7% body mass, and were in negative nitrogen balance. Mean retention times of two particle markers on the plant diet (27.5 and 27.0 h) were more than double those on the insect diet (12.4 and 11.2 h), and on both diets the mean retention time of the fluid digesta marker was greater than those of the particle markers, indicating consistent selective retention of fluid digesta in the gut. It was seen radiographically than in mealwormfed bandicoots major sites of digesta retention were the distal colon and rectum, whereas in the sweet potato-fed animals the caecum and proximal colon were principal sites. It was concluded that retention of plant material in the caecum and proximal colon (the main sites of microbial digestion) and the preferential retention of fluid digesta (together with bacteria and small feed particles) in the caecum were important factors in the ability of bandicoots to switch between insect and plant foods, depending on relative availabilities, and thus to exploit nutritionally unpredictable environments.Abbreviations ADF acid-detergent fibre - bm body mass - Co-ED-TA cobalt-ethylenediaminetetra-acetic acid - CWC cell wall constituents - DE digestable energy - dm dry matter - EUN endogenous urinary nitrogen - ICP inductively-coupled plasma atomic emission spectroscopy - MFN metabolic faecal nitrogen - MRT mean retention time - NDF neutral-detergent fibre - ww wet weight     

Digesta Passage,Digestibility and Behavior in Captive Gorillas Under Two Dietary Regimens

Gorilla adaptation has been debated in recent years given the wide variation among diets of gorillas in different habitats. Gorillas are the largest of living primates, have large colons and should be capable of processing tough foods. Preliminary captive studies have suggested that they may well have long average gut retention times relative to smaller hominoids, which should facilitate digestive efficiency in their wild counterparts. Indeed, wild gorillas consume large amounts of fibrous foods as staples or fall-back foods across their range, in response to habitat-related or seasonal changes in fruit availability. Fluctuations in diet might be matched by changes in digesta passage and digestibility, with possible selective retention of harder to digest items. We further studied digestive processes via chemical cobalt and chromium markers to track liquid and solids, as they passed through the guts of gorillas at the San Francisco Zoo (SFZ). In addition, we examined the effects of variation in captive diets on intake, digesta passage, digestion and behavior. The SFZ gorillas exhibited high digestibility coefficients, and gut passage was long relative to those of smaller-bodied hominoids. The results permit us to understand more fully the relationships of digestive processes to adaptation and dietary flexibility in the wild and to inform the development of dietary recommendations to improve the well-being of captive gorillas.     

Differential design for hopping in two species of wallabies.

Hindlimb musculoskeletal anatomy and steady speed over ground hopping mechanics were compared in two species of macropod marsupials, tammar wallabies and yellow-footed rock wallabies (YFRW). These two species are relatively closely related and are of similar size and general body plan, yet they inhabit different environments with presumably different musculoskeletal demands. Tammar wallabies live in relatively flat, open habitat whereas yellow-footed rock wallabies inhabit steep cliff faces. The goal of this study was to explore musculoskeletal differences between tammar wallabies and yellow-footed rock wallabies and determine how these differences influence each species' hopping mechanics. We found the cross-sectional area of the combined ankle extensor tendons of yellow-footed rock wallabies was 13% greater than that of tammar wallabies. Both species experienced similar ankle joint moments during steady-speed hopping, however due to a lower mechanical advantage at this joint, tammar wallabies produced 26% more muscle force. Thus, during moderate speed hopping, yellow-footed rock wallabies operated with 38% higher tendon safety factors, while tammar wallabies were able to store 73% more elastic strain energy (2.18 J per leg vs. 1.26 J in YFRW). This likely reflects the differing demands of the environments inhabited by these two species, where selection for non-steady locomotor performance in rocky terrain likely requires trade-offs in locomotor economy.     

Does Feeding Competition Influence Tammar Wallaby Time Allocation?

Animals may aggregate to reduce predation risk, but this potentially incurs the cost of increased competition. We studied the degree to which competition for food influenced the time tammar wallabies (Macropus eugenii) allocate to foraging and vigilance by experimentally manipulating access to food, while holding other factors constant. Groups of six wallabies were observed when they had access to either one or six non‐depleting bins of supplemental food. Food availability had no effect on the time allocated to foraging, looking or affiliative interactions, and this was true whether individuals or groups were treated as the unit of analysis. However, wallabies engaged in substantially more aggressive acts in the high‐competition treatment. These results, when combined with other findings, suggest that the moderately social tammar wallaby receives an antipredator benefit by aggregating with conspecifics which is not reduced significantly by foraging competition.     

Lactation in the tammar wallaby (Macropus eugenii). II. Intake of milk components and maternal allocation of energy

The intake of milk components (total solids, carbohydrate, protein, lipid, energy) by suckling tammar wallabies ( Macropus eugenii ), from peak lactation to independence, was measured using a double-isotope dilution technique and chemical analysis of milk samples. The time of peak intake of milk solids (day 256 of lactation or 1126 g of offspring weight) was similar to that for whole milk. Peak intake of carbohydrate occurred earlier than this (235 days) and peak intakes of protein and lipid occurred later (262 days and 266 days, respectively). Intake of gross energy peaked at 262 days and represented a maternal yield of about 207 KJ.Kg^-0.75.d^-1. This is much lower than peak lactational energy yields in most other mammals, but the duration of lactation is longer in tammars than in other mammals. Total output of energy in milk by tammar mothers was 63 MJ, and this would require an intake of about 98 MJ of metabolizable energy in food. This requirement, which is equivalent to 21 MJ/kg of maternal weight, is similar to those calculated for sheep and cattle, suggesting that there are not large differences between marsupial and placental herbivores in terms of weight-related allocation of energy to reproduction. We suggest that a strategy of minimizing the peak energetic demand of lactation may be an important adaptation for a small, primarily grazing mammal due to size-related physiological constraints on elevating herbage intake and the increasing risk from predators when grazing time is increased.     

Effects of dietary fibre on digesta passage, nutrient digestibility, and gastrointestinal tract morphology in the granivorous Mongolian gerbil (Meriones unguiculatus)

To investigate digestive tract performance in Mongolian gerbils (Meriones unguiculatus), food intake and digestibility, digesta passage rate, and gastrointestinal tract morphology were measured in captive animals fed low- or high-fibre diets. We used two markers (Co-ethylene diamine tetra-acetic acid for solutes and Cr-mordanted cell walls for particles) to measure differential passage rates of digesta fractions in order to test for the presence of a colonic separation mechanism (CSM). Although dry-matter intakes on the high-fibre diet did not differ from those on the low-fibre diet, digestibilities of dry matter, neutral-detergent fibre, acid-detergent fibre, crude protein, and crude fat were all significantly lower on the high-fibre diet. Gross energy intake on the high-fibre diet also did not differ from that on the low-fibre diet, but energy lost in faeces was much higher than on the low-fibre diet; thus, energy digestibility and digestible energy intake were significantly lower on the high-fibre diet. The lengths and dry-tissue masses of all segments of the gastrointestinal tract tended to enlarge in response to increased dietary fibre, but only the total tract contents, contents of the small intestine, and length and dry-tissue mass of the caecum increased significantly. The mean retention time (MRT) of the particle marker was significantly greater than that of the solute marker on the low-fibre but not the high-fibre diet; the solute/particle differential retention ratio was 0.62 on the low-fibre diet and 0.90 on the high-fibre diet. Thus, there was no evidence for selective retention of the solute marker on either diet. The MRT of the particle marker was significantly lower on the high-fibre diet and in the same direction as the MRT of the solute marker. These results suggest that the granivorous Mongolian gerbil has no CSM but can adjust its digestive tract capacity to accommodate greater quantities of low-quality food.     

Ingestion of crude oil: effects on digesta retention times and nutrient uptake in captive river otters

Studies following the Exxon Valdez oil spill in Prince William Sound, Alaska indicated that river otters (Lontra canadensis) from oiled regions displayed symptoms of degraded health, including reduced body weight. We examined the fate of ingested oil in the digestive tract and its effects on gut function in captive river otters. Fifteen wild-caught males were assigned to three groups, two of which were given weathered crude oil in food (i.e., control, 5 ppm day⁻¹, and 50 ppm day⁻¹) under controlled conditions at the Alaska Sealife Center. Using glass beads as non-specific digesta markers and stable isotope analysis, we determined the effects of ingested oil on retention time and nutrient uptake. Our data indicated that oil ingestion reduced marker retention time when we controlled for activity and meal size. Fecal isotope ratios suggested that absorption of lipids in the oiled otters might have been affected by reduced retention time of food. In addition, a dilution model indicated that as much as 80% of ingested oil was not absorbed in high-dose animals. Thus, while the ingestion of large quantities of weathered crude oil appears to reduce absorption of oil hydrocarbons and may alleviate systemic effects, it may concurrently affect body condition by impacting digestive function. Accepted: 18 May 2000     

Influence of two different diets on fluid and particle retention time Javan langur (Trachypithecus auratus auratus)

In captivity, langurs (foregut fermenting primates) often suffer from digestive disorders, and in particular display soft stools on diets with a high proportion of fruits, vegetables, and grain products. In this study, we tested whether the improvement in feces consistency expected after an omission of vegetables from a conventional diet was also accompanied by a change in the excretion pattern as measured by the mean retention time (MRT). Two adult Javan langurs were kept together and offered a conventional diet of pellets, browse, vegetables, or a diet consisting of pellets and browse only. MRT were measured with cobalt-EDTA as the fluid and chromium oxide as the particle marker before, during, and after the test diet, collecting the feces of both animals together. MRTs of fluids and particles were longer (47/49 h) on the test diet as compared to the usual diet (42/43 h). Feces consistency improved during the test period. The test period was marked by an increased fiber intake; however, on the conventional diet, dietary water intake exceeded the calculated water flux for these animals distinctively, due to the high proportion of vegetables. Therefore, it cannot be decided whether the increase of fiber level alone or also a reduction of the excessive water intake was responsible for the changes observed. Comparing the results of this study to other studies on ingesta retention in foregut fermenting primates, no correlation between the body mass of the animals and the MRTs measured is evident, which is in accordance with observations in other groups of foregut fermenters.