Scaling at different ontogenetic stages: Gastrointestinal tract contents of a marsupial foregut fermenter, the western grey kangaroo Macropus fuliginosus melanops.

Prominent ontogenetic changes of the gastrointestinal tract (GIT) should occur in mammals whose neonatal diet of milk differs from that of adults, and especially in herbivores (as vegetation is particularly distinct from milk), and even more so in foregut fermenters, whose forestomach only becomes functionally relevant with vegetation intake. Due to the protracted lactation in marsupials, ontogenetic differences can be particularly well investigated in this group. Here, we report body mass (BM) scaling relationships of wet GIT content mass in 28 in-pouch young (50 g to 3 kg) and 15 adult (16-70 kg) western grey kangaroos Macropus fuliginosus melanops. Apart from the small intestinal contents, in-pouch young and adults did not differ in the scaling exponents ('slope' in log-log plots) but did differ in the scaling factor ('intercept'), with an implied substantial increase in wet GIT content mass during the out-of-pouch juvenile period. In contrast to forestomach contents, caecum contents were elevated in juveniles still in the pouch, suggestive of fermentative digestion of milk and intestinal secretion residues, particularly in the caecum. The substantial increase in GIT contents (from less than 1 to 10-20% of BM) was associated mainly with the increase in forestomach contents (from 25 to 80% of total GIT contents) and a concomitant decrease in small intestine contents (from 50 to 8%), emphasizing the shifting relevance of auto-enzymatic and allo-enzymatic (microbial) digestion. There was a concomitant increase in the contents-to-tissue ratio of the fermentation chambers (forestomach and caecum), but this ratio generally did not change for the small intestine. Our study not only documents significant ontogenetic changes in digestive morpho-physiology, but also exemplifies the usefulness of intraspecific allometric analyses for quantifying these changes.

Ontogenetic scaling of the gastrointestinal tract of a marsupial foregut fermenter, the western grey kangaroo Macropus fuliginosus melanops.

As an animal grows, the relative sizes of their organs may grow proportionately or disproportionately, depending on ontogenetic changes in function. If organ growth is proportional (isometric), then the exponent of the scaling equation is 1.0. Relative decreases or increases in size result in exponents less than 1 (hypoallometric) or greater than 1 (hyperallometric). Thus, the empirical exponent can indicate potential changes in function. The entire gastrointestinal tract (GIT) of the foregut-fermenting western grey kangaroo Macropus fuliginosus melanops exhibited biphasic allometry across five orders of magnitude body mass (Mb; 52.0 g-70.5 kg). Prior to weaning at around 12 kg Mb, the entire empty GIT mass scaled with hyperallometry (Mb1.13), shifting to hypoallometry (Mb0.80) post-weaning. In addition, there were varying patterns of hyper-, hypo-, and isometric scaling for select GIT organs, with several displaying phase shifts associated with major life-history events, specifically around exit from the maternal pouch and around weaning. Mass of the kangaroo forestomach, the main fermentation site, scaled with hyperallometry (Mb1.16) before the stage of increased vegetation intake, and possibly after this stage (Mb1.12; P = 0.07), accompanied by a higher scaling factor (elevation of the curve) probably associated with more muscle for processing fibrous vegetation. The acid hindstomach mass showed hyperallometry (Mb1.15) before weaning, but hypoallometry (Mb0.58) post-weaning, presumably associated with decreasing intake of milk. Small intestine mass and length each scaled isometrically throughout ontogeny, with no discernible breakpoints at any life-history stage. The caecum and colon mass were steeply hyperallometric early in-pouch life (Mb1.59-1.66), when the young were ectothermic, hairless, and supported solely by milk. After around 295 g Mb, caecum mass remained hyperallometric (Mb1.14), possibly supporting its early development as a nidus for microbial populations to provide for secondary fermentation in this organ after the young transition from milk to vegetation.

Cellular life from the three domains and viruses are transcriptionally active in a hypersaline desert community.

Microbial communities play essential roles in the biosphere and understanding the mechanisms underlying their functional adaptations to environmental conditions is critical for predicting their behaviour. This aspect of microbiome function has not been well characterized in natural high-salt environments. To address this knowledge gap, and to build a general framework relating the genomic and transcriptomic components in a microbiome, we performed a meta-omic survey of extremophile communities inhabiting halite (salt) nodules in the Atacama Desert. We found that the major phyla of this halophilic community have different levels of total transcriptional activity, at the selected time-points, and that different metabolic pathways were activated in their transcriptomes. We report that a novel Dolichomastix alga-the only eukaryote found in this system-was the most active community member. It produced the vast majority of the community's photosynthetic transcripts despite being outnumbered by Cyanobacteria. The divergence in the transcriptional landscapes of these segregated communities, compared with the relatively stable metagenomic functional potential, suggests that microbiomes in each salt nodule undergo unique transcriptional adjustments to adapt to local conditions. We also report the characterization of several previously unknown halophilic viruses, many of which exhibit transcriptional activity indicative of host infection.

Environmental Factors Driving Spatial Heterogeneity in Desert Halophile Microbial Communities.

Spatial heterogeneity in microbial communities is observed in all natural ecosystems and can stem from both adaptations to local environmental conditions as well as stochastic processes. Extremophile microbial communities inhabiting evaporitic halite nodules (salt rocks) in the Atacama Desert, Chile, are a good model ecosystem for investigating factors leading to microbiome heterogeneity, due to their diverse taxonomic composition and the spatial segregation of individual nodules. We investigated the abiotic factors governing microbiome composition across different spatial scales, allowing for insight into the factors that govern halite colonization from regional desert-wide scales to micro-scales within individual nodules. We found that water availability and community drift account for microbiome assembly differently at different distance scales, with higher rates of cell dispersion at the smaller scales resulting in a more homogenous composition. This trend likely applies to other endoliths, and to non-desert communities, where dispersion between communities is limited. At the intra-nodule scales, a light availability gradient was most important in determining the distribution of microbial taxa despite intermixing by water displacement via capillary action.

Regulatory Noncoding Small RNAs Are Diverse and Abundant in an Extremophilic Microbial Community.

Regulatory small RNAs (sRNAs) play large-scale and essential roles in many cellular processes across all domains of life. Microbial sRNAs have been extensively studied in model organisms, but very little is known about the dynamics of sRNA synthesis and their roles in the natural environment. In this study, we discovered hundreds of intergenic (itsRNAs) and antisense (asRNAs) sRNAs expressed in an extremophilic microbial community inhabiting halite nodules (salt rocks) in the Atacama Desert. For this, we built SnapT, a new sRNA annotation pipeline that can be applied to any microbial community. We found asRNAs with expression levels negatively correlated with that of their overlapping putative target and itsRNAs that were conserved and significantly differentially expressed between 2 sampling time points. We demonstrated that we could perform target prediction and correlate expression levels between sRNAs and predicted target mRNAs at the community level. Functions of putative mRNA targets reflected the environmental challenges members of the halite communities were subjected to, including osmotic adjustments to a major rain event and competition for nutrients.IMPORTANCE Microorganisms in the natural world are found in communities, communicating and interacting with each other; therefore, it is essential that microbial regulatory mechanisms, such as gene regulation affected by small RNAs (sRNAs), be investigated at the community level. This work demonstrates that metatranscriptomic field experiments can link environmental variation with changes in RNA pools and have the potential to provide new insights into environmental sensing and responses in natural microbial communities through noncoding RNA-mediated gene regulation.

Halophilic microbial community compositional shift after a rare rainfall in the Atacama Desert.

Understanding the mechanisms underlying microbial resistance and resilience to perturbations is essential to predict the impact of climate change on Earth's ecosystems. However, the resilience and adaptation mechanisms of microbial communities to natural perturbations remain relatively unexplored, particularly in extreme environments. The response of an extremophile community inhabiting halite (salt rocks) in the Atacama Desert to a catastrophic rainfall provided the opportunity to characterize and de-convolute the temporal response of a highly specialized community to a major disturbance. With shotgun metagenomic sequencing, we investigated the halite microbiome taxonomic composition and functional potential over a 4-year longitudinal study, uncovering the dynamics of the initial response and of the recovery of the community after a rainfall event. The observed changes can be recapitulated by two general modes of community shifts-a rapid Type 1 shift and a more gradual Type 2 adjustment. In the initial response, the community entered an unstable intermediate state after stochastic niche re-colonization, resulting in broad predicted protein adaptations to increased water availability. In contrast, during recovery, the community returned to its former functional potential by a gradual shift in abundances of the newly acquired taxa. The general characterization and proposed quantitation of these two modes of community response could potentially be applied to other ecosystems, providing a theoretical framework for prediction of taxonomic and functional flux following environmental changes.

An ecophysiologically informed model of seed dispersal by orangutans: linking animal movement with gut passage across time and space.

Fauna-mediated ecosystem service provision (e.g. seed dispersal) can be difficult to quantify and predict because it is underpinned by the shifting niches of multiple interacting organisms. Such interactions are especially complex in tropical ecosystems, including endangered peat forests of Central Borneo, a biodiversity hot spot and home to the critically endangered orangutan (Pongo pygmaeus wurmbii). We combined studies of the digestive physiology of captive orangutans in Australia with detailed field studies of wild orangutans in the Natural Laboratory of Peat-Swamp Forest of Sabangau, Central Kalimantan, Indonesia. By measuring the gut transit time (TT) of indigestible seed mimics (beads) in captivity and applying this as a temporal constraint to movement data of wild orangutans, we developed a mechanistic, time-explicit spatial model to project the seed dispersal patterns by these large-bodied, arboreal frugivores. We followed seven orangutans and established home range kernels using Time Local Convex Hull (T-LoCoH) modelling. This allowed us to model individual orangutan movements and to adjust these models according to gut transit times to estimate seed dispersal kernels. Female movements were conservative (core ranges of 55 and 52 ha in the wet and dry seasons, respectively) and revisitation rates to the same location of n = 4 in each 24-h block. Male movements were more unpredictable, yielding fragmented core ranges and revisitation rates to the same location of only 1.2 times each 24 h; males also demonstrated large disjunctions where they moved rapidly over long distances and were frequently lost from view. Seed dispersal kernels were nested predictably within the core ranges of females, but not males. We used the T-LoCoH approach to analyse movement ecology, which offered a powerful tool to predict the primary deposition of seeds by orangutans, thereby providing a reliable method for making a priori predictions of seed dispersal dynamics by other frugivores in novel ecosystems.

Effect of captivity on morphology: negligible changes in external morphology mask significant changes in internal morphology.

Captive breeding programmes are increasingly relied upon for threatened species management. Changes in morphology can occur in captivity, often with unknown consequences for reintroductions. Few studies have examined the morphological changes that occur in captive animals compared with wild animals. Further, the effect of multiple generations being maintained in captivity, and the potential effects of captivity on sexual dimorphism remain poorly understood. We compared external and internal morphology of captive and wild animals using house mouse (Mus musculus) as a model species. In addition, we looked at morphology across two captive generations, and compared morphology between sexes. We found no statistically significant differences in external morphology, but after one generation in captivity there was evidence for a shift in the internal morphology of captive-reared mice; captive-reared mice (two generations bred) had lighter combined kidney and spleen masses compared with wild-caught mice. Sexual dimorphism was maintained in captivity. Our findings demonstrate that captive breeding can alter internal morphology. Given that these morphological changes may impact organismal functioning and viability following release, further investigation is warranted. If the morphological change is shown to be maladaptive, these changes would have significant implications for captive-source populations that are used for reintroduction, including reduced survivorship.

Phenotypic plasticity in the common garden snail: big guts and heavier mucus glands compete in snails faced with the dual challenge of poor diet and coarse substrate.

Phenotypic plasticity allows animals to manage environmental challenges. Studies aimed at quantifying plasticity often focus on one challenge, such as diet, and one organ system, such the gastrointestinal tract, but this approach may not adequately reflect how plasticity could buffer multiple challenges. Thus, we investigated the outcomes of a dual challenge experiment that fed land snails either a high-fibre (low quality) or a low-fibre (high quality) diet, and simultaneously exercised them daily over 1.2 m on either a smooth surface of polyvinyl chloride (PVC) or a rough sandpaper. By the end of 20 days, snails fed the poor quality diet had a longer crop and oesophagus and a heavier intestine and rectum than those offered a low-fibre diet. Additionally, high-fibre fed snails had a smaller spermoviduct and oviduct. When also exercised on sandpaper, high-fibre fed snails had a smaller digestive gland, a main energy store, than those exercised on PVC. All snails exercised on sandpaper had a heavier pedal mucus gland, used a loping gait and used less mucus than those on PVC plastic, but there was no difference in the average speed of snails on either surface, supporting the conclusion that loping is a mucus conserving gait. Notably, snails faced with both a diet and substrate challenge had a smaller kidney, which could directly effect fecundity. This demonstrates that our dual challenge approach has potential for evaluating the costs and limits of the plasticity necessary to fully appreciate the evolutionary significance of plasticity in snails and other species.

Methane production by two non-ruminant foregut-fermenting herbivores: The collared peccary (Pecari tajacu) and the pygmy hippopotamus (Hexaprotodon liberiensis).

Methane (CH4) production varies between herbivore species, but reasons for this variation remain to be elucidated. Here, we report open-circuit chamber respiration measurements of CH4 production in four specimens each of two non-ruminant mammalian herbivores with a complex forestomach but largely differing in body size, the collared peccary (Pecari tajacu, mean body mass 17kg) and the pygmy hippopotamus (Hexaprotodon liberiensis, 229kg) fed lucerne-based diets. In addition, food intake, digestibility and mean retention times were measured in the same experiments. CH4 production averaged 8 and 72L/d, 18 and 19L/kg dry matter intake, and 4.0 and 4.2% of gross energy intake for the two species, respectively. When compared with previously reported data on CH4 production in other non-ruminant and ruminant foregut-fermenting as well as hindgut-fermenting species, it is evident that neither the question whether a species is a foregut fermenter or not, or whether it ruminates or not, is of the relevance previously suggested to explain variation in CH4 production between species. Rather, differences in CH4 production between species on similar diets appear related to species-specific differences in food intake and digesta retention kinetics.

Decreasing methane yield with increasing food intake keeps daily methane emissions constant in two foregut fermenting marsupials, the western grey kangaroo and red kangaroo.

Fundamental differences in methane (CH4) production between macropods (kangaroos) and ruminants have been suggested and linked to differences in the composition of the forestomach microbiome. Using six western grey kangaroos (Macropus fuliginosus) and four red kangaroos (Macropus rufus), we measured daily absolute CH4 production in vivo as well as CH4 yield (CH4 per unit of intake of dry matter, gross energy or digestible fibre) by open-circuit respirometry. Two food intake levels were tested using a chopped lucerne hay (alfalfa) diet. Body mass-specific absolute CH4 production resembled values previously reported in wallabies and non-ruminant herbivores such as horses, and did not differ with food intake level, although there was no concomitant proportionate decrease in fibre digestibility with higher food intake. In contrast, CH4 yield decreased with increasing intake, and was intermediate between values reported for ruminants and non-ruminant herbivores. These results correspond to those in ruminants and other non-ruminant species where increased intake (and hence a shorter digesta retention in the gut) leads to a lower CH4 yield. We hypothesize that rather than harbouring a fundamentally different microbiome in their foregut, the microbiome of macropods is in a particular metabolic state more tuned towards growth (i.e. biomass production) rather than CH4 production. This is due to the short digesta retention time in macropods and the known distinct 'digesta washing' in the gut of macropods, where fluids move faster than particles and hence most likely wash out microbes from the forestomach. Although our data suggest that kangaroos only produce about 27% of the body mass-specific volume of CH4 of ruminants, it remains to be modelled with species-specific growth rates and production conditions whether or not significantly lower CH4 amounts are emitted per kg of meat in kangaroo than in beef or mutton production.

Is "cooling then freezing" a humane way to kill amphibians and reptiles?

What is the most humane way to kill amphibians and small reptiles that are used in research? Historically, such animals were often killed by cooling followed by freezing, but this method was outlawed by ethics committees because of concerns that ice-crystals may form in peripheral tissues while the animal is still conscious, putatively causing intense pain. This argument relies on assumptions about the capacity of such animals to feel pain, the thermal thresholds for tissue freezing, the temperature-dependence of nerve-impulse transmission and brain activity, and the magnitude of thermal differentials within the bodies of rapidly-cooling animals. A review of published studies casts doubt on those assumptions, and our laboratory experiments on cane toads (Rhinella marina) show that brain activity declines smoothly during freezing, with no indication of pain perception. Thus, cooling followed by freezing can offer a humane method of killing cane toads, and may be widely applicable to other ectotherms (especially, small species that are rarely active at low body temperatures). More generally, many animal-ethics regulations have little empirical basis, and research on this topic is urgently required in order to reduce animal suffering.

Long-term changes in food availability mediate the effects of temperature on growth, development and survival in striped marsh frog larvae: implications for captive breeding programmes.

Food availability and temperature are known to trigger phenotypic change, but the interactive effects between these factors are only beginning to be considered. The aim of this study was to examine the independent and interactive effects of long-term stochastic food availability and water temperature on larval survivorship, growth and development of the striped marsh frog, Limnodynastes peronii. Larval L. peronii were reared in conditions of either constant or stochastic food availability and in water at three different temperatures (18, 22 and 26°C), and effects on survival, growth and development were quantified. Over the experimental period, larval growth rate was highest and survivorship lowest at the warmest temperature. However, changes in food availability mediated the effects of temperature, with slower larval growth and higher survivorship in stochastic food availability treatments. Tadpoles in the stochastic food availability treatments did not reach metamorphosis during the experimental period, suggesting that developmental stasis may have been induced by food restriction. Overall, these results demonstrate that changes in food availability alter the effects of water temperature on survival, growth and development. From an applied perspective, understanding how environmental factors interact to cause phenotypic change may assist with amphibian conservation by improving the number of tadpoles generated in captive breeding programmes.

Fibre-induced feed sorting in King Quail (Coturnix chinensis): behavioural plasticity elicited by a physiological challenge.

We examined the effect of an abrupt change in diet fibre content on the feed intake, gastrointestinal morphology and utilisation of gastroliths by a small (ca. 40 g body mass) herbivorous bird, the King Quail (Coturnix chinensis). King Quail were acclimated for 14 days on a low-fibre (LF) pullet starter diet. Following acclimation, half the quail population was immediately switched to a 23% wood-shaving diluted high-fibre (HF) diet for a further 14 days. Contrary to expectations, we found no differences in feed intake, gut morphology or gastrolith mass between the LF- and HF-fed quail. However, when switched from the LF to HF diet, the quail commenced feed-sorting behaviours that permitted HF-fed animals to maintain body condition (mass, abdominal fat mass) without adjustments to intestinal organ sizes or gastrolith mass. Feed sorting was initiated only after exposure to the HF diet, which corresponded with an immediate reduction in food intake, suggesting that the sorting behaviour was cued by a physiological challenge associated with the HF diet. This challenge apparently induced preferential sorting behaviour and was possibly due to abrupt changes in the rate of food passage, impacting satiation or other internal cues.

Applications and implications of ecological energetics.

The ecological processes that are crucial to an animal's growth, survival, and reproductive fitness have energetic costs. The imperative for an animal to meet these costs within the energetic constraints of the environment drives many aspects of animal ecology and evolution, yet has largely been overlooked in traditional ecological paradigms. The field of 'ecological energetics' is bringing comparative physiology out of the laboratory and, for the first time, is becoming broadly accessible to field ecologists addressing real-world questions at many spatial and temporal scales. In an era of unprecedented global environmental challenges, ecological energetics opens up the tantalising prospect of a more predictive, mechanistic understanding of the drivers of threatened species decline, delivering process-based modelling approaches to natural resource management.

Is gastrointestinal plasticity in king quail (Coturnix chinensis) elicited by diet-fibre or diet-energy dilution?

Phenotypic plasticity of organ size allows some animals to manage fluctuations of resource quality or availability. Here, we examined the phenotypic plasticity of the gastrointestinal tract of king quail (Coturnix chinensis) in a diet-fibre manipulation study. Quail were offered either a control low-fibre (high-quality) food (8.5% neutral-detergent fibre; NDF), or one of two experimental diets of higher fibre contents of 16% NDF (i.e. low-quality food). To examine whether phenotypic plasticity of organ size was associated with the fibre content per se, or as a consequence of diluting the diet energy contents by adding fibre, one of the high-fibre feeds was 'balanced' with additional energy to match that of the low-fibre control diet. Total empty dry mass of the gastrointestinal tract was significantly heavier among birds offered the unbalanced high-fibre diet as compared with those offered the control diet, with birds offered the fibrous but energy-balanced diet having guts of intermediate size. The heavier entire-gut mass (dry) of quail offered the unbalanced high-fibre diet was associated mainly with these birds having significantly heavier gizzards. Notably, the larger gizzard in the birds offered the unbalanced high-fibre diet was associated with marked increases in their metabolisability (digestion) of diet fibre. Our findings suggest that the available energy in the diet may be more important for eliciting phenotypic changes in the gut of these herbivorous birds rather than simple physical effects of diet fibre on feed intakes or on muscular compensation to fibrous ingesta.

Physiology in conservation translocations.

Conservation translocations aim to restore species to their indigenous ranges, protect populations from threats and/or reinstate ecosystem functions. They are particularly important for the conservation and management of rare and threatened species. Despite tremendous efforts and advancement in recent years, animal conservation translocations generally have variable success, and the reasons for this are often uncertain. We suggest that when little is known about the physiology and wellbeing of individuals either before or after release, it will be difficult to determine their likelihood of survival, and this could limit advancements in the science of translocations for conservation. In this regard, we argue that physiology offers novel approaches that could substantially improve translocations and associated practices. As a discipline, it is apparent that physiology may be undervalued, perhaps because of the invasive nature of some physiological measurement techniques (e.g. sampling body fluids, surgical implantation). We examined 232 publications that dealt with translocations of terrestrial vertebrates and aquatic mammals and, defining 'success' as high or low, determined how many of these studies explicitly incorporated physiological aspects into their protocols and monitoring. From this review, it is apparent that physiological evaluation before and after animal releases could progress and improve translocation/reintroduction successes. We propose a suite of physiological measures, in addition to animal health indices, for assisting conservation translocations over the short term and also for longer term post-release monitoring. Perhaps most importantly, we argue that the incorporation of physiological assessments of animals at all stages of translocation can have important welfare implications by helping to reduce the total number of animals used. Physiological indicators can also help to refine conservation translocation methods. These approaches fall under a new paradigm that we term 'translocation physiology' and represent an important sub-discipline within conservation physiology generally.

Energy in-equivalence in Australian marsupials: evidence for disruption of the continent's mammal assemblage, or are rules meant to be broken?

The energy equivalence rule (EER) is a macroecological hypothesis that posits that total population energy use (PEU) should be independent of species body mass, because population densities and energy metabolisms scale with body mass in a directly inverse manner. However, evidence supporting the EER is equivocal, and the use of basal metabolic rate (BMR) in such studies has been questioned; ecologically-relevant indices like field metabolic rate (FMR) are probably more appropriate. In this regard, Australian marsupials present a novel test for the EER because, unlike eutherians, marsupial BMRs and FMRs scale differently with body mass. Based on either FMR or BMR, Australian marsupial PEU did not obey an EER, and scaled positively with body mass based on ordinary least squares (OLS) regressions. Importantly, the scaling of marsupial population density with body mass had a slope of -0.37, significantly shallower than the expected slope of -0.75, and not directly inverse of body-mass scaling exponents for BMR (0.72) or FMR (0.62). The findings suggest that the EER may not be a causal, universal rule, or that for reasons not yet clear, it is not operating for Australia's unique native fauna.

Caecal abnormality in a layer hen (Gallus gallus forma domestica) without deficits in digestive performance or egg productivity.

We report a case of a layer hen (Gallus gallus forma domestica) with deviation in the morphology of the caecum, and unique opportunity to investigate the digestive performance of the animal compared with normal hens. In a study investigating digestive and reproductive performance, an atypical caecal arrangement was found in a hen that was unremarkable in regards to body mass, digestive performance and egg productivity in comparison to other hens fed a similar diet. Examination of the gastrointestinal tract revealed a singular tubular outgrowth from the ileo-caecal junction, rather than the typical paired outgrowths. The single caecal duct bifurcated into two separate blind-ended sacs. Similar caecal deviations have been described in adult and juveniles, but no indications of animal performance were reported in these cases. We conclude that if the presence of an abnormal caecal arrangement reduces digestive abilities they were not obvious, and some compensatory mechanism/s may exist. Alternatively, the abnormal caecal arrangement of our hen might function adequately, such that no compensation in feed intakes or reduced egg productivity was required or observed.

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.