How do food passage rate and assimilation differ between herbivorous lizards and nonruminant mammals?

Intersexual patterns of the digestive tract and body size are opposed in a large bird

The appropriate structure of the digestive tract is crucial for individual adaptation to ecological conditions. In birds, the length of the small intestine, responsible for food absorption, is generally believed to be positively correlated with body size. In this study, we investigated the variation in small intestine length in the White Stork (Ciconia ciconia), a monomorphic species without visible sexual dimorphism, but characterized by differing parental efforts, which can be reflected by the small intestine lengths between the sexes. We examined the relationship between small intestine length and body size within the sexes. Our findings show that male White Storks have significantly shorter small intestines than females, despite having larger body sizes than the latter. Furthermore, we found a significant relationship between body size and small intestine length, but it was of a different nature in the two sexes. Males exhibited a previously unreported phenomenon, whereby increasing body size was associated with shortening small intestines, whereas females exhibited the opposite pattern. These novel findings shed light on the anatomical adaptations of the digestive tract in birds.

A Critical Review on the Dosing and Safety of Antifungals Used in Exotic Avian and Reptile Species

Antifungals are used in exotic avian and reptile species for the treatment of fungal diseases. Dose extrapolations across species are common due to lack of species-specific pharmacological data. This may not be ideal because interspecies physiological differences may result in subtherapeutic dosing or toxicity. This critical review aims to collate existing pharmacological data to identify antifungals with the most evidence to support their safe and effective use. In the process, significant trends and gaps are also identified and discussed. An extensive search was conducted on PubMed and JSTOR, and relevant data were critically appraised. Itraconazole or voriconazole showed promising results in Japanese quails, racing pigeons and inland bearded dragons for the treatment of aspergillosis and CANV-related infections. Voriconazole neurotoxicity manifested as seizures in multiple penguins, but as lethargy or torticollis in cottonmouths. Itraconazole toxicity was predominantly hepatotoxicity, observed as liver abnormalities in inland bearded dragons and a Parson's chameleon. Differences in formulations of itraconazole affected various absorption parameters. Non-linearities in voriconazole due to saturable metabolism and autoinduction showed opposing effects on clearance, especially in multiple-dosing regimens. These differences in pharmacokinetic parameters across species resulted in varying elimination half-lives. Terbinafine has been used in dermatomycoses, especially in reptiles, due to its keratinophilic nature, and no significant adverse events were observed. The use of fluconazole has declined due to resistance or its narrow spectrum of activity.

Reptilian digestive efficiency: Past, present, and future

Digestion and assimilation of nutrients and energy is central to survival. At its most basic level, investigations of digestion in animals must examine digestive efficiency, or how much of a given meal (i.e., energy) or a specific nutrient an organism can acquire from its food. There are many studies examining this in reptiles, but there is large variation in methodology, and thus, in the conclusions drawn from the gathered data. The majority rely on ratio-based analyses that can jeopardize the reliability of their findings. Therefore, we reviewed the literature to identify common themes in the digestive efficiency data on reptiles. Due to the sheer number of available studies, we largely focused on lizards, but included data on all reptilian groups. As an example of what the current data can reveal, we performed a meta-analysis of digestive efficiency in lizards as a function of temperature using regression analyses. We detected a weak positive trend of soluble carbohydrate digestibility as a function of temperature, but no similar trend in broad-scale digestive efficiency, and propose that these patterns be reevaluated with non-ratio data. We conclude with calls to end conducting analyses on ratios and instead employ covariate methods, for more studies of reptilian digestive efficiency and related processes using consistent methodology, more representation of each population (e.g., many studies focus on males only), and more detailed studies examining the effects of temperature on digestion (since the current data are inconclusive).

Using eDNA metabarcoding to understand the effect of fire on the diet of small mammals in a woodland ecosystem

Food acquisition is a fundamental process that drives animal distribution and abundance, influencing how species respond to changing environments. Disturbances such as fire create significant shifts in available dietary resources, yet, for many species, we lack basic information about what they eat, let alone how they respond to a changing resource base. In order to create effective management strategies, faunal conservation in flammable landscapes requires a greater understanding of what animals eat and how this change following a fire. What animals eat in postfire environments has received little attention due to the time-consuming methodologies and low-resolution identification of food taxa. Recently, molecular techniques have been developed to identify food DNA in scats, making it possible to identify animal diets with enhanced resolution. The primary aim of this study was to utilize eDNA metabarcoding to obtain an improved understanding of the diet of three native Australian small mammal species: yellow-footed antechinus (Antechinus flavipes), heath mouse (Pseudomys shortridgei), and bush rat (Rattus fuscipes). Specifically, we sought to understand the difference in the overall diet of the three species and how diet changed over time after fire. Yellow-footed antechinus diets mostly consisted of moths, and plants belonging to myrtles and legume families while bush rats consumed legumes, myrtles, rushes, and beetles. Heath mouse diet was dominated by rushes. All three species shifted their diets over time after fire, with most pronounced shifts in the bush rats and least for heath mice. Identifying critical food resources for native animals will allow conservation managers to consider the effect of fire management actions on these resources and help conserve the species that use them.

Factors shaping the abundance and diversity of the gut archaeome across the animal kingdom

Archaea are common constituents of the gut microbiome of humans, ruminants, and termites but little is known about their diversity and abundance in other animals. Here, we analyse sequencing and quantification data of archaeal and bacterial 16S rRNA genes from 250 species of animals covering a large taxonomic spectrum. We detect the presence of archaea in 175 animal species belonging to invertebrates, fish, amphibians, birds, reptiles and mammals. We identify five dominant gut lineages, corresponding to Methanobrevibacter, Methanosphaera, Methanocorpusculum, Methanimicrococcus and "Ca. Methanomethylophilaceae". Some archaeal clades, notably within Methanobrevibacter, are associated to certain hosts, suggesting specific adaptations. The non-methanogenic lineage Nitrososphaeraceae (Thaumarchaeota) is frequently present in animal samples, although at low abundance, but may have also adapted to the gut environment. Host phylogeny, diet type, fibre content, and intestinal tract physiology are major drivers of the diversity and abundance of the archaeome in mammals. The overall abundance of archaea is more influenced by these factors than that of bacteria. Methanogens reducing methyl-compounds with H2 can represent an important fraction of the overall methanogens in many animals. Together with CO2-reducing methanogens, they are influenced by diet and composition of gut bacteria. Our results provide key elements toward our understanding of the ecology of archaea in the gut, an emerging and important field of investigation.

Advance social information allows red crossbills (Loxia curvirostra) to better conserve body mass and intestinal mass during food stress

Animals prepare for fluctuations in resources through advance storage of energy, planned reduction in energy costs or by moving elsewhere. Unpredictable fluctuations in food, however, may be particularly challenging if animals cannot avoid negative impacts on body condition. Social information may help animals to cope with unpredictable resources if cues from individuals with low foraging success give advance warning about deteriorating conditions. This study investigates the impact of social information on behaviour and physiology of food-restricted captive red crossbills (Loxia curvirostra). Birds were restricted to two short feeding periods per day to simulate a decline in resources and were given social information from food-restricted neighbours either before (i.e. predictive) or during (i.e. parallel) the food-restriction period. Focal birds better conserved body mass during food restriction if social information was predictive of the decline in resources. Crossbills with predictive information ate more food, had larger intestinal mass and better conserved pectoral muscle size at the end of the restriction period compared to those with parallel social information. These data suggest that birds can use social information to alter behavioural and physiological responses during food shortage in ways that may confer an adaptive advantage for survival.

Effect of slug mycophagy on Tuber aestivum spores

Truffles in the genus Tuber produce subterranean fruiting bodies that are not able to actively discharge their spores in the environment. For this reason, truffles depend on mycophagous animals for reproduction. Fungus consumption (mycophagy) is a behaviour typical of both vertebrates and invertebrates. Mammals, especially rodents, are the most studied group of mycophagists and have been found to consume a great variety of fungi. Among invertebrates, mycophagy is documented in arthropods, but rarely in molluscs. In our study we assessed the effect on the morphology and mycorrhizal colonization of Tuber aestivum spores after passage through the gut of slugs (Deroceras invadens) and, for comparison, of a house mouse (Mus musculus). Light, scanning electron and atomic force microscopy revealed that the digestion, especially by slugs, freed spores from the asci and modified their morphology. These are believed to be the reasons why we observed an improvement in oak mycorrhization with the slug and rodent ingested spores in comparison to a fresh spore inoculation. We also demonstrated by molecular barcoding that slugs' guts sampled on a Tuber melanosporum truffle ground contain spores from this species and Tuber brumale, further suggesting that some invertebrates are efficient Tuber spore dispersers.

Less need for differentiation? Intestinal length of reptiles as compared to mammals

Although relationships between intestinal morphology between trophic groups in reptiles are widely assumed and represent a cornerstone of ecomorphological narratives, few comparative approaches actually tested this hypothesis on a larger scale. We collected data on lengths of intestinal sections of 205 reptile species for which either body mass (BM), snout-vent-length (SVL) or carapax length (CL) was recorded, transforming SVL or CL into BM if the latter was not given, and analyzed scaling patterns with BM and SVL, accounting for phylogeny, comparing three trophic guilds (faunivores, omnivores, herbivores), and comparing with a mammal dataset. Length-BM relationships in reptiles were stronger for the small than the large intestine, suggesting that for the latter, additional factors might be relevant. Adding trophic level did not consistently improve model fit; only when controlling for phylogeny, models indicated a longer large intestine in herbivores, due to a corresponding pattern in lizards. Trophic level effects were highly susceptible to sample sizes, and not considered strong. Models that linked BM to intestine length had better support than models using SVL, due to the deviating body shape of snakes. At comparable BM, reptiles had shorter intestines than mammals. While the latter finding corresponds to findings of lower tissue masses for the digestive tract and other organs in reptiles as well as our understanding of differences in energetic requirements between the classes, they raise the hitherto unanswered question what it is that reptiles of similar BM have more than mammals. A lesser effect of trophic level on intestine lengths in reptiles compared to mammals may stem from lesser selective pressures on differentiation between trophic guilds, related to the generally lower food intake and different movement patterns of reptiles, which may not similarly escalate evolutionary arms races tuned to optimal agility as between mammalian predators and prey.

Flexibility of feeding movements in pigs: effects of changes in food toughness and stiffness on the timing of jaw movements

In mammals, chewing movements can be modified, or flexible, in response to changes in food properties. Variability between and within food in the temporal characteristics of chewing movements can impact chewing frequency and rhythmicity, which in turn may affect food breakdown, energy expenditure and tooth wear. Here, we compared total chewing cycle duration and intra-cycle phase durations in pigs chewing on three foods varying in toughness and stiffness: apples (low toughness, low stiffness), carrots (high toughness, low stiffness), and almonds (high toughness, high stiffness). We also determined whether within-food variability in timing parameters is modified in response to changes in food properties. X-ray Reconstruction Of Moving Morphology (XROMM) demonstrates that the timing of jaw movements are flexible in response to changes in food properties. Within each food, pigs also exhibited flexibility in their ability to vary cycle parameters. The timing of jaw movements during processing of high-toughness foods is more variable, potentially decreasing chewing rhythmicity. In contrast, low-toughness foods result in jaw movements that are more stereotyped in their timing parameters. In addition, the duration of tooth-food-tooth contact is more variable during the processing of low-stiffness foods compared with tough or stiff foods. Increased toughness is suggested to alter the timing of the movements impacting food fracture whereas increased stiffness may require a more cautious control of jaw movements. This study emphasizes that flexibility in biological movements in response to changes in conditions may not only be observed in timing but also in the variability of their timing within each condition.

Physiological and microbial adjustments to diet quality permit facultative herbivory in an omnivorous lizard

While herbivory is a common feeding strategy in a number of vertebrate classes, less than 4% of squamate reptiles feed primarily on plant material. It has been hypothesized that physiological or microbial limitations may constrain the evolution of herbivory in lizards. Herbivorous lizards exhibit adaptations in digestive morphology and function that allow them to better assimilate plant material. However, it is unknown whether these traits are fixed or perhaps phenotypically flexible as a result of diet. Here, we maintained a naturally omnivorous lizard, Liolaemus ruibali, on a mixed diet of 50% insects and 50% plant material, or a plant-rich diet of 90% plant material. We compared parameters of digestive performance, gut morphology and function, and gut microbial community structure between the two groups. We found that lizards fed the plant-rich diet maintained nitrogen balance and exhibited low minimum nitrogen requirements. Additionally, lizards fed the plant-rich diet exhibited significantly longer small intestines and larger hindguts, demonstrating that gut morphology is phenotypically flexible. Lizards fed the plant-rich diet harbored small intestinal communities that were more diverse and enriched in Melainabacteria and Oscillospira compared with mixed diet-fed lizards. Additionally, the relative abundance of sulfate-reducing bacteria in the small intestine significantly correlated with whole-animal fiber digestibility. Thus, we suggest that physiological and microbial limitations do not sensu stricto constrain the evolution of herbivory in lizards. Rather, ecological context and fitness consequences may be more important in driving the evolution of this feeding strategy.

Herbivory and body size: allometries of diet quality and gastrointestinal physiology, and implications for herbivore ecology and dinosaur gigantism

Digestive physiology has played a prominent role in explanations for terrestrial herbivore body size evolution and size-driven diversification and niche differentiation. This is based on the association of increasing body mass (BM) with diets of lower quality, and with putative mechanisms by which a higher BM could translate into a higher digestive efficiency. Such concepts, however, often do not match empirical data. Here, we review concepts and data on terrestrial herbivore BM, diet quality, digestive physiology and metabolism, and in doing so give examples for problems in using allometric analyses and extrapolations. A digestive advantage of larger BM is not corroborated by conceptual or empirical approaches. We suggest that explanatory models should shift from physiological to ecological scenarios based on the association of forage quality and biomass availability, and the association between BM and feeding selectivity. These associations mostly (but not exclusively) allow large herbivores to use low quality forage only, whereas they allow small herbivores the use of any forage they can physically manage. Examples of small herbivores able to subsist on lower quality diets are rare but exist. We speculate that this could be explained by evolutionary adaptations to the ecological opportunity of selective feeding in smaller animals, rather than by a physiologic or metabolic necessity linked to BM. For gigantic herbivores such as sauropod dinosaurs, other factors than digestive physiology appear more promising candidates to explain evolutionary drives towards extreme BM.

Methodological Considerations in the Analysis of Fecal Glucocorticoid Metabolites in Tufted Capuchins (Cebus apella)

Analysis of fecal glucocorticoid (GC) metabolites has recently become the standard method to monitor adrenocortical activity in primates noninvasively. However, given variation in the production, metabolism, and excretion of GCs across species and even between sexes, there are no standard methods that are universally applicable. In particular, it is important to validate assays intended to measure GC production, test extraction and storage procedures, and consider the time course of GC metabolite excretion relative to the production and circulation of the native hormones. This study examines these four methodological aspects of fecal GC metabolite analysis in tufted capuchins (Cebus apella). Specifically, we conducted an adrenocorticotrophic hormone (ACTH) challenge on one male and one female capuchin to test the validity of four GC enzyme immunoassays (EIAs) and document the time course characterizing GC metabolite excretion in this species. In addition, we compare a common field-friendly technique for extracting fecal GC metabolites to an established laboratory extraction methodology and test for effects of storing "field extracts" for up to 1 yr. Results suggest that a corticosterone EIA is most sensitive to changes in GC production, provides reliable measures when extracted according to the field method, and measures GC metabolites which remain highly stable after even 12 mo of storage. Further, the time course of GC metabolite excretion is shorter than that described yet for any primate taxa. These results provide guidelines for studies of GCs in tufted capuchins, and underscore the importance of validating methods for fecal hormone analysis for each species of interest.

Giant lizards occupied herbivorous mammalian ecospace during the Paleogene greenhouse in Southeast Asia

Mammals dominate modern terrestrial herbivore ecosystems, whereas extant herbivorous reptiles are limited in diversity and body size. The evolution of reptile herbivory and its relationship to mammalian diversification is poorly understood with respect to climate and the roles of predation pressure and competition for food resources. Here, we describe a giant fossil acrodontan lizard recovered with a diverse mammal assemblage from the late middle Eocene Pondaung Formation of Myanmar, which provides a historical test of factors controlling body size in herbivorous squamates. We infer a predominately herbivorous feeding ecology for the new acrodontan based on dental anatomy, phylogenetic relationships and body size. Ranking body masses for Pondaung Formation vertebrates indicates that the lizard occupied a size niche among the larger herbivores and was larger than most carnivorous mammals. Paleotemperature estimates of Pondaung Formation environments based on the body size of the new lizard are approximately 2–5°C higher than modern. These results indicate that competitive exclusion and predation by mammals did not restrict body size evolution in these herbivorous squamates, and elevated temperatures relative to modern climates during the Paleogene greenhouse may have resulted in the evolution of gigantism through elevated poikilothermic metabolic rates and in response to increases in floral productivity.

Paracellular absorption is relatively low in the herbivorous Egyptian spiny-tailed lizard, Uromastyx aegyptia

Absorption of small water-soluble nutrients in vertebrate intestines occurs both by specific, mediated transport and by non-specific, passive, paracellular transport. Although it is apparent that paracellular absorption represents a significant route for nutrient absorption in many birds and mammals, especially small, flying species, its importance in ectothermic vertebrates has not previously been explored. Therefore, we measured fractional absorption (ƒ) and absorption rate of three paracellular probes (arabinose, L-rhamnose, cellobiose) and of 3-O-methyl D-glucose (absorbed by both mediated and paracellular pathways) by the large herbivorous lizard, Uromastyx aegyptia, to explore the relative importance of paracellular and mediated transport in an ectothermic, terrestrial vertebrate. Fractional absorption of 3-O-methyl D-glucose was high (ƒ = 0.73±0.04) and similar to other vertebrates; ƒ of the paracellular probes was relatively low (arabinose ƒ = 0.31±0.03, L-rhamnose ƒ = 0.19±0.02, and cellobiose ƒ = 0.14±0.02), and decreased with molecular mass, a pattern consistent with other vertebrates. Paracellular absorption accounted for approximately 24% of total 3-O-methyl D-glucose uptake, indicating low reliance on this pathway for these herbivorous lizards, a pattern similar to that found in other terrestrial vertebrates, and different from small flying endotherms (both birds and bats).

Experience matters: prior exposure to plant toxins enhances diversity of gut microbes in herbivores

For decades, ecologists have hypothesised that exposure to plant secondary compounds (PSCs) modifies herbivore-associated microbial community composition. This notion has not been critically evaluated in wild mammalian herbivores on evolutionary timescales. We investigated responses of the microbial communities of two woodrat species (Neotoma bryanti and N. lepida). For each species, we compared experienced populations that independently converged to feed on the same toxic plant (creosote bush, Larrea tridentata) to naïve populations with no exposure to creosote toxins. The addition of dietary PSCs significantly altered gut microbial community structure, and the response was dependent on previous experience. Microbial diversity and relative abundances of several dominant phyla increased in experienced woodrats in response to PSCs; however, opposite effects were observed in naïve woodrats. These differential responses were convergent in experienced populations of both species. We hypothesise that adaptation of the foregut microbiota to creosote PSCs in experienced woodrats drives this differential response.

Seasonal variations of body mass, thermogenesis and digestive tract morphology in Apodemus chevrieri in Hengduan mountain region

Seasonal changes in an animal’s morphology, physiology, and behavior are considered to be an adaptive strategy for survival and reproductive success. We hypothesize that Apodemus chevrieri will change their thermogenesis seasonally and serum leptin will change with body mass or body fat mass. Seasonal variations in body mass (BM), basal metabolic rate (BMR), nonshivering thermogenesis (NST), digestive tract morphology, serum leptin and uncoupling protein 1 (UCP1) were measured in wild-trapped A. chevrieri in Hengduan mountain region. The results showed that the body weight of A. chevrieri was lowest in winter and highest in summer. Decreased BM in the winter was accompanied by increased energy intake and enhanced NST and UCP1 as well as by decreased body fat mass, adjusted digestive tract morphology and reduced levels of circulating leptin. Further, serum leptin were positively correlated with body weight and body fat mass, and negatively correlated with energy intake and UCP1 contents. These data suggest that wild A. chevrieri do not depend on a decrease in BM, but instead increase their thermogenic capacity to cope with cold stress. Leptin may be involved in the seasonal regulation in energy balance and thermogenesis in field A. chevrieri.

Methane output of tortoises: its contribution to energy loss related to herbivore body mass

An increase in body mass (M) is traditionally considered advantageous for herbivores in terms of digestive efficiency. However, recently increasing methane losses with increasing M were described in mammals. To test this pattern in non-mammal herbivores, we conducted feeding trails with 24 tortoises of various species (M range 0.52–180 kg) fed a diet of grass hay ad libitum and salad. Mean daily dry matter and gross energy intake measured over 30 consecutive days scaled to M^{0.75 (95%CI 0.64–0.87)} and M^{0.77 (95%CI 0.66–0.88)}, respectively. Methane production was measured over two consecutive days in respiration chambers and scaled to M^{1.03 (95%CI 0.84–1.22)}. When expressed as energy loss per gross energy intake, methane losses scaled to 0.70 (95%CI 0.47–1.05) M^{0.29 (95%CI 0.14–0.45)}. This scaling overlaps in its confidence intervals to that calculated for nonruminant mammals 0.79 (95%CI 0.63–0.99) M^{0.15 (95%CI 0.09–0.20)}, but is lower than that for ruminants. The similarity between nonruminant mammals and tortoises suggest a common evolution of the gut fauna in ectotherms and endotherms, and that the increase in energetic losses due to methane production with increasing body mass is a general allometric principle in herbivores. These findings add evidence to the view that large body size itself does not necessarily convey a digestive advantage.

To chew or not to chew: fecal particle size in herbivorous reptiles and mammals

A major difference between reptile and mammalian herbivores is that the former do not masticate their food. Actually, food particle size reduction by chewing is usually considered one of the adaptations facilitating the higher metabolic rates of mammals. However, quantitative comparisons of ingesta particle size between the clades have, to our knowledge, not been performed so far. We measured mean fecal particle size (MPS) in 79 captive individuals of 14 reptile herbivore species (tortoises, lizards, and Corucia zebrata) by wet sieving and compared the results with a mammalian dataset. MPS increased with body mass in both clades, but at a significantly higher level in reptiles. Limited evidence in free-ranging and captive individuals of Testudo hermanni indicates that in reptiles, the ability to crop food and food particle size significantly influence fecal particle size. The opportunistic observation of a drastic particle size difference between stomach and intestinal contents corroborates findings that in reptiles, in contrast to terrestrial mammals, significant ingesta particle size reduction does occur in the gastrointestinal tract, most likely owing to microbial action during very long ingesta retention. Whether behavioral adaptations to controlling ingesta particle size, such as deliberate small bite sizes, are adaptive strategies in reptiles remains to be investigated.

Chewing variation in lepidosaurs and primates

Mammals chew more rhythmically than lepidosaurs. The research presented here evaluated possible reasons for this difference in relation to differences between lepidosaurs and mammals in sensorimotor systems. Variance in the absolute and relative durations of the phases of the gape cycle was calculated from kinematic data from four species of primates and eight species of lepidosaurs. The primates exhibit less variance in the duration of the gape cycle than in the durations of the four phases making up the gape cycle. This suggests that increases in the durations of some gape cycle phases are accompanied by decreases in others. Similar effects are much less pronounced in the lepidosaurs. In addition, the primates show isometric changes in gape cycle phase durations, i.e. the relative durations of the phases of the gape cycle change little with increasing cycle time. In contrast, in the lepidosaurs variance in total gape cycle duration is associated with increases in the proportion of the cycle made up by the slow open phase. We hypothesize that in mammals the central nervous system includes a representation of the optimal chew cycle duration maintained using afferent feedback about the ongoing state of the chew cycle. The differences between lepidosaurs and primates do not lie in the nature of the sensory information collected and its feedback to the feeding system, but rather the processing of that information by the CNS and its use feed-forward for modulating jaw movements and gape cycle phase durations during chewing.

Evidence for a tradeoff between retention time and chewing efficiency in large mammalian herbivores

Large body size is thought to produce a digestive advantage through different scaling effects of gut capacity and food intake, with supposedly longer digesta retention times in larger animals. However, empirical tests of this framework have remained equivocal, which we hypothesize is because previous comparative studies have not included digesta particle size. Larger particles require more time for digestion, and if digesta particle size increases with body mass, it could explain the lack of digestive advantage in larger herbivores. We combine data on body mass, food intake, digesta retention and digestibility with data on faecal particle size (as a proxy for digesta particle size) in 21 mammalian herbivore species. Multiple regression shows that fibre digestibility is independent of body mass but dependent on digesta retention and particle size; the resulting equation indicates that retention time and particle size can compensate for each other. Similarly, digestible food intake is independent of body mass, but dependent on food intake, digesta retention, and particle size. For mammalian herbivores, increasing digesta retention and decreasing digesta particle size are viable strategies to enhance digestive performance and energy intake. Because the strategy of increased digesta retention is usually linked to reduced food intake, the high selective pressure to evolve a more efficient dentition or a physiological particle separation mechanism that facilitates repeated mastication of digesta (rumination) becomes understandable.

Allometry of visceral organs in living amniotes and its implications for sauropod dinosaurs

Allometric equations are often used to extrapolate traits in animals for which only body mass estimates are known, such as dinosaurs. One important decision can be whether these equations should be based on mammal, bird or reptile data. To address whether this choice will have a relevant influence on reconstructions, we compared allometric equations for birds and mammals from the literature to those for reptiles derived from both published and hitherto unpublished data. Organs studied included the heart, kidneys, liver and gut, as well as gut contents. While the available data indicate that gut content mass does not differ between the clades, the organ masses for reptiles are generally lower than those for mammals and birds. In particular, gut tissue mass is significantly lower in reptiles. When applying the results in the reconstruction of a sauropod dinosaur, the estimated volume of the coelomic cavity greatly exceeds the estimated volume of the combined organ masses, irrespective of the allometric equation used. Therefore, substantial deviation of sauropod organ allometry from that of the extant vertebrates can be allowed conceptually. Extrapolations of retention times from estimated gut contents mass and food intake do not suggest digestive constraints on sauropod dinosaur body size.

A case of non-scaling in mammalian physiology? Body size, digestive capacity, food intake, and ingesta passage in mammalian herbivores

As gut capacity is assumed to scale linearly to body mass (BM), and dry matter intake (DMI) to metabolic body weight (BM(0.75)), it has been proposed that ingesta mean retention time (MRT) should scale to BM(0.25) in herbivorous mammals. We test these assumptions with the most comprehensive literature data collations (n=74 species for gut capacity, n=93 species for DMI and MRT) to date. For MRT, only data from studies was used during which DMI was also recorded. Gut capacity scaled to BM(1.06). In spite of large differences in feeding regimes, absolute DMI (kg/d) scaled to BM(0.76) across all species tested. Regardless of this allometry inherent in the dataset, there was only a very low allometric scaling of MRT with BM(0.14) across all species. If species were divided according to the morphophysiological design of their digestive tract, there was non-significant scaling of MRT with BM(0.04) in colon fermenters, BM(0.08) in non-ruminant foregut fermenters, BM(0.06) in browsing and BM(0.04) in grazing ruminants. In contrast, MRT significantly scaled to BM(0.24) (CI 0.16-0.33) in the caecum fermenters. The results suggest that below a certain body size, long MRTs cannot be achieved even though coprophagy is performed; this supports the assumption of a potential body size limitation for herbivory on the lower end of the body size range. However, above a 500 g-threshold, there is no indication of a substantial general increase of MRT with BM. We therefore consider ingesta retention in mammalian herbivores an example of a biological, time-dependent variable that can, on an interspecific level, be dissociated from a supposed obligatory allometric scaling by the morphophysiological design of the digestive tract. We propose that very large body size does not automatically imply a digestive advantage, because long MRTs do not seem to be a characteristic of very large species only. A comparison of the relative DMI (g/kg(0.75)) with MRT indicates that, on an interspecific level, higher intakes are correlated to shorter MRTs in caecum, colon and non-ruminant foregut fermenters; in contrast, no significant correlation between relative DMI and MRT is evident in ruminants.

Symbiotic fermentation, digesta passage, and gastrointestinal morphology in bullfrog tadpoles (Rana catesbeiana)

Relative to other herbivorous vertebrates, the nutritional ecology and digestive physiology of anuran larvae remain poorly understood. Our objective was to compare gut structure and inhabitants, digesta passage, and microbial fermentation in bullfrog tadpoles (Rana catesbeiana) to those in other herbivores. Bullfrog tadpole gastrointestinal tracts were long and voluminous, with an enlarged colon that harbored a diverse symbiotic community. The transit time for particulate markers passing through bullfrog tadpoles was 6 h, the median retention time was 8-10 h, and gut clearance was 10-14 h postingestion. Relatively high levels of short-chain fatty acids in the hindgut of tadpoles indicated active microbial fermentation in this gut region. This report represents the first account of gastrointestinal fermentation in the class Amphibia. On the basis of in vitro fermentation assays, we estimated that microbial fermentation in the hindgut provides 20% of the total daily energy requirement of bullfrog tadpoles. These tadpoles also exhibited coprophagy, a practice that provides important nutritive gains in other herbivores. The physiological and behavioral characteristics of these tadpoles are remarkably similar to those of other small-bodied, hindgut-fermenting vertebrates, suggesting convergent digestive strategies among a broad range of herbivorous taxa.

Ontogenetic Variation in Digestion by the Herbivorous Lizard Ctenosaura pectinata

I tested the hypothesis that an animal with an ontogenetic diet shift must have different digestive efficiencies for foods that correspond to its diet shift, so that nutrient and energy extraction are maximized. The iguanine lizard Ctenosaura pectinata undergoes an ontogenetic diet shift from eating insects as a juvenile to plants as an adult. When fed six different pure foods from the natural diets of different age classes, C. pectinata assimilated nutrients and energy differently depending on food type and age class. Extraction of energy and nutrients in insect larvae was maximized by juvenile lizards. Calcium, phosphorus, and energy were readily assimilated from flowers and fruit by immature and adult lizards. Magnesium levels were highest in leaves and were extracted by immature and adult lizards, but xenobiotic effects of one plant leaf (Croton suberosus), eaten by adults, killed juvenile lizards. Although juvenile C. pectinata ate some flowers (Senna wislizenii) naturally, they were less efficient at digesting cell walls from these plant parts than were older lizards. Ontogenetic changes in ctenosaur digestive physiology were not the result of a trade-off involving ecological costs of different foods; rather, each age class preferred a diet that maximized its physiological benefit.

Ingestion of juniper foliage reduces metabolic rates in woodrat (Neotoma) herbivores

Ingestion of plant secondary compounds by herbivores is predicted to increase resting or basal metabolic rates. We tested this hypothesis with two species of woodrat herbivores, Neotoma stephensi and Neotoma albigula, consuming diets of juniper (Juniperus monosperma), which is rich in plant secondary compounds. In nature, N. stephensi specializes on juniper, whereas N. albigula consumes a variety of plant species including juniper. We measured resting metabolic rates (RMR) of woodrats on control, 25% juniper and a treatment containing the maximum tolerable dose of juniper (50% juniper for N. albigula and 70% juniper for N. stephensi). Ingestion of a juniper diet resulted in decreased RMR in both species of woodrats. We propose several potential mechanisms for metabolic depression of Neotoma on juniper diets. Our novel results underscore the need for more studies utilizing plant-based diets to determine the general effect of plant secondary compounds on metabolic rates of herbivores.

Dietary fiber influences nutrient utilization, growth and dry matter intake of green iguanas (Iguana iguana)

Herbivory is an uncommon feeding strategy in lizards. Appropriate diet formulations for captive lizards should be based on performance measures, yet few data are available on the effect of plant fiber on food intake, nutrient utilization and growth of captive herbivorous lizards. This study was conducted to determine the effect of three levels of dietary fiber on dry matter intake, nutrient and energy metabolizability and growth rate of the green iguana (Iguana iguana). Twenty-one captive iguanas were fed nutritionally complete diets containing three levels of dietary fiber: 19, 24, and 27% neutral detergent fiber. The iguanas were fed each diet for at least 12 wk, and total excreta were collected for 11.3 +/- 4.0 d (means +/- , range of 7 to 25 d). Diets and excreta were analyzed for dry matter, organic matter, gross energy, neutral detergent fiber, acid detergent fiber, and acid detergent lignin. The study was designed as a Latin square crossover. Across all diets, dry matter intake was proportional to body mass1.0 (BM). Growth rate was greater (P < 0. 05) when iguanas were fed the low and medium fiber diets (2.2 and 2. 4 g/d, respectively) than when fed the high fiber diet (1.4 g/d). However, mean daily dry matter intake of the three diets [7.2 g/(d. kg BM)] was not different. In general, digestibility of fiber fractions and the metabolizability of dietary energy decreased (P < 0.05) as the level of dietary fiber increased. These data suggest that a diet containing less than 27% neutral detergent fiber should be fed if rapid growth is to be sustained during intensive captive production of green iguanas.

Fibre digestion and digesta retention time in guinea-pigs (Cavia porcellus), degus (Octodon degus) and leaf-eared mice (Phyllotis darwini)

1. Digestibilities of feed and turnover time (1/k), Transit time (TT) and mean retention time (MRT: 1/k + TT) of fluid and particle markers were measured in the guinea-pig (Cavia porcellus), degu (Octodon degus) and leaf-eared mouse (Phyllotis darwini) fed a diet containing 50% alfalfa. 2. The digestibility of fibre and the retention time of digesta were highest in the guinea-pig followed by the degu and lowest in the leaf-eared mouse. 3. The difference in the retention time of digesta, resulting from the variation in the digestibility of fibre, between the three animals can be considered to be related to their body mass.