Specific dynamic action of a large carnivorous lizard, Varanus albigularis

Specific dynamic action: the energy cost of digestion or growth?

The physiological processes underlying the post-prandial rise in metabolic rate, most commonly known as the 'specific dynamic action' (SDA), remain debated and controversial. This Commentary examines the SDA response from two opposing hypotheses: (i) the classic interpretation, where the SDA represents the energy cost of digestion, versus (ii) the alternative view that much of the SDA represents the energy cost of growth. The traditional viewpoint implies that individuals with a reduced SDA should grow faster given the same caloric intake, but experimental evidence for this effect remains scarce and inconclusive. Alternatively, we suggest that the SDA reflects an organism's efficacy in allocating the ingested food to growth, emphasising the role of post-absorptive processes, particularly protein synthesis. Although both viewpoints recognise the trade-offs in energy allocation and the dynamic nature of energy distribution among physiological processes, we argue that equating the SDA with 'the energy cost of digestion' oversimplifies the complexities of energy use in relation to the SDA and growth. In many instances, a reduced SDA may reflect diminished nutrient absorption (e.g. due to lower digestive efficiency) rather than increased 'free' energy available for somatic growth. Considering these perspectives, we summarise evidence both for and against the opposing hypotheses with a focus on ectothermic vertebrates. We conclude by presenting a number of future directions for experiments that may clarify what the SDA is, and what it is not.

Cardiovascular responses and the role of the neurohumoral cardiac regulation during digestion in the herbivorous lizard Iguana iguana

Carnivorous reptiles exhibit an intense metabolic increment during digestion, which is accompanied by several cardiovascular adjustments responsible for meeting the physiological demands of the gastrointestinal system. Postprandial tachycardia, a well-documented phenomenon in these animals, is mediated by the withdrawal of vagal tone associated with the chronotropic effects of non-adrenergic and non-cholinergic (NANC) factors. However, herbivorous reptiles exhibit a modest metabolic increment during digestion and there is no information about postprandial cardiovascular adjustments. Considering the significant impact of feeding characteristics on physiological responses, we investigated cardiovascular and metabolic responses, as well as the neurohumoral mechanisms of cardiac control, in the herbivorous lizard Iguana iguana during digestion. We measured oxygen consumption rate (O2), heart rate (fH), mean arterial blood pressure (MAP), myocardial activity, cardiac autonomic tone, fH/MAP variability and baroreflex efficiency in both fasting and digesting animals before and after parasympathetic blockade with atropine followed by double autonomic blockade with atropine and propranolol. Our results revealed that the peak of O2 in iguanas was reached 24 h after feeding, accompanied by an increase in myocardial activity and a subtle tachycardia mediated exclusively by a reduction in cardiac parasympathetic activity. This represents the first reported case of postprandial tachycardia in digesting reptiles without the involvement of NANC factors. Furthermore, this withdrawal of vagal stimulation during digestion may reduce the regulatory range for short-term fH adjustments, subsequently intensifying the blood pressure variability as a consequence of limiting baroreflex efficiency.

Dehydrated snakes reduce postprandial thermophily

Transient thermophily in ectothermic animals is a common response during substantial physiological events. For example, ectotherms often elevate body temperature after ingesting a meal. In particular, the increase in metabolism during the postprandial response of pythons - known as specific dynamic action - is supported by a concurrent increase in preferred temperature. The objective of this study was to determine whether hydration state influences digestion-related behavioral thermophily. Sixteen (8 male and 8 female) Children's pythons (Antaresia childreni) with surgically implanted temperature data loggers were housed individually and provided with a thermal gradient of 25-45°C. Body temperature was recorded hourly beginning 6 days prior to feeding and for 18 days post-feeding, thus covering pre-feeding, postprandial and post-absorptive stages. Each snake underwent this 24 day trial twice, once when hydrated and once when dehydrated. Our results revealed a significant interaction between temperature preference, digestive stage and hydration state. Under both hydrated and dehydrated conditions, snakes similarly increased their body temperature shortly after consuming a meal, but during the later days of the postprandial stage, snakes selected significantly lower (∼1.5°C) body temperature when they were dehydrated compared with when they were hydrated. Our results demonstrate a significant effect of hydration state on postprandial thermophily, but the impact of this dehydration-induced temperature reduction on digestive physiology (e.g. passage time, energy assimilation) is unknown and warrants further study.

Phenotypic plasticity in the common snapping turtle (Chelydra serpentina): long-term physiological effects of chronic hypoxia during embryonic development

Studies of embryonic and hatchling reptiles have revealed marked plasticity in morphology, metabolism, and cardiovascular function following chronic hypoxic incubation. However, the long-term effects of chronic hypoxia have not yet been investigated in these animals. The aim of this study was to determine growth and postprandial O2 consumption (V̇o2), heart rate (fH), and mean arterial pressure (Pm, in kPa) of common snapping turtles (Chelydra serpentina) that were incubated as embryos in chronic hypoxia (10% O2, H10) or normoxia (21% O2, N21). We hypothesized that hypoxic development would modify posthatching body mass, metabolic rate, and cardiovascular physiology in juvenile snapping turtles. Yearling H10 turtles were significantly smaller than yearling N21 turtles, both of which were raised posthatching in normoxic, common garden conditions. Measurement of postprandial cardiovascular parameters and O2 consumption were conducted in size-matched three-year-old H10 and N21 turtles. Both before and 12 h after feeding, H10 turtles had a significantly lower fH compared with N21 turtles. In addition, V̇o2 was significantly elevated in H10 animals compared with N21 animals 12 h after feeding, and peak postprandial V̇o2 occurred earlier in H10 animals. Pm of three-year-old turtles was not affected by feeding or hypoxic embryonic incubation. Our findings demonstrate that physiological impacts of developmental hypoxia on embryonic reptiles continue into juvenile life.

Resting metabolic rate and heat increment of feeding in juvenile South American fur seals (Arctocephalus australis)

Bio-energetic models used to characterize an animal's energy budget require the accurate estimate of different variables such as the resting metabolic rate (RMR) and the heat increment of feeding (HIF). In this study, we estimated the in air RMR of wild juvenile South American fur seals (SAFS; Arctocephalus australis) temporarily held in captivity by measuring oxygen consumption while at rest in a postabsorptive condition. HIF, which is an increase in metabolic rate associated with digestion, assimilation and nutrient interconversion, was estimated as the difference in resting metabolic rate between the postabsorptive condition and the first 3.5h postprandial. As data were hierarchically structured, linear mixed effect models were used to compare RMR measures under both physiological conditions. Results indicated a significant increase (61%) for the postprandial RMR compared to the postabsorptive condition, estimated at 17.93±1.84 and 11.15±1.91mL O2 min(-1)kg(-1), respectively. These values constitute the first estimation of RMR and HIF in this species, and should be considered in the energy budgets for juvenile SAFS foraging at-sea.

Energetic costs of digestion in Australian crocodiles

We measured standard metabolic rate (SMR) and the metabolic response to feeding in the Australian crocodiles, Crocodylus porosus and C. johnsoni. Both species exhibit a response that is characterised by rapidly increasing metabolism that peaks within 24 h of feeding, a postfeeding metabolic peak (peak O2) of 1.4–2.0 times SMR, and a return to baseline metabolism within 3–4 days after feeding. Postfeeding metabolism does not significantly differ between species, and crocodiles fed intact meals have higher total digestive costs (specific dynamic action; SDA) than those fed homogenised meals. Across a more than 100-fold range of body size (0.190 to 25.96 kg body mass), SMR, peak O2, and SDA all scale with body mass to an exponent of 0.85. Hatchling (≤1 year old) C. porosus have unexpectedly high rates of resting metabolism, and this likely reflects the substantial energetic demands that accompany the rapid growth of young crocodilians.

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.

Determinants and modeling of specific dynamic action for the Common Garter Snake (Thamnophis sirtalis)

Specific dynamic action (SDA), the cumulative energy expended on digestion and assimilation, can contribute significantly to individual daily energy expenditure (DEE). The Common Garter Snake (Thamnophis sirtalis (L., 1758)) is a widely distributed generalist predator that could experience considerable variation in SDA and hence the contribution of SDA to DEE. We examined the effects of meal size, meal type, and body temperature on the postprandial metabolic response and SDA of the Eastern Garter Snake ( Thamnophis sirtalis sirtalis (L., 1758)) and generated predictive models of SDA based on meal size, body mass, and body temperature, and separately based on meal energy. Postprandial peak in oxygen consumption, duration of significantly elevated oxygen consumption, and SDA increased with increasing meal size (5%–45% of body mass). Postprandial metabolic response digesting six different equal-size prey items varied significantly; vertebrate prey generated larger SDA responses than soft-bodied invertebrates. Increases in experimental temperature (15–35 °C) yielded a matched increase in postprandial peak in oxygen consumption and decrease in digestive duration. For a 1-month hypothetical feeding history for an adult T. sirtalis, the cumulative predicted SDA varied by 4.5% among the three models (minimal, intermediate, and maximal intake of prey) and averaged 8%, 22%, and 38% of DEE, respectively.

Acetaminophen as an oral toxicant for Nile monitor lizards (Varanus niloticus) and Burmese pythons (Python molurus bivittatus)

Context. Invasive species are a growing global problem. Biological invasions can result in numerous harmful impacts on local ecologies, and non-native herpetofauna are frequently ignored. Nile monitor lizards (Varanus niloticus) and Burmese pythons (Python molurus bivittatus, recently reassessed as Python bivittatus bivittatus), have become established in southern Florida. Both are large, semi-aquatic predators that pose serious threats to a variety of threatened and endangered species, as well as to the unique ecology of the area. Aims. Acetaminophen (CAS#103-90-2), a lethal oral toxicant for the invasive brown treesnake (Boiga irregularis) on Guam, was investigated as a possible toxicant in juvenile Burmese pythons and Nile monitors. Methods. Dead neonatal mouse (DNM) baits containing 0, 10, 20, or 40 mg acetaminophen were force-fed to Nile monitors, whereas DNM containing doses of 0, 20, 40, or 80 mg were freely consumed by Burmese pythons. Subjects were frequently observed post-treatment for general condition and position, with special attention paid to activity (if any), behaviour, respiration, bleeding, emesis, ataxia, and mortality. Key results. In Nile monitors, acetaminophen doses of 10, 20, or 40 mg resulted in 0, 50 and 100% mortality, respectively. In Burmese pythons, doses of 20, 40, or 80 mg resulted in 14.3, 85.7 and 100% mortality, respectively. No mortality was observed in control individuals of either species. A negative correlation between dosage (mg kg–1) and time-to-death was observed in both species. Dosages ranging from 522 to 2438 mg kg–1 and 263 to 703 mg kg–1 were uniformly lethal to monitors and pythons, respectively. Neither species exhibited signs of pain or discomfort following acetaminophen treatment. Conclusions. Acetaminophen is an effective toxicant in juvenile Nile monitors and Burmese pythons. Further investigation into acetaminophen toxicity in adults of these species is merited. Implications. Although further investigation into adult lethal dosages and strategies to optimise bait deployment while minimising secondary hazards is required, acetaminophen may have a role to play in the control of these invasive species in Florida.

Bioenergetic modeling reveals that Chinese green tree vipers select postprandial temperatures in laboratory thermal gradients that maximize net energy intake

With bioenergetic modeling, we tested the hypothesis that reptiles maximize net energy gain by postprandial thermal selection. Previous studies have shown that Chinese green tree vipers (Trimeresurus s. stejnegeri) have postprandial thermophily (mean preferred temperature T(p) for males =27.8 degrees C) in a linear thigmothermal gradient when seclusion sites and water existed. With some published empirical models of digestion associated factors for this snake, we calculated the average rate (E(net)) and efficiency (K(net)) of net energy gain from possible combinations of meal size, activity level, and feeding frequency at each temperature. The simulations consistently revealed that E(net) maximizes at the T(p) of these snakes. Although the K(net) peaks at a lower temperature than E(net), the value of K(net) remains high (>=0.85 in ratio to maximum) at the peak temperature of E(net). This suggested that the demands of both E(net) and K(net) can be attained by postprandial thermal selection in this snake. In conclusion, the data support our prediction that postprandial thermal selection may maximize net energy gain.

Specific dynamic action: a review of the postprandial metabolic response

For more than 200 years, the metabolic response that accompanies meal digestion has been characterized, theorized, and experimentally studied. Historically labeled "specific dynamic action" or "SDA", this physiological phenomenon represents the energy expended on all activities of the body incidental to the ingestion, digestion, absorption, and assimilation of a meal. Specific dynamic action or a component of postprandial metabolism has been quantified for more than 250 invertebrate and vertebrate species. Characteristic among all of these species is a rapid postprandial increase in metabolic rate that upon peaking returns more slowly to prefeeding levels. The average maximum increase in metabolic rate stemming from digestion ranges from a modest 25% for humans to 136% for fishes, and to an impressive 687% for snakes. The type, size, composition, and temperature of the meal, as well as body size, body composition, and several environmental factors (e.g., ambient temperature and gas concentration) can each significantly impact the magnitude and duration of the SDA response. Meals that are large, intact or possess a tough exoskeleton require more digestive effort and thus generate a larger SDA than small, fragmented, or soft-bodied meals. Differences in the individual effort of preabsorptive (e.g., swallowing, gastric breakdown, and intestinal transport) and postabsorptive (e.g., catabolism and synthesis) events underlie much of the variation in SDA. Specific dynamic action is an integral part of an organism's energy budget, exemplified by accounting for 19-43% of the daily energy expenditure of free-ranging snakes. There are innumerable opportunities for research in SDA including coverage of unexplored taxa, investigating the underlying sources, determinants, and the central control of postprandial metabolism, and examining the integration of SDA across other physiological systems.

Metabolic and digestive response to food ingestion in a binge-feeding lizard, the Gila monster (Heloderma suspectum)

The gastrointestinal tract possesses the capacity to change in form and function in response to fasting and feeding. Such plasticity can be dramatic for species that naturally experience long episodes of fasting between large meals (e.g. sit-and-wait foraging snakes, estivating anurans). By contrast, for active foraging species that feed more frequently on smaller meals, gastrointestinal responses are more modest in magnitude. The Gila monster Heloderma suspectum is an active foraging lizard that feeds infrequently on meals weighing up to one-third of its body mass. Additionally, Gila monsters possess a species-specific salivary peptide, exendin-4, which may be involved in the regulation of metabolic and digestive performance. To investigate the adaptive postprandial response of Gila monsters and the potential regulatory role of exendin-4, we measured metabolic and intestinal responses to feeding in the presence or absence of circulating exendin-4. Following the consumption of rodent or egg meals equivalent to 10% of lizard body mass, metabolic rates peaked at 4.0- to 4.9-fold of standard metabolic rates and remained elevated for 5-6 days. Specific dynamic action of these meals (43-60 kJ) was 13-18% of total meal energy. Feeding triggered significant increases in mucosal mass, enterocyte width and volume, and the upregulation of D-glucose uptake rates and aminopeptidase-N activity. Total intestinal uptake capacity for L-leucine, L-proline and D-glucose were significantly elevated within 1-3 days after feeding. Whereas the absence of circulating exendin-4 had no impact on postprandial metabolism or the postprandial response of intestinal structure and nutrient uptake, it significantly increased intestinal aminopeptidase-N activity. Within the continuum of physiological responses to feeding and fasting, Gila monsters occupy an intermediate position in experiencing moderate, though significant, regulation of intestinal performance with feeding.

Adaptive regulation of digestive performance in the genus Python

The adaptive interplay between feeding habits and digestive physiology is demonstrated by the Burmese python, which in response to feeding infrequently has evolved the capacity to widely regulate gastrointestinal performance with feeding and fasting. To explore the generality of this physiological trait among pythons, we compared the postprandial responses of metabolism and both intestinal morphology and function among five members of the genus Python: P. brongersmai, P. molurus, P. regius, P. reticulatus and P. sebae. These infrequently feeding pythons inhabit Africa, southeast Asia and Indonesia and vary in body shape from short and stout (P. brongersmai) to long and slender (P. reticulatus). Following the consumption of rodent meals equaling 25% of snake body mass, metabolic rates of pythons peaked at 1.5 days at levels 9.9- to 14.5-fold of standard metabolic rates before returning to prefeeding rates by day 6-8. Specific dynamic action of these meals (317-347 kJ) did not differ among species and equaled 23-27% of the ingested energy. For each species, feeding triggered significant upregulation of intestinal nutrient transport and aminopeptidase-N activity. Concurrently, intestinal mass doubled on average for the five species, in part due to an 85% increase in mucosal thickness, itself a product of 27-59% increases in enterocyte volume. The integrative response of intestinal functional upregulation and tissue hypertrophy enables each of these five python species, regardless of body shape, to modulate intestinal performance to meet the demands of their large infrequent meals.

Metabolic and blood gas dependence on digestive state in the Savannah monitor lizard Varanus exanthematicus: an assessment of the alkaline tide

A large alkaline tide (up to 20 mmol l(-1) increase in bicarbonate concentration [HCO3-] with an accompanied increase in blood pH) has previously been reported for some carnivorous reptiles within 24 h after ingesting a large meal. This phenomenon has been attributed to the secretion of large amounts of H+ ions into the stomach, which is required for digestion of large prey items. To test the generality of this phenomenon in carnivorous reptiles, this study quantified the metabolic and acid-base status of the Savannah monitor lizard, Varanus exanthematicus, during digestion at 35 degrees C. Following a meal of approximately 10% of body mass, V(O2) and V(CO2) were measured continuously and arterial pH, blood gases and strong ions were measured every 8 h for 5 days. During peak digestion (24 h post feeding), V(O2) and V(CO2) increased to approximately threefold fasting values (V(O2), 0.95-2.57 ml min(-1) kg(-1); V(CO2) 0.53-1.63 ml min(-1) kg(-1)) while respiratory exchange ratio (R) remained constant (0.62-0.73). During digestion, arterial P(CO2) increased (from 4.6 kPa to 5.8 kPa), and [HCO3-] also increased (from 24.1 mmol l(-1) to 40.3 mmol l(-1)). In contrast to early studies on crocodilians, arterial pH in V. exanthematicus remained relatively stable during digestion (7.43-7.56). Strong ions contributed little to the acid-base compensation during the alkalosis. Collectively the data indicate that the metabolic alkalosis associated with H+ secretion (as indicated by increased plasma bicarbonate) is partially compensated by a respiratory acidosis.

Specific dynamic action of ambystomatid salamanders and the effects of meal size, meal type, and body temperature

The past decade has witnessed a dramatic increase in studies of amphibian and reptile specific dynamic action (SDA). These studies have demonstrated that SDA, the summed energy expended on meal digestion and assimilation, is affected significantly by meal size, meal type, and body size and to some extent by body temperature. While much of this attention has been directed at anuran and reptile SDA, we investigated the effects of meal size, meal type, and body temperature on the postprandial metabolic responses and the SDA of the tiger salamander (Ambystoma tigrinum tigrinum). We also compared the SDA responses among six species of Ambystoma salamanders representing the breadth of Ambystoma phylogeny. Postprandial peaks in VO(2) and VO(2), duration of elevated metabolism, and SDA of tiger salamanders increased with the size of cricket meals (2.5%-12.5% of body mass). For A. tigrinum, as for other ectotherms, a doubling of meal size results in an approximate doubling of SDA, a function of equal increases in peak VO(2) and duration. For nine meal types of equivalent size (5% of body mass), the digestion of hard-bodied prey (crickets, superworms, mealworms, beetles) generated larger SDA responses than the digestion of soft-bodied prey (redworms, beetle larvae). Body temperature affected the profile of postprandial metabolism, increasing the peak and shortening the duration of the profile as body temperature increased. SDA was equivalent among three body temperatures (20 degrees, 25 degrees, and 30 degrees C) but decreased significantly at 15 degrees C. Comparatively, the postprandial metabolic responses and SDA of Ambystoma jeffersonianum, Ambystoma maculatum, Ambystoma opacum, Ambystoma talpoideum, Ambystoma texanum, and the conspecific Ambystoma tigrinum mavortium digesting cricket meals that were 5% of their body mass were similar (independent of body mass) to those of A. t. tigrinum. Among the six species, standard metabolic rate, peak postprandial VO(2), and SDA scaled with body mass with mass exponents of 0.72, 0.78, and 1.05, respectively.

Metabolic Response to Feeding inTupinambis merianae: Circadian Rhythm and a Possible Respiratory Constraint

The diurnal tegu lizard Tupinambis merianae exhibits a marked circadian variation in metabolism that is characterized by the significant increase in metabolism during part of the day. These increases in metabolic rate, found in the fasting animal, are absent during the first 2 d after meal ingestion but reappear subsequently, and the daily increase in metabolic rate is added to the increase in metabolic rate caused by digestion. During the first 2 d after feeding, priority is given to digestion, while on the third and following days, the metabolic demands are clearly added to each other. This response seems to be a regulated response of the animal, which becomes less active after food ingestion, rather than an inability of the respiratory system to support simultaneous demands at the beginning of digestion. The body cavity of Tupinambis is divided into two compartments by a posthepatic septum (PHS). Animals that had their PHS surgically removed showed no significant alteration in the postprandial metabolic response compared to tegus with intact PHS. The maximal metabolic increment during digestion, the relative cost of meal digestion, and the duration of the process were virtually unaffected by the removal of the PHS.

Specific dynamic action: a century of investigation

Specific dynamic action (SDA) is the term used to refer to the increased metabolic expenditure that occurs in postprandial animals. Postprandial increases in metabolism were first documented in animals over two hundred years ago, and have since been observed in every species thus far examined. Ironically, the ubiquity of this physiological response to feeding understates its complex nature. This review is designed to summarize both classical and modern hypotheses regarding the causality of SDA as well as to review important findings from the past century of scientific research into SDA. A secondary aim of this work is to emphasize the importance of carefully designed experiments and systematic hypothesis testing to make more rapid progress in understanding the physiological processes that contribute to SDA. I also identify three areas in SDA research that deserve more detailed investigation. The first area is identification of the causality of SDA in 'model' organisms. The second area is characterization of SDA responses in novel species. The third area is exploration of the ecological and potential evolutionary significance of SDA in energy budgets of animals.

Metabolic response to feeding in the Chinese striped-necked turtle, Ocadia sinensis

We measured oxygen consumption in juvenile Chinese striped-necked turtles (Ocadia sinensis) after they ingested food, either as a single meal or as double meals, to examine the influence of meal type and feeding frequency on specific dynamic action (SDA). Temporal variation in oxygen consumption after feeding was evident in the ingesting turtles but not in the unfed control turtles. In the single-meal experiment, the peak metabolic rate and the integrated SDA response (the whole energetic cost for the processes of digestion) both did not differ between turtles ingesting mealworms and shrimps when the influence of variation in ingested energy was removed, and the time to reach peak metabolic rate was not affected by meal type and the amount of food ingested. Turtles in the double-meal experiment ingested more energy and hence had a prolonged duration of SDA response than did those in the single-meal experiment, but the integrated SDA response did not differ between both experimental treatments when the influence of variation in ingested energy was removed. Our results show that meal type and feeding frequency have important consequences on the SDA response of juvenile O. sinensis. As the integrated SDA response remained remarkably constant either between turtles ingesting different food or between turtles ingesting the same food but at different frequencies when the influence of variation in ingested energy was removed, we therefore conclude that the energetic cost associated with ingestion is primarily determined by energy content of food ingested in juvenile O. sinensis.

Effects of prey type on specific dynamic action, growth, and mass conversion efficiencies in the horned frog, Ceratophrys cranwelli

To be most energetically profitable, predators should ingest prey with the maximal nutritional benefit while minimizing the cost of processing. Therefore, when determining the quality of prey items, both the cost of processing and nutritional content must be considered. Specific dynamic action (SDA), the increase in metabolic rate associated with feeding in animals, is a significant processing cost that represents the total cost of digestion and assimilation of nutrients from prey. We examined the effects of an invertebrate diet (earthworms) and a vertebrate diet (newborn mice) on mass conversion efficiencies, growth, and SDA in the Chacoan horned frog, Ceratophrys cranwelli. We found the earthworm diet to be significantly lower in lipid, protein, and energy content when compared to the diet of newborn mice. Growth and mass conversion efficiencies were significantly higher in frogs fed newborn mice. However, mean SDA did not differ between frogs fed the two diets, a finding that contradicts many studies that indicate SDA increases with the protein content of the meal. Together, our results indicate that future studies evaluating the effect of meal type on bioenergetics of herpetofauna are warranted and may provide significant insight into the underlying factors driving SDA.

The effect of meal composition on specific dynamic action in burmese pythons (Python molurus)

We quantified the specific dynamic action (SDA) resulting from the ingestion of various meal types in Burmese pythons (Python molurus) at 30 degrees C. Each snake was fed a series of experimental meals consisting of amino acid mixtures, simple proteins, simple or complex carbohydrates, or lipids as well as meals of whole animal tissue (chicken breast, beef suet, and mouse). Rates of oxygen consumption were measured for approximately 4 d after feeding, and the increment above standard metabolic rate was determined and compared to energy content of the meals. While food type (protein, carbohydrate, and lipid) had a general influence, SDA was highly dependent on meal composition (i.e., amino acid composition and carbohydrate structure). For chicken breast and simple carbohydrates, the SDA coefficient was approximately one-third the energetic content of the meal. Lard, suet, cellulose, and starch were not digested and did not produce measurable SDA. We conclude that the cost of de novo protein synthesis is an important component of SDA after ingestion of protein meals because (1) simple proteins, such as gelatin and collagen, did not stimulate levels of SDA attained after consumption of complete protein, (2) incomplete mixtures of amino acids failed to elicit the SDA of a complete mixture, and (3) the inhibition of de novo protein synthesis with the drug cycloheximide caused a more than 70% decrease in SDA. Stomach distension and mechanical digestion of intact prey did not cause measurable SDA.

Influence of food type on specific dynamic action of the Chinese skink Eumeces chinensis

We used the Chinese skink (Eumeces chinensis) as an experimental model to study influence of food type on specific dynamic action (SDA) of feeding. Thirty-three adult males collected from a natural population were divided equally into three (one control and two experimental) groups. We starved all skinks at 30 degrees C for 3 days and then provided the experimental skinks with a single meal consisting of either mealworms or meat [the flesh of the bullfrog (Rana catesbeiana)]. Food ingested by skinks of the two experimental groups differed in lipid content and lean dry mass but not in total dry mass and energy. Defecation following feeding occurred slightly earlier in skinks ingesting mealworms (mean=41.7 h) than in those ingesting meat (mean=47.7 h), but the difference was not significant. Analyses of variance (ANOVAs) with repeated measures showed that temporal variation in oxygen consumption over 72 h after feeding was evident in the experimental skinks but not in the control ones. Oxygen consumption was higher in the experimental skinks than in the control ones during the time interval between 4.5 and 36 h after feeding. The peak metabolic rate was greater but occurred later in skinks ingesting meat than in those ingesting mealworms. The estimated amounts of oxygen consumed by mealworm-fed, meat-fed and unfed skinks at 30 degrees C over 72 h after feeding were 356.5, 393.8 and 295.2 mL, respectively. Our results provide a support for the previous prediction that SDA is affected by types of food ingested by animals as skinks ingesting mealworms and meat differed in the time to reach a peak metabolic rate, the level of the peak metabolic rate and the magnitude of the SDA effect.

Eat and run: prioritization of oxygen delivery during elevated metabolic states

The principal function of the cardiopulmonary system is the matching of oxygen and carbon dioxide transport to the metabolic requirements of different tissues. Increased oxygen demands (VO2), for example during physical activity, result in a rapid compensatory increase in cardiac output and redistribution of blood flow to the appropriate skeletal muscles. These cardiovascular changes are matched by suitable ventilatory increments. This matching of cardiopulmonary performance and metabolism during activity has been demonstrated in a number of different taxa, and is universal among vertebrates. In some animals, large increments in aerobic metabolism may also be associated with physiological states other than activity. In particular, VO2 may increase following feeding due to the energy requiring processes associated with prey handling, digestion and ensuing protein synthesis. This large increase in VO2 is termed "specific dynamic action" (SDA). In reptiles, the increase in VO2 during SDA may be 3-40-fold above resting values, peaking 24-36 h following ingestion, and remaining elevated for up to 7 days. In addition to the increased metabolic demands, digestion is associated with secretion of H+ into the stomach, resulting in a large metabolic alkalosis (alkaline tide) and a near doubling in plasma [HCO3-]. During digestion then, the cardiopulmonary system must meet the simultaneous challenges of an elevated oxygen demand and a pronounced metabolic alkalosis. This paper will compare and contrast the patterns of cardiopulmonary response to similar metabolic increments in these different physiological states (exercise and/or digestion) in a variety of reptiles, including the Burmese python, Python morulus, savannah monitor lizard, Varanus exanthematicus, and American alligator Alligator mississipiensis.

Cost of ventilation and effect of digestive state on the ventilatory response of the tegu lizard

We performed simultaneous measurements of ventilation, oxygen uptake and carbon dioxide production in the South American lizard, Tupinambis merianae, equipped with a mask and maintained at 25 degrees C. Ventilation of resting animals was stimulated by progressive exposure to hypercapnia (2, 4 and 6%) or hypoxia (15, 10, 8 and 6%) in inspired gas mixture. This was carried out in both fasting and digesting animals. The ventilatory response to hypercapnia and hypoxia were affected by digestive state, with a more vigorous ventilatory response in digesting animals compared to fasting animals. Hypoxia doubled total ventilation while hypercapnia led to a four-fold increase in total ventilation both accomplished through an increase in tidal volume. Oxygen uptake remained constant during all hypercapnic exposures while there was an increase during hypoxia. Cost of ventilation was estimated to be 17% during hypoxia but less than 1% during hypercapnia. Our data indicate that ventilation can be greatly elevated at a small energetic cost.

Intersexual differences in energy expenditure of Anolis carolinensis lizards during breeding and postbreeding seasons

Although the amount of energy that males and females invest in reproduction is an integral component of theories explaining the evolution of particular mating strategies, few studies have actually determined the amount of energy that each sex allocates to reproduction. We compared how energy is expended by male and female Anolis carolinensis lizards during both the breeding and postbreeding seasons. We used laboratory respirometry to determine resting metabolic rates (RMRs) of inactive, freshly captured lizards and the doubly labeled water technique to determine field metabolic rates (FMRs) of free-ranging lizards. Both RMRs and FMRs were influenced by body mass but not by sex. Season did not influence FMRs; however, RMRs of both sexes increased approximately 40% from the breeding to the postbreeding season. The seasonal increase in RMRs was attributed to a postreproductive increase in feeding rate and specific dynamic action. We used RMRs, FMRs, and thermal profiles of lizards to calculate energy budgets for breeding and postbreeding seasons. Energy budgets partitioned daily field energy (DFE; calculated from FMRs) into daily activity energy (DAE) and daily resting energy (DRE; calculated from RMRs). Energy expended for reproduction was estimated as DAE during the breeding season plus egg production (for females). Despite males having 40% greater body mass, females expended 46% more energy for reproduction than did males (906 and 619 J/d, respectively). Total metabolizable energy (TME=DFE+egg production for females) expended during the breeding season was similar for males and females (1,280 and 1,365 J/d, respectively). Although TME of females decreased 44% from the breeding to the postbreeding season (1,365 vs. 766 J/d), TME of males was similar during both seasons (1,280 vs. 1,245 J/d). There were both seasonal and sexual differences in DRE and DAE. Compared with most lizards from semiarid/desert habitats, A. carolinensis in a temperate habitat expends more total energy during the breeding season, allocates more energy to eggs, and appears to have more total energy available for reproduction.

Hysteresis of heart rate and heat exchange of fasting and postprandial savannah monitor lizards (Varanus exanthematicus)

Reptiles are ectothermic, but regulate body temperatures (T(b)) by behavioural and physiological means. Body temperature has profound effects on virtually all physiological functions. It is well known that heating occurs faster than cooling, which seems to correlate with changes in cutaneous perfusion. Increased cutaneous perfusion, and hence elevated cardiac output, during heating is reflected in an increased heart rate (f(H)), and f(H), at a given T(b), is normally higher during heating compared to cooling ('hysteresis of heart rate'). Digestion is associated with an increased metabolic rate. This is associated with an elevated f(H) and many species of reptiles also exhibited a behavioural selection of higher T(b) during digestion. Here, we examine whether digestion affects the rate of heating and cooling as well as the hysteresis of heart rate in savannah monitor lizards (Varanus exanthematicus). Fasting lizards were studied after 5 days of food deprivation while digesting lizards were studied approximately 24 h after ingesting dead mice that equalled 10% of their body mass. Heart rate was measured while T(b) increased from 28 to 38 degrees C under a heat lamp and while T(b) decreased during a subsequent cooling phase. The lizards exhibited hysteresis of heart rate, and heating occurred faster than cooling. Feeding led to an increased f(H) (approximately 20 min(-1) irrespective of T(b)), but did not affect the rate of temperature change during heating or cooling. Therefore, it is likely that the increased blood flows during digestion are distributed exclusively to visceral organs and that the thermal conductance remains unaffected by the elevated metabolic rate during digestion.

Energetic cost of a meal in a frequent feeding lizard

Specific dynamic action (SDA) describes the rise in metabolism following feeding in animals and represents the energetic cost of digesting and assimilating a meal. The overall energetic cost of feeding may depend on whether or not an animal is post-absorptive at the time of feeding. The aim of this study was to compare the energetic cost of SDA due to feeding frequently compared with infrequently in the eastern water skink, Eulamprus quoyii. For similar quantities of food, repeated feeding incurred an energetic cost equal to 8.8% of the metabolizable energy of the meal (25,220 J), while single feeding incurred an energetic cost of 9.4% of the metabolizable energy of the meal (26,072 J). Experimental lizards maintained a rise in (VO2) that was on average 1.8 times greater than the (VO2) of the unfed controls over a 50-h interval as a result of feeding frequently. This prolonged rise in metabolism resulting from frequent feeding does not result in a higher energetic cost of SDA compared with that resulting from infrequent single feeding.

Effects of body mass, meal size, fast length, and temperature on specific dynamic action in the timber rattlesnake (Crotalus horridus)

Detailed analysis of animal energy budgets requires information on the cost of digestion (specific dynamic action [SDA]), which can represent a significant proportion of ingested energy (up to 30% in infrequent feeders). We studied the effects of snake mass, temperature (25 degrees and 30 degrees C), fasting time (1 and 5 mo), and prey size (10%-50% of snake mass) on SDA in 26 timber rattlesnakes (Crotalus horridus). We used flow-through respirometry to measure hourly CO(2) production rates (VCO2) for 1 d before and up to 17 d after feeding. Crotalus horridus, like previously studied viperids and boids, show large and ecologically relevant increases in metabolism due to feeding. Depending on treatment and individual, VCO2 increased to 2.8-11.8 times the resting metabolic rate within 12-45 h postfeeding and decreased to baseline within 4.3-15.4 d. Significant effects of snake mass, meal mass, and fast length were detected. Increased temperature decreased the time required to complete the process but had little effect on total energy expended on SDA. Energy expended on SDA increased with increasing fast length, snake mass, and prey mass. Considering all of our data, we found that a simple allometric relationship explained 96.7% of the variation in total CO(2) production during SDA. Calculations suggest that energy devoted to SDA may approach 20% of the total annual energy budget of snakes in nature. Discrepancies between our data and some previous studies draw attention to the fact that the measurement, expression, and analysis of SDA may be sensitive to several methodological and statistical assumptions.

Effects of meal size, meal type, body temperature, and body size on the specific dynamic action of the marine toad, Bufo marinus

Specific dynamic action (SDA), the accumulated energy expended on all physiological processes associated with meal digestion, is strongly influenced by features of both the meal and the organism. We assessed the effects of meal size, meal type, body temperature, and body size on the postprandial metabolic response and calculated SDA of the marine toad, Bufo marinus. Peak postprandial rates of O(2) consumption (.V(O2)) and CO(2) production (.V(CO2)) and SDA increased with meal size (5%-20% of body mass). Postprandial metabolism was impacted by meal type; the digestion of hard-bodied superworms (Zophobas larva) and crickets was more costly than the digestion of soft-bodied earthworms and juvenile rats. An increase in body temperature (from 20 degrees to 35 degrees C) altered the postprandial metabolic profile, decreasing its duration and increasing its magnitude, but did not effect SDA, with the cost of meal digestion remaining constant across body temperatures. Allometric mass exponents were 0.69 for standard metabolic rate, 0.85 for peak postprandial .V(O2), and 1.02 for SDA; therefore, the factorial scope of peak postprandial .V(O2) increased with body mass. The mass of nutritive organs (stomach, liver, intestines, and kidneys) accounted for 38% and 20% of the variation in peak postprandial .V(O2) and SDA, respectively. Toads forced to exercise experienced 25-fold increases in .V(O2) much greater than the 5.5-fold increase experience during digestion. Controlling for meal size, meal type, and body temperature, the specific dynamic responses of B. marinus are similar to those of the congeneric Bufo alvarius, Bufo boreas, Bufo terrestris, and Bufo woodhouseii.

Structural flexibility of the small intestine and liver of garter snakes in response to feeding and fasting

Garter snakes Thamnophis sirtalis parietalis feed frequently but also tolerate extended periods of fasting when food is unavailable. We studied the dynamics, reversibility and repeatability of size changes of the small intestine and liver using ultrasonography. We employed light and transmission electron microscopy and flow cytometry to study the tissue mechanism that drives this flexibility. We compared garter snakes that fed every other day,snakes that fed once a week and fasting snakes. In all feeding trials, the size of the small intestine and the liver increased rapidly after feeding. Constantly feeding snakes maintained an elevated level of organ size, while snakes that were fed only once a week showed a marked up- and downregulation of organ size. Histology revealed the mucosal epithelium to be a transitional epithelium that can change cell configuration considerably to accommodate organ size changes. Upregulation of small intestine and liver size was always associated with the incorporation of lipid droplets into enterocytes and hepatocytes. Cell proliferation was not involved in upregulation of organ size. In contrast, cell proliferation increased during downregulation of organ size, indicating that cells worn out during digestion were replaced. The dynamics of flexibility and the functional features of the tissue were the same as described for the Burmese python Python molurus bivittatus. We suggest that garter snakes employ the same energetically cheap mechanism of organ size regulation as pythons, which allows for rapid, repeated and reversible size changes with no cell proliferation involved. Comparative evidence suggests that the transitional mucosal epithelium is an ancestral character of snakes and that feeding ecology is not directly related to the cytological features of the mucosal epithelium.

Oxygen consumption and the energetics of island-dwelling Florida cottonmouth snakes

We measured oxygen consumption (Vo(2)) to estimate standard metabolic rates (SMR) in cottonmouth snakes (Agkistrodon piscivorus conanti) from Seahorse Key and the adjacent peninsula of northern Florida. The island population is unusual because adult snakes feed on fish that are dropped by colonial nesting birds, and food resources are temporally limited relative to that of mainland populations. We found no differences in SMR between island and mainland snakes at any of four experimental temperatures (15 degrees -30 degrees C), suggesting that any adjustments to energy limitations involve other aspects of physiology or behavior. As with other viperid species, the SMR of cottonmouths is about one-half of that expected from interspecific allometric regressions previously reported for snakes generally. Allometric mass exponents of SMR averaged 0.76 and were not affected by temperature. We found that Vo(2) increased with temperature (Q(10) = 2.4-2.8) and was elevated 29% during scotophase compared with photophase. Neonates exhibited elevated Vo(2)compared with older juveniles of similar size, apparently due to assimilation of yolk that is present in the neonatal gut. In adult snakes, specific dynamic action (SDA) following feeding resulted in four- to eightfold increases in Vo(2), with magnitude and duration related positively to relative meal size. The total energy devoted to SDA increased with meal size and averaged 32.8%+/-4.4% of total ingested energy. We estimate that a nonreproductive 500-g adult cottonmouth at Seahorse Key uses 3,656 kJ of assimilated energy annually for maintenance and activity, which requires ingestion of approximately 1 kg of fish.

Regulation of digestive performance: a proposed adaptive response

Among snakes a correlation exists between feeding habits (frequent or infrequent) and the magnitude by which digestive performance is regulated (modest or large). This paper investigates whether the observed regulation of digestive performance is an adaptation to feeding habits and therefore, a product of natural selection. Using data on metabolic and intestinal responses to feeding for amphibians and reptiles, it is attempted to show the selective advantage and independent origin of either modestly or widely regulating gut performance. In an energetic model, snakes that naturally feed frequently on small meals benefit (from lower energy output) from modestly regulating gut performance as opposed to widely regulating gut performance. Likewise, the model suggests an energetic benefit for infrequently-feeding snakes secondary to the wide regulation of gut performance. This benefit is a function of long spans of fasting with a down-regulated gut (thereby incurring a lower standard metabolic rate) and the occasionally incursion of a costly up-regulation of the gut. In a comparison across several distantly-related lineages of amphibians and reptiles, frequently-feeding species all exhibit small postprandial responses in metabolism and intestinal nutrient transport capacities. In contrast, frogs and snakes that routinely fast for long periods independently experience five- to 30-fold increases in metabolism and intestinal performance with feeding. Among amphibians and reptiles the evidence presented supports the hypothesis that the extent by which the gut is regulated is an adaptive trait that evolved with divergence in feeding habits and energy budgets. In finishing, the foundations, caveats, and suggested future tests of this adaptive hypothesis are presented.

Evolution of regulatory responses to feeding in snakes

Do animal species that normally consume large meals at long intervals evolve to down-regulate their metabolic physiology while fasting and to up-regulate it steeply on feeding? To test this hypothesis, we compared postfeeding regulatory responses in eight snake species: four frequent feeders on small meals and four infrequent feeders on large meals. For each species, we measured factorial changes in metabolic rate, in activities and capacities of five small intestinal brush border nutrient transporters, and in masses of eight organs that function in nutrient processing after consumption of a rodent meal equivalent to 25% of the snake's body mass. It turned out that, compared with frequent feeders, infrequent feeders digest that meal more slowly; have lower metabolic rates, organ masses, and nutrient uptake rates and capacities while fasting; have higher energy expenditure during digestion; and have higher postfeeding factorial increases in metabolic rate, organ masses, and nutrient uptake rates and capacities. These conclusions, which conform to the hypothesis mentioned above, remain after phylogeny has been taken into account. The small organ masses and low nutrient transporter activities during fasting contribute to the low fasting metabolism of infrequent feeders. Quantitative calculations of partial energy budgets suggest that energy savings drive the evolution of low mass and activities of organs during fasting and of large postfeeding regulatory responses in infrequent feeders. We propose further tests of this hypothesis among other snake species and among other ectotherms.

Effects of feeding on metabolism, gas transport, and acid-base balance in the bullfrog Rana catesbeiana

Massive feeding in ectothermic vertebrates causes changes in metabolism and acid-base and respiratory parameters. Most investigations have focused on only one aspect of these complex changes, and different species have been used, making comparison among studies difficult. The purpose of the present study was, therefore, to provide an integrative study of the multiple physiological changes taking place after feeding. Bullfrogs (Rana catesbeiana) partly submerged in water were fed meals (mice or rats) amounting to approximately (1)/(10) of their body weight. Oxygen consumption increased and peaked at a value three times the predigestive level 72-96 h after feeding. Arterial PO(2) decreased slightly during digestion, whereas hemoglobin-bound oxygen saturation was unaffected. Yet, arterial blood oxygen content was pronouncedly elevated because of a 60% increase in hematocrit, which appeared mediated via release of red blood cells from the spleen. Gastric acid secretion was associated with a 60% increase in plasma HCO3(-) concentration ([HCO3(-)]) 48 h after feeding. Arterial pH only increased from 7.86 to 7.94, because the metabolic alkalosis was countered by an increase in PCO(2) from 10.8 to 13.7 mm Hg. Feeding also induced a small intracellular alkalosis in the sartorius muscle. Arterial pH and HCO3(-) returned to control values 96-120 h after feeding. There was no sign of anaerobic energy production during digestion as plasma and tissue lactate levels remained low and intracellular ATP concentration stayed high. However, phosphocreatine was reduced in the sartorius muscle and ventricle 48 h after feeding.

Nutrition of captive reptiles

In reptile practice, most nutritional problems arise from improper husbandry, including poor feeding management and provision of imbalanced diets. Each reptile species has its own requirements for temperature and humidity, space and social interaction, lighting, and habitat components. Failure, to provide these requirements often results in failure to thrive and secondary nutritional disorders. Common dietary problems include deficiencies of energy, calcium, vitamin D3, vitamin A, and fiber.