Diet selection and digestion in Iguana iguana: the importance of age and nutrient requirements

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).

Vertebrates as uninfected disseminators of helminth eggs and larvae

The passive dispersal of non-mobile organisms by vertebrates (zoochory) is a common mechanism used to explain their often widespread distribution. Transport occurs either internally via the vertebrate digestive tract (endozoochory), or externally be adhering to skin, feathers or fur (ectozoochory), and its success is due to both physiological and ecological factors associated with the disseminating 'hosting' animal. Helminth eggs and larvae are generally non-mobile stages that are largely dependent on the movement of another animal, typically a host, for geographical dissemination. Studies on the zoochory of helminths by vertebrates are extensive and particularly long-standing, stretching back to the 19th century, although this literature is often overlooked when considering the biogeography of parasites. This review assesses the potential of helminths to be dispersed passively by zoochory examining evidence from both laboratory and field studies. The physiological dynamics of the vertebrate intestines and skin surface as hostile environments, as well as the characteristics of eggs and larvae which may facilitate successful transport are evaluated. The various mechanisms of helminth endo- and ectozoochory are presented and the likelihood of long-distance dispersal determined. It is concluded that zoochory is a potentially important means of disseminating parasites.

Quantitative analyses of squamate dentition demonstrate novel morphological patterns

Squamates are ideal subjects for investigating relationships between diet and dental patterns because they exhibit wide dietary diversity, marked variation in dental shape, and are taxonomically abundant. Despite this, well-established links between diet and dental morphology are primarily qualitative in nature, with specific patterns of squamate dental complexity remaining largely unknown. Here, we use quantitative methods and a broad taxonomic dataset to quantify key patterns in squamate dental morphology, including re-examining the relationship between dentition and diet, testing for differences in complexity between dentigerous elements, and exploring the effect of ontogenetic dietary shifts in dental complexity in two iguanid genera. Our findings support previous research by demonstrating that species consuming more plant material possess more complex teeth. We did not find significant complexity differences between the left and right dentigerous elements nor the upper and lower jaws, with the exception of Amblyrhynchus cristatus, the marine iguana, which possesses significantly more complex dentary teeth than premaxillary and maxillary teeth. We find discordant patterns when testing for dental complexity changes through ontogeny. Amblyrhynchus, which is primarily herbivorous throughout its lifetime, increases dental complexity through ontogeny, whereas Ctenosaura, which is generally insectivorous as juveniles and herbivorous as adults, decreases dental complexity. Although preliminary, this research documents and quantifies novel patterns of squamate dental complexity and exhibits the possibilities for further research on the diversity of squamate dental morphology.

Being hunted high and low: do differences in nocturnal sleeping and diurnal resting sites of howler monkeys (Alouatta nigerrima and Alouatta discolor) reflect safety from attack by different types of predator?

Predation risk is important in influencing animal behaviour. We investigated how the choice of nocturnal sleeping and diurnal resting sites by two species of primates was influenced by the most likely forms of attack (diurnal raptors and nocturnal felids). We recorded vertical and horizontal patterns of occupancy for 47 sleeping and 31 resting sites, as well as the presence of lianas or vines on trees. We compared the heights of trees used as resting or sleeping sites by the monkeys with those of 200 forest trees that the monkeys did not use. Trees used as nocturnal sleeping sites were taller than those used as diurnal resting sites, and taller than trees that the monkeys did not use. However, while trees used as diurnal resting sites were not significantly taller than non-used trees, diurnal resting sites were located on branches closer to the ground, closer to the main trunk of the tree and in trees with more lianas/vines than nocturnal sleeping sites. The differences in site location can be explained by the type of predator most likely to attack at a particular time: raptors in the day and felids at night.

Hunters or farmers? Microbiome characteristics help elucidate the diet composition in an aquatic carnivorous plant

BACKGROUND

Utricularia are rootless aquatic carnivorous plants which have recently attracted the attention of researchers due to the peculiarities of their miniaturized genomes. Here, we focus on a novel aspect of Utricularia ecophysiology-the interactions with and within the complex communities of microorganisms colonizing their traps and external surfaces.

RESULTS

Bacteria, fungi, algae, and protozoa inhabit the miniature ecosystem of the Utricularia trap lumen and are involved in the regeneration of nutrients from complex organic matter. By combining molecular methods, microscopy, and other approaches to assess the trap-associated microbial community structure, diversity, function, as well as the nutrient turn-over potential of bacterivory, we gained insight into the nutrient acquisition strategies of the Utricularia hosts.

CONCLUSIONS

We conclude that Utricularia traps can, in terms of their ecophysiological function, be compared to microbial cultivators or farms, which center around complex microbial consortia acting synergistically to convert complex organic matter, often of algal origin, into a source of utilizable nutrients for the plants.

The relationship between diet and tooth complexity in living dentigerous saurians

Living saurian reptiles exhibit a wide range of diets, from carnivores to strict herbivores. Previous research suggests that the tooth shape in some lizard clades correlates with diet, but this has not been tested using quantitative methods. I investigated the relationship between phenotypic tooth complexity and diet in living reptiles by examining the entire dentary tooth row in over 80 specimens comprising all major dentigerous saurian clades. I quantified dental complexity using orientation patch count rotated (OPCR), which discriminates diet in living and extinct mammals, where OPCR-values increase with the proportion of dietary plant matter. OPCR was calculated from high-resolution CT-scans, and I standardized OPCR-values by the total number of teeth to account for differences in tooth count across taxa. In contrast with extant mammals, there appears to be greater overlap in tooth complexity values across dietary groups because multicusped teeth characterize herbivores, omnivores, and insectivores, and because herbivorous skinks have relatively simple teeth. In particular, insectivorous lizards have dental complexities that are very similar to omnivores. Regardless, OPCR-values for animals that consume significant amounts of plant material are higher than those of carnivores, with herbivores having the highest average dental complexity. These results suggest reptilian tooth complexity is related to diet, similar to extinct and extant mammals, although phylogenetic history also plays a measurable role in dental complexity. This has implications for extinct amniotes that display a dramatic range of tooth morphologies, many with no modern analogs, which inhibits detailed dietary reconstructions. These data demonstrate that OPCR, when combined with additional morphological data, has the potential to be used to reconstruct the diet of extinct amniotes. J. Morphol. 278:500-522, 2017. © 2017 Wiley Periodicals, Inc.

Incubation temperature and clutch effects on initial body sizes and growth rates in green iguana hatchlings (Iguana iguana)

Large initial body size and rapid early growth rate are important in many species, both because predation rates decline as individuals grow and because females that attain a larger adult body size are more fecund. To identify possible factors contributing to size and growth rate variation in hatchling green iguanas, we artificially incubated six clutches at three constant temperatures to test for effects of incubation temperature and/or clutch effects on initial size and growth rate. Higher incubation temperatures resulted in significantly shorter incubation periods but did not influence initial body size. There were significant differences among clutches in egg size, and also in initial hatchling body size, even after correcting for differences in egg size among clutches. A subset of hatchlings from each nest was reared in semi-natural conditions for four months, with individuals from the high incubation temperature condition exhibiting the slowest longer-term growth rates. No clutch effects were detected in the growth rate analyses. The observed variation in early growth rate of juvenile iguanas seems to be selectively important and this variation may be due in part to the conditions the eggs experience during incubation, but clutch effects in this study were limited to egg size and initial hatchling body size variation, but were not found for subsequent growth rates.

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.

Diet of Amazon river turtles (Podocnemididae): a review of the effects of body size, phylogeny, season and habitat

Amazon rivers can be divided into three groups (black, white and clear waters) according to the origin of their sediment, dissolved nutrient content, and vegetation. White water rivers have high sediment loads and primary productivity, with abundant aquatic and terrestrial plant life. In contrast, black water rivers are acid and nutrient-poor, with infertile floodplains that support plant species exceptionally rich in secondary chemical defences against herbivory. In this study, we reviewed available information on the diet of Amazon sideneck river turtles (Family Podocnemididae). Our aim was to test the relationship between water type and diet of podocnemidids. We also took into account the effects of season, size, age, sex and phylogeny. Based on our review, turtles of this family are primarily herbivorous but opportunistic, consuming from 46 to 99% (percent volume) of vegetable matter depending on species, sex, season and location. There was no significant correlation between the maximum carapace size of a species and vegetable matter consumed. When the available information on diet, size and habitat was arranged on the podocnemidid phylogeny, no obvious evolutionary trend was evident. The physicochemical properties of the inhabited water type indirectly influence the average volume of total vegetable matter consumed. Species with no specialised stomach adaptations for herbivory consumed smaller amounts of hard to digest vegetable matter (i.e. leaves, shoots and stems). We propose that turtles with specialized digestive tracts may have an advantage in black water rivers where plant chemical defences are more common. Despite limitations of the published data our review highlights the overall pattern of diet in the Podocnemididae and flags areas where more studies are needed.

Anatomical and physiological changes associated with a recent dietary shift in the lizard Podarcis sicula

Dietary shifts have played a major role in the evolution of many vertebrates. The idea that the evolution of herbivory is physiologically constrained in squamates is challenged by a number of observations that suggest that at least some lizards can overcome the putative physiological difficulties of herbivory on evolutionary and even ecological timescales. We compared a number of morphological and physiological traits purportedly associated with plant consumption between two island populations of the lacertid lizard Podarcis sicula. Previous studies revealed considerable differences in the amount of plant material consumed between those populations. We continued the investigation of this study system and explored the degree of divergence in morphology (dentition, gut morphology), digestive performance (gut passage time, digestive efficiency), and ecology (endosymbiont density). In addition, we also performed a preliminary analysis of the plasticity of some of these modifications. Our results confirm and expand earlier findings concerning divergence in the morphology of feeding structures between two island populations of P. sicula lizards. In addition to the differences in skull dimensions and the prevalence of cecal valves previously reported, these two recently diverged populations also differ in aspects of their dentition (teeth width) and the lengths of the stomach and small intestine. The plasticity experiment suggests that at least some of the changes associated with a dietary shift toward a higher proportion of plant material may be plastic. Our results also show that these morphological changes effectively translate into differences in digestive performance: the population with the longer digestive tract exhibits longer gut passage time and improved digestive efficiency.

Ontogeny of digestion in Daphnia: implications for the effectiveness of algal defenses

Models of feeding and digestion predict that increased body size should result in longer gut passage time and improved assimilation efficiency. We examined the implications of digestion theory for size-structured interactions in a generalist zooplankton herbivore by studying the relationships between body size, ingestion rate, gut passage time (GPT), assimilation efficiency (AE), and growth rate in a clone of Daphnia pulex feeding on seven taxa of green algae that differed in digestibility. We also tested the effect of varying food concentration on GPT and AE while keeping body size constant. Food quality varied markedly among algal taxa, with mean juvenile growth rates at high food concentrations (1-2 mg/L) ranging from 0.10 to 0.61 d(-1). Juvenile growth rate for high food concentrations was highly correlated with juvenile AE (r2 = 0.96), verifying the importance of digestibility for food quality. AE, measured with 14C-labeled algae, increased with increasing age and body size for each of four digestion-resistant taxa but did not vary with age and body size for three readily digested algae. GPT decreased with decreasing body size, supporting the hypothesis that shorter GPT in juveniles leads to lower AE for digestion-resistant resources. Lower food concentrations led to increased GPT and improved AE for juveniles feeding on two digestion-resistant algae, providing further support for a role of longer gut retention in overcoming digestion defenses. The results suggest that increased abundance of digestion-resistant food will lead to growth and recruitment bottlenecks for juvenile herbivores, but that the effectiveness of digestion defenses will be decreased when large-bodied grazers predominate and when low food concentrations result in longer gut passage times. Gut processing constraints may favor either high concentrations of slow-growing, digestion-resistant resources or low concentrations of fast-growing, undefended resources.

Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource

Although rapid adaptive changes in morphology on ecological time scales are now well documented in natural populations, the effects of such changes on whole-organism performance capacity and the consequences on ecological dynamics at the population level are often unclear. Here we show how lizards have rapidly evolved differences in head morphology, bite strength, and digestive tract structure after experimental introduction into a novel environment. Despite the short time scale ( approximately 36 years) since this introduction, these changes in morphology and performance parallel those typically documented among species and even families of lizards in both the type and extent of their specialization. Moreover, these changes have occurred side-by-side with dramatic changes in population density and social structure, providing a compelling example of how the invasion of a novel habitat can evolutionarily drive multiple aspects of the phenotype.

Ontogenetic Diet Shifts and Digestive Constraints in the Omnivorous Freshwater Turtle Trachemys scripta

Many reptiles undergo an ontogenetic diet shift from carnivory to herbivory. In this study, we used the yellow-bellied slider turtle, Trachemys scripta, as a model to evaluate whether juvenile turtles are carnivorous because physiological constraints preclude herbivory. We conducted feeding trials in which we fed juvenile and adult turtles a duckweed plant, Lemna valdiviana, or a freshwater grass shrimp, Palaemontes paludosus, for 5 wk. During the trials, we measured mass-specific intake, digestibility, and digestible intake for both size classes, as well as juvenile growth. At the end of the trials, we measured the nutrient composition of the juvenile turtles. Juveniles fed shrimp grew 3.2 times faster than those fed duckweed and had equivalent lipid stores. Digestive processing in juveniles was extremely efficient on the shrimp diet, with higher mass-specific intakes than adults and very high digestibilities (97%). Juveniles digested duckweed as well as adults did; however, their intake of this diet was limited, possibly by the time required for fermentation. We concluded that although juveniles can process plant material, an animal diet allows for greater juvenile growth, which in turtles is linked to higher survivorship and increased future reproductive success.

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.

The growth pattern of the African tortoise Geochelone pardalis and other chelonians

Growth rings were measured in dead African leopard tortoises, Geochelone pardalis, collected in the seasonal tropics of Zimbabwe over an 11-year period. A series of Ford-Walford plots using growth measured from annuli showed that growth fitted a logistic by mass curve best, logistic by length and Gompertz curves less well, and a Bertalanffy curve least well. The Bertalanffy curve, often fitted to growth of chelonians, is characterised by particularly high growth rates of juveniles compared with larger individuals. It is suggested that this growth pattern is likely to be found in species showing a marked decrease in diet quality with size. This hypothesis is supported by a review of growth patterns in chelonians: Bertalanffy curves are associated with an omnivorous (and thus potentially variable) diet and other growth patterns with an obligate carnivorous or herbivorous diet. Geochelone pardalis in Zimbabwe showed significant sexual size dimorphism, the mean asymptotic mass of females being 1.7 times that of males, unlike populations with larger body sizes to the north and south. Annual survival estimated from age-frequency distributions was significantly higher in males (0.80) than in females (0.72), the difference being sufficient to account for the male-biased sex ratio of live animals.

Response to dietary dilution in an omnivorous freshwater turtle: implications for ontogenetic dietary shifts

Several species of freshwater turtles in the family Emydidae undergo an ontogenetic dietary shift; as juvenile turtles mature, they change from a primarily carnivorous to a primarily herbivorous diet. It has been hypothesized that this shift results from an unfavorable ratio of gut capacity to metabolic rate that prevents small reptiles from processing adequate volumes of plant material to meet their energetic demands. Effects of dietary dilution on intake were evaluated in two size classes of red-eared sliders (Trachemys scripta elegans) to test whether small reptiles have a lower capacity to compensate for low-quality diets through increased intake than do larger conspecifics. Artificial diets with an inert diluent were offered to two size classes of turtles, and mass-specific intakes of dry matter, energy, and nitrogen were calculated. Both small (28.7+/-4.9 g body mass, mean mass+/-SD) and large (1,230+/-94 g body mass) turtles compensated for dietary dilution and maintained constant energy and nitrogen intakes on diets with lower energy content than common aquatic plants. Thus, body size did not affect the ability to respond to nutritional dilution, which suggests that processing limitations imposed by small body size do not constrain juveniles from adopting an herbivorous diet.

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.

Iguana iguana: a model species for studying the ontogeny of behavior/hormone interactions

Several features of the green iguana, Iguana iguana, make this species an excellent subject for biological research, especially with respect to collecting comparative data on behavior/hormone interactions. This species thrives on captivity, and behavioral interactions appear substantially similar to those observed in natural populations, given effective captive environments. Because hatchlings do not necessarily associate with adult conspecifics in wild populations, behavioral patterns exhibited by and among juveniles in either natural conditions or captivity are probably unlearned behaviors. The species is large enough to obtain blood samples at frequent intervals at an early age. Because the species is herbivorous the maturation process can be controlled by the caloric base made available to individual lizards.

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

What digestive adaptations permit herbivorous nonruminant mammals to sustain much higher metabolic rates than herbivorous lizards, despite gross similarity in digestive anatomy and physiology? We approached this question by comparing four herbivorous species eating the same diet of alfalfa pellets: two lizards (chuckwalla and desert iugana) and two mammals (desert woodrat and laboratory mouse). The mammals had longer small and large intestines, greater intestinal surface area, much higher (by an order of magnitude) food intake normalized to metabolic live mass, and much faster food passage times (a few hours instead of a few days). Among both reptiles and mammals, passage times increase with body size and are longer for herbivores than for carnivores. The herbivorous lizards, despite these much slower passage times, had slightly lower apparent digestive efficiencies than the mammals. At least for chuckwallas, this difference from mammals was not due to differences in body temperature regime. Comparisons of chuckwallas and woodrats in their assimilation of various dietary components showed that the woodrat's main advantage lay in greater assimilation of the dietary fiber fraction. Woodrats achieved greater fiber digestion despite shorter residence time, but possibly because of a larger fermentation chamber, coprophagy, and/or different conditions for microbial fermentation. We conclude with a comparative overview of digestive function in herbivorous lizards and mammals, and with a list of four major unsolved questions.