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).

Diet influences latitudinal gradients in life-history traits, but not reproductive output, in ectotherms

Aim

Latitudinal gradients in life-history traits are apparent in many taxa and are expected to be strong for ectotherms that have temperature-driven constraints on performance and fitness. The strength of these gradients, however, should also be affected by diet. Because diet type (carnivory, omnivory, herbivory) influences accessibility to nutrition and assimilation efficiency, we aim to study how diet affects latitudinal gradients in lifetime reproductive output and the underlying life-history traits in ectotherms.

Location

Global.

Time period

Recent.

Major taxa studied

Lizards (Reptilia, Squamata, Sauria).

Methods

We used empirical (352 species) and phylogenetically imputed data (563 species) to analyse the interactive effects of latitude and diet on life-history traits (longevity, age at maturity, reproductive life span, hatchling mass, clutch/brood size, clutch/brood frequency, female mass) and lifetime reproductive output of lizards.

Results

Lifetime reproductive output does not significantly differ in lizards across diet types, and only carnivores exhibit a small increase at higher latitudes. Diet type, however, influences latitudinal patterns of individual life-history traits. Carnivores exhibit a shift towards 'slower-paced' life histories at higher latitudes for most traits (increased longevity, age at maturity, reproductive life span, and decreased clutch frequency). By contrast, herbivores either display 'faster-paced' life histories (reduction in reproductive life span, hatchling mass, female mass) or no change (clutch frequency, clutch size, age at maturity) at higher latitudes. Omnivores exhibit intermediate and muted latitudinal patterns.

Main conclusions

We suggest that the nutritional challenges of herbivory, compounded by thermal constraints at higher latitudes, may explain differences in life-history characteristics of herbivorous ectotherms. Intermediate patterns exhibited by omnivores highlight how flexibility in diet can buffer environmental challenges at higher latitudes. Our results indicate that lizards with different diet types display various trends in their life histories across latitudes, which eventually balance out to result in similar reproductive outputs throughout their lifetime, with little benefits to carnivory.

Integrating bioenergetics and conservation biology: thermal sensitivity of digestive performance in Eastern Collared Lizards (Crotaphytus collaris) may affect population persistence

Information on bioenergetics can provide valuable insight into the ecology, life history and population dynamics of organisms. For ectothermic animals, thermal sensitivity of digestion is an important determinant of net assimilated energy budgets. A recent study in the Ozark Mountains indicated that eastern collared lizards (Crotaphytus collaris) restricted to encroached glades (characterized by woody vegetation encroachment) experience reduced environmental heat loads and have reduced age-specific growth and reproductive rates compared to populations in intact glades. To assess the potential impact of reduced body temperatures on assimilation rates of C. collaris in encroached glades, we conducted feeding trials across four temperature treatments (28, 31, 34 and 37°C). We tested for temperature effects on voluntary feeding rates, passage times, apparent assimilated energy (AE) and metabolizable energy (ME). Passage times decreased and voluntary feeding rates increased significantly with increasing temperature. Consumption explained the majority of variance in AE and ME, followed by the effect of temperature treatments. Using data on voluntary feeding rates, passage times and ME as a function of temperature, we estimated over a 10-fold increase in predicted daily assimilated energy across temperature treatments (28°C = 0.58 kJ/day, 31°C = 1.20 kJ/day, 34°C = 4.30 kJ/day, 37°C = 7.95 kJ/day). Thus, lower heat loads in encroached glades may cause reduced body temperature and result in restricted energy assimilation rates. Our study provides a novel approach to the integration of bioenergetics and conservation and shows the efficacy of using information on digestive performance to investigate underlying mechanisms in a conservation context.

Proximal duodenoileal anastomosis for treatment of small intestinal obstruction and volvulus in a green iguana (Iguana iguana)

CASE DESCRIPTION A 13-year-old female green iguana (Iguana iguana) was examined because of a 6-day history of vomiting, anorexia, and lethargy and a 4-day history of decreased fecal and urate output. CLINICAL FINDINGS Physical examination revealed a distended abdomen, signs of depression, pallor, tachycardia, harsh lung sounds, and vomiting. Abdominal radiographs revealed gas distention of the stomach and small intestine with fluid lines evident on the lateral view. Plasma biochemical analysis indicated hypochloremic metabolic alkalosis, hyperglycemia, and hyperuricemia. TREATMENT AND OUTCOME Exploratory laparotomy confirmed a diagnosis of small intestinal entrapment and 170° volvulus involving approximately 80% (20 to 30 cm) of the small intestine. The portion of the small intestine extending from the middle portion of the duodenum to the caudal extent of the ileum was resected, and end-to-end anastomosis of the remaining small intestine was performed. The iguana recovered without apparent complications and was reportedly doing well 1 year after surgery. CLINICAL RELEVANCE Findings suggested that iguanas, as hindgut fermenters, may tolerate > 70% resection of the small intestine with a good outcome and no clinical evidence of residual gastrointestinal dysfunction.

Effect of lysine addition on growth of black iguana (Ctenosaura pectinata)

The effects of the addition of lysine to commercial feed given to captive black iguana (Ctenosaura pectinata) were evaluated in terms of growth and feed digestibility. Twenty-eight-day-old black iguana with an initial weight of 5.5 ± 0.3 g were housed individually in cages measuring 45 × 45 × 45 cm. The experiment lasted 150 days. The ambient temperature ranged from 28 to 35°C with a relative humidity of 60 to 95%. Treatments consisted of the addition of different percentages of lysine to the feed (0.0, 0.1, 0.2, and 0.3%, dry matter [DM] base). There was a linear response (P < 0.01) in daily gain (68, 112, 118, and 151 mg/d) and daily intake (251, 289, 297, and 337 mg/d) for levels from 0 to 0.3%, respectively, as well in the growth in head size, snout-vent length, and total length. The digestibility of DM, neutral detergent fiber, and acid detergent fiber were reduced linearly (P < 0.01) as lysine levels increased. Intake and digestibility were negatively correlated (r = -0.74; P < 0.001). It is concluded that the addition of lysine to the black iguana diet in the first months of life is important to stimulate growth and intake.

Phylogenetic analysis of the fecal microbial community in herbivorous land and marine iguanas of the Galápagos Islands using 16S rRNA-based pyrosequencing

Herbivorous reptiles depend on complex gut microbial communities to effectively degrade dietary polysaccharides. The composition of these fermentative communities may vary based on dietary differences. To explore the role of diet in shaping gut microbial communities, we evaluated the fecal samples from two related host species--the algae-consuming marine iguana (Amblyrhynchus cristatus) and land iguanas (LI) (genus Conolophus) that consume terrestrial vegetation. Marine and LI fecal samples were collected from different islands in the Galápagos archipelago. High-throughput 16S rRNA-based pyrosequencing was used to provide a comparative analysis of fecal microbial diversity. At the phylum level, the fecal microbial community in iguanas was predominated by Firmicutes (69.5±7.9%) and Bacteroidetes (6.2±2.8%), as well as unclassified Bacteria (20.6±8.6%), suggesting that a large portion of iguana fecal microbiota is novel and could be involved in currently unknown functions. Host species differed in the abundance of specific bacterial groups. Bacteroides spp., Lachnospiraceae and Clostridiaceae were significantly more abundant in the marine iguanas (MI) (P-value>1E-9). In contrast, Ruminococcaceae were present at >5-fold higher abundance in the LI than MI (P-value>6E-14). Archaea were only detected in the LI. The number of operational taxonomic units (OTUs) in the LI (356-896 OTUs) was >2-fold higher than in the MI (112-567 OTUs), and this increase in OTU diversity could be related to the complexity of the resident bacterial population and their gene repertoire required to breakdown the recalcitrant polysaccharides prevalent in terrestrial plants. Our findings suggest that dietary differences contribute to gut microbial community differentiation in herbivorous lizards. Most importantly, this study provides a better understanding of the microbial diversity in the iguana gut; therefore facilitating future efforts to discover novel bacterial-associated enzymes that can effectively breakdown a wide variety of complex polysaccharides.

Digestive performance in five Mediterranean lizard species: effects of temperature and insularity

Temperature sensitivity of digestive processes has important ramifications for digestive performance in ectothermic vertebrates. We conducted a comparative analysis of temperature effects on digestive processes [gut passage times (GPTs) and apparent digestive efficiencies (ADEs)] in five lacertid lizards occurring in insular (Podarcis erhardii, P. gaigeae), and mainland (P. muralis, P. peloponnesiaca, Lacerta graeca) Mediterranean environments. GPTs were negatively correlated to temperature with mainland taxa having 10-20% longer GPTs than island taxa. In contrast to previous studies that estimate ADEs using bomb calorimetry, we compare ADEs by analyzing discrete efficiencies for lipids, sugars and proteins at three temperature regimes (20, 25, and 30 degrees C); each of these categories produces different results. ADEs for lipids and sugars showed a monotonic increase with temperature whereas ADEs for proteins decreased with temperature. Island taxa had consistently higher ADEs than their mainland counterparts for lipids and for proteins but not for sugars. They are characterized by superior energy acquisition abilities despite significantly shorter GPTs. Their increased digestive performance relative to the mainland species appears to allow them to maximize energy acquisition in unproductive island environments where food availability is spatially and seasonally clustered.

Correlating diet and digestive tract specialization: examples from the lizard family Liolaemidae

A range of digestive tract specializations were compared among dietary categories in the family Liolaemidae to test the hypothesis that herbivores require greater gut complexity to process plant matter. Additionally, the hypothesis that herbivory favors the evolution of larger body size was tested. Lastly, the association between diet and hindgut nematodes was explored. Herbivorous liolaemids were larger relative to omnivorous and insectivorous congeners and consequently had larger guts. In addition, small intestine length of herbivorous liolaemids was disproportionately longer than that of congeners. Significant interaction effects between diet and body size among organ dimensions indicate that increases in organ size occur to a greater extent in herbivores than other diet categories. For species with plant matter in their guts, there was a significant positive correlation between the percentage of plant matter consumed and small intestine length. Herbivorous liolaemids examined in this study lacked the gross morphological specializations (cecum and colonic valves) found in herbivores in the families Iguanidae and Agamidae. A significantly greater percentage of herbivorous species had nematodes in their gut. Of the species with nematodes, over 95% of herbivores had nematodes only in the hindgut. Prevalence of nematodes in the hindgut of herbivores was 2 x that of omnivores and 4 x that of insectivores.

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.

Biochemical and Microbiological Evidence for Fermentative Digestion in Free‐Living Land Iguanas (Conolophus pallidus) and Marine Iguanas (Amblyrhynchus cristatus) on the Galápagos Archipelago

Herbivorous lizards are potentially capable of high digestive efficiency, but the presence of an indigenous microbial population has been implied from measurements of activity rather than directly studied. This study is the first to provide direct biochemical and microbiological evidence for fermentative digestion in free-living land iguanas (Conolophus pallidus) and marine iguanas (Amblyrhynchus cristatus) from the Galapagos archipelago. In marine iguanas, the stomach and large capacious colon contained ca. 32% and 60%, respectively, of the weight of total gut content. Total volatile fatty acid concentration was ca. 150 and 180 mM, respectively, for marine and land iguanas. Molar proportions of acetate, propionate, and butyrate (80.3%, 9.5%, and 3.5%) in land iguana fecal samples were similar to those for marine iguanas. Examination of fecal samples using confocal and transmission electron microscopy, as well as cultivable counts, revealed a dense and diverse population of bacteria, with spores prominent. Total culturable counts of anaerobes (2.22x10(8) g(-1) wet weight of fecal material) outnumbered aerobes on average by a factor of ca. 700. Combined, these results strongly support the contention that these unique herbivorous lizards are largely dependent on the presence and metabolic activities of a resident bacterial population in order to hydrolyze and ferment plant polymers that are indigestible to the host.

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.

Differential resource use, growth, and the ontogeny of social relationships in the green iguana

Growth rates, resource use, and ontogeny of behavior patterns were examined in captive groups of juvenile green iguanas (Iguana iguana). Four groups were housed in large arenas where supplemental heat and perch sites were limited, whereas two control groups were housed in similar arenas without limited resources. Growth, frequency and types of displays, behavioral interactions, and the use of resources were monitored. By 35-70 days of age, male hatchlings in arenas with limited resources could be classified into two groups: rapid growing dominant individuals and slow growing subordinate individuals. Growth and dominance were not correlated with size of individuals at hatching. Digestive efficiency of males was related to dominance and access to limited resources, but all lizards exhibited a similar digestive efficiency when maintained at a constant ambient temperature. Dominant males used the resources, especially supplemental heat sources, twice as often as subordinate males. The dominant/subordinate relationships and accompanying skew in body size were not evident in the control groups, or in females of any group. At 105 days, control groups were introduced to conditions with limited resources, resulting in increased aggressive interactions and divergence in growth rates. Onset of adult behavioral patterns was related to body size, and the species-specific signature display was observed in dominant males up to several months before subordinate males. These results indicate that male green iguanas establish a dominance hierarchy immediately posthatching when resources are limited, and as a consequence, both physiological and behavioral maturation are delayed in subordinate individuals.