Adaptive responses to feeding in Burmese pythons: pay before pumping

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.

Reproductive strategies in snakes

Snakes of both sexes display remarkable flexibility and diversity in their reproductive tactics. Many features of reproduction in female snakes (such as reproductive mode and frequency, seasonality and multiple mating) allow flexible maternal control. For example, females can manipulate not only the genotypes of their offspring (through mate choice or enhanced sperm competition) but also the phenotypes of their offspring (through allocation 'decisions', behavioural and physiological thermoregulation, and nest-site selection). Reliance on stored energy ('capital') to fuel breeding results in low frequencies of female reproduction and, in extreme cases, semelparity. A sophisticated vomeronasal system not only allows male snakes to locate reproductive females by following scent trails, but also facilitates pheromonally mediated mate choice by males. Male-male rivalry takes diverse forms, including female mimicry and mate guarding; combat bouts impose strong selection for large body size in males of some species. Intraspecific (geographical) variation and phenotypic plasticity in a wide array of reproductive traits (offspring size and number; reproductive frequency; incidence of multiple mating; male tactics such as mate guarding and combat; mate choice criteria) provide exceptional opportunities for future studies.

Changes in the rate of CO2 release following feeding in the insect Rhodnius prolixus

We describe for the first time changes in the rate of CO2 release (as a surrogate of metabolic rate) in the terminal larval stage of the insect Rhodnius prolixus following a blood meal and during the molt leading to the adult stage. These data are presented on a whole-animal basis as well as per gram wet and dry weight. We have also used techniques that allow us to describe the rate of release per gram of actual body tissue (i.e., removing the weight of the remaining bloodmeal in the gut and the metabolically inactive portion of the cuticle). While the metabolic rate of the whole animal rises approximately 10-fold in 15 d following feeding, the rate per gram of dry body mass rises only twofold. We use these data to provide insights into the relative contributions of tissue growth and increases in metabolic intensity to the massive increases in metabolic rate observed in these insects following feeding. Our analyses indicate that the majority of nutrient uptake occurs in the first 4 d following feeding. It is well known in this species that day 4 following feeding is the end of a critical period for the insect in determining whether it will proceed to the next molt. Our results indicate that the insects may be able to make this decision based on nutrients already transported into the body. We examined the "down regulation" of metabolism observed in the latter stages of the molt cycle in this insect. We express these changes on both a per animal and per gram basis and demonstrate that this down regulation extends even into the adult stage before feeding. Using a comparison of the allometric relationships of metabolic rate to mass in insects and ticks, we demonstrate that unfed R. prolixus show a marked decrease in metabolic rate compared to other insects, while fed Rhodnius are similar in metabolic rate to other insects. Rhodnius has a markedly higher metabolic rate (as do all insects) than that found in ticks.

Gastric function and its contribution to the postprandial metabolic response of the Burmese python Python molurus

The large intact prey ingested by Burmese pythons require considerable processing by the stomach before passage into the small intestine. To investigate the function and cost of gastric digestion and its contribution to postprandial metabolic response for the Burmese python, I examined the rate of gastric digestion, the postprandial profile of gastric pH and the effects of decreasing gastric workload on the metabolic cost of digestion, referred to as specific dynamic action (SDA). Ingested meal mass (equivalent to 25% of snake body mass) was reduced by 18% within 1 day postfeeding, by which time intragastric pH had decreased from 7.5 to 2. Gastric pH was maintained at 1.5 for the next 5-7 days, after which it returned to 7.5. The SDA generated by digesting an intact rat meal was reduced by 9.1%, 26.0%, 56.5% and 66.8%, respectively, when pythons were fed steak, ground rat, liquid diet or ground rat directly infused into the small intestine. The production of HCl and enzymes and other gastric functions represent an estimated 55% of the python's SDA generated from the digestion of an intact rodent meal. Additional contributors to SDA include protein synthesis (estimated 26%), gastrointestinal upregulation (estimated 5%) and the activities of the pancreas, gallbladder, liver, kidneys and intestines during digestion (estimated 14%). Operating on a 'pay before pumping' principle, pythons must expend endogenous energy in order to initiate acid production and other digestive processes before ingested nutrients can be absorbed and channeled into metabolic pathways.

Temperature and meal size effects on the postprandial metabolism and energetics in a boid snake

We investigated the combined effect of meal size and temperature on the aerobic metabolism and energetics of digestion in Boa constrictor amarali. Oxygen uptake rates (Vd2;o2) and the duration of the digestion were determined in snakes fed with meals equaling to 5%, 10%, 20%, and 40% of the snake's body mass at 25 degrees and 30 degrees C. The maximum Vd2;o2 values attained during digestion were greater at 30 degrees C than at 25 degrees C. Both maximal Vd2;o2 values and the duration of the specific dynamic action (SDA) were attained sooner at 30 degrees C than at 25 degrees C. Therefore, the temperature effect on digestion in Boa is characterized by the shortening of the SDA duration at the expense of increased Vd2;o2. Energy allocated to SDA was not affected by meal size but was greater at 25 degrees C compared to 30 degrees C. This indicates that a postprandial thermophilic response can be advantageous not only by decreasing the duration of digestion but also by improving digestive efficiency. Maximal Vd2;o2 and SDA duration increased with meal size at both temperatures.

Luminal nutrient signals for intestinal adaptation in pythons

Python intestine responds rapidly to luminal nutrients by increasing mass and upregulating nutrient transport. Candidates for luminal signals triggering those responses include mechanical stimulation, single or several dietary nutrients, and endogenous secretions. To identify signals, we infused into the python's small intestine either a nonnutrient solution (saline) or a single- or multinutrient solution. Python intestine failed to respond trophically or functionally to luminal infusions of saline, glucose, lipid, or bile. Infusion of amino acids and peptides, with or without glucose, induced an intermediate response. Infusion of nutritionally complete liquid formula or natural diet induced full intestinal response. Intact meals triggered full intestinal responses without pancreatic or biliary secretions, whereas direct cephalic and gastric stimulation failed to elicit any response. Hence neither physical stimulation (cephalic, gastric, or intestinal) nor the luminal presence of glucose, lipids, or bile can induce intestinal response; instead, a combination of nutrients is required (even without pancreaticobiliary secretions), the most important being amino acids and peptides. This is understandable because pythons, as carnivores, have a high-protein diet.

Increased blood oxygen affinity during digestion in the snakePython molurus

Many snakes exhibit large increases in metabolic rate during digestion that place extensive demands on efficient oxygen transport. In the present study,we describe blood oxygen affinity following three weeks of fasting and 48 h after feeding in the Burmese python (Python molurus). We also report simultaneous measurements of arterial blood gases and haematological parameters. Arterial blood was obtained from chronically implanted catheters,and blood oxygen-dissociation curves were constructed from oxygencontent measurements at known oxygen partial pressure(PO2) values at 2% and 5% CO2. Arterial pH remained constant at approximately 7.6 after feeding, but digestion was associated with an approximately 6 mmol l-1 increase in [HCO3-], while CO2 partial pressure(PCO2) increased from 2.21±0.13 kPa in fasted animals to 2.89±0.17 kPa at 48 h after feeding. Blood oxygen affinity in vivo was predicted on the basis of pH in vivoand the blood oxygen-dissociation curves obtained in vitro. The blood oxygen affinity in vivo increased during digestion, with P50 values decreasing from 4.58±0.11 kPa to 3.53±0.24 kPa. This increase was associated with a significant decrease in the red blood cell [NTP]/[Hb4] ratio (relationship between the concentrations of organic phosphates and total haemoglobin) and a significant decrease in mean cellular haemoglobin content, which is indicative of swelling of the red blood cells. Our data for blood oxygen affinity and arterial oxygen levels, together with previously published values of oxygen uptake and blood flows, allow for a quantitative evaluation of oxygen transport during digestion. This analysis shows that a large part of the increased metabolism during digestion is supported by an increased venous extraction, while arterial PO2(PaO2) and haemoglobin saturation do not vary with digestive status. Thus, we predict that venous PO2 (PvO2) is reduced from a fasting value of 5.2 kPa to 1.6 kPa during digestion.

Can methane suppression during digestion of woody and leafy browse compensate for energy costs of detoxification of plant secondary compounds? A test with muskoxen fed willows and birch

Digestion and metabolism of woody and leafy browse requires detoxification of plant secondary compounds that can incur an energy cost. Browse, however, inhibits methane (CH(4)) production and therefore could offset some costs of detoxification. We measured an index of heat increment of feeding (HIFi) and CH(4) production in muskoxen (Ovibos moschatus) given a single test meal (at 10 g/kg BM(0.75)) composed of hay mixed with one of three browse species (Willow: Salix alaxensis, S. pulchra; Birch: Betula nana). Detoxification cost was estimated as HIFi of browse diet-HIFi of hay diet and CH(4) compensation as CH(4) production of hay diet-CH(4) production of browse diet. CH(4) compensation was noted in 47% of 15 trials in which a detoxification cost was evident; six trials were with woody browse and one with leafy browse. Separate controls were responsible for the difference in CH(4) compensation for leafy browse vs. woody browse. Detoxification costs for twigs and leaves of B. nana were underestimated because of their low digestibility. In only one of six treatments was CH(4) compensation documented for B. nana. We conclude that energy saved by CH(4) suppression was small (<6%) compared with detoxification costs.

Effects of temperature on the metabolic response to feeding in Python molurus

As ectothermic vertebrates, reptiles undergo diurnal and seasonal changes in body temperature, which affect many biological functions. In conjunction with a general review regarding the effects of temperature on digestion in reptiles, we describe the effects of various temperatures (20-35 degrees C) on the metabolic response to digestion in the Burmese python (Python molurus). The snakes were fed mice amounting to 20% of their body weight and gas exchange (oxygen uptake and CO(2) production) were measured until digestion had ended and gas exchange returned to fasting levels. Elevated temperature was associated with a faster and larger metabolic increase after ingestion, and the time required to return to fasting levels was markedly longer at low temperature. The factorial increase between fasting oxygen consumption (VO(2)) and maximal VO(2) during digestion was, however, similar at all temperatures studied. Furthermore, the integrated SDA response was not affected by temperature suggesting the costs associated with digestion are temperature-independent. Other studies on reptiles show that digestive efficiency is only marginally affected by temperature and we conclude that selection of higher body temperatures during digestion (postprandial thermophilic response) primarily reduces the time required for digestion.

Effects of digestive status on the reptilian gut

Reptiles, including the Burmese python, Python molurus bivittatus, that feed at infrequent intervals show a prominent increase in gastrointestinal mass, metabolism and brush border transport rates after feeding. Current knowledge and theories around these phenomena, as well as studies on the innervation of the reptilian gut, are summarised in this review. Little is known about the putative changes in the nervous and humoral control systems of the gut, and it is not known whether feeding affects innervation and motility of the stomach and intestine. Using immunohistochemistry, we have investigated possible up/down regulation of several neurotransmitters in specimens that had been fasted for a minimum of 3 weeks and specimens that had ingested a large meal 2 days before the experiments were conducted. There were no major changes in the innervation by nerves containing calcitonin gene-related peptide (CGRP), galanin, nitric oxide synthase (NOS), pituitary adenylate cyclase-activating polypeptide (PACAP), somatostatin (SOM), substance P/neurokinin A (SP/NKA), or vasoactive intestinal polypeptide (VIP)-like immunoreactivity. Nor did we find any differences in the effect of substance P (stomach and intestine), galanin (intestine), or bradykinin (intestine) on motility in strip preparations from the gut wall. A significant increase in dry weight of the intestine was obtained 48 h after feeding. We conclude that although there are considerable changes in gut thickness and absorptive properties after feeding, the smooth muscle and its control appear little affected.

Intracardiac flow separation in anin situperfused heart from Burmese pythonPython molurus

The heart of non-crocodilian reptiles has two separate atria that receive blood from the systemic and pulmonary circulations. The ventricle is not fully divided, but is compartmentalised into two chambers (cavum dorsale and cavum pulmonale) by a muscular ridge that runs from the apex to the base of the ventricle. The muscular ridge is small in turtles, but is well developed in varanid lizards and many species of snakes. These anatomical differences correlate with an effective blood flow separation in varanid lizards, whereas turtles can exhibit very large cardiac shunts. Very little is known about the cardiac shunt patterns in other groups of reptiles.

Here we characterise cardiac performance and flow dynamics in the Burmese python (Python molurus) using an in situ perfused heart preparation. The pericardium remained intact and the two atria were perfused separately (Ringer solution), and the two systemic and the pulmonary outflows were independently cannulated. Right and left atrial filling pressures and ventricular outflow pressures of the pulmonary and systemic vessels could be manipulated independently, permitting the establishment of large experimental intraventricular pressure gradients across the muscular ridge. The maximal power output generated by the systemic side of the ventricle exceeded the maximal power output that was generated by the cavum pulmonale that perfuse the pulmonary circulation. Furthermore, systemic flow could be generated against a higher outflow pressure than pulmonary flow. Perfusate entering the right atrium was preferentially distributed into the pulmonary circulation,whereas perfusate into the left atrium was distributed to the systemic circulation.

Our study indicates that the well-developed muscular ridge can separate the cavum systemic and pulmonary sides of the heart to prevent mixing of systemic and pulmonary flows. Therefore, the heart of Python appears to exhibit a large degree of ventricular flow separation as previously described for varanid lizards. We speculate that the ventricular separation has evolved in response to the need of maintaining high oxygen delivery while protecting the pulmonary circulation from oedema as result of high vascular pressures.

The effects of fasting duration on the metabolic response to feeding in Python molurus: an evaluation of the energetic costs associated with gastrointestinal growth and upregulation

The oxygen uptake of Python molurus increases enormously following feeding, and the elevated metabolism coincides with rapid growth of the gastrointestinal organs. There are opposing views regarding the energetic costs of the gastrointestinal hypertrophy, and this study concerns the metabolic response to feeding after fasting periods of different duration. Since mass and function of the gastrointestinal organs remain elevated for several days after feeding, the metabolic increment following a second meal given soon after the first can reveal whether the metabolic costs relate to the upregulation of gastrointestinal organs or merely the metabolic cost of processing a meal. Eight juvenile pythons were kept on a regular feeding regime for 6 mo after hatching. At the beginning of the metabolic measurements, they were fed mice (20% of body mass), and the metabolic response to similarly sized meals was determined following 3, 5, 7, 14, 21, 30, and 60 d of fasting. Our data show that the metabolic response following feeding was large, ranging from 21% to 35% of ingested energy (mean=27%), but the metabolic response seems independent of fasting duration. Hence, the extraordinarily large cost of digestion in P. molurus does not appear to correlate with increased function and growth of gastrointestinal organs but must be associated with other physiological processes.

The alkaline tide phenomenon

The parietal cell is capable of secreting high concentrations of hydrochloric acid into the lumen of the stomach. The apical membrane of this cell contains K+H+ ATPase, which is responsible for proton transport into the lumen. Potassium and chloride channels are also present. The basolateral membrane of the parietal cell possesses transporters that maintain intracellular homeostasis. Specifically, large amounts of bicarbonate that are generated by carbonic anhydrase must be removed from the cell to prevent alkalinization. Efflux of bicarbonate into the blood after acid secretion can be detected and is known as the alkaline tide. Determination of the alkaline tide has been used to measure acid secretion. In this review, we summarize the published data.

Quantitative evolutionary design of nutrient processing: glucose

Quantitative evolutionary design involves the numerical relationships, evolved through natural selection, of biological capacities to each other and to natural loads. Here we study the relation of nutrient-processing capacities of the intestine and of organs beyond it (such as liver and kidneys) to each other and to natural loads of nutrients normally consumed. To control experimentally the rate of nutrient delivery to organs beyond the intestine, we administered nutrients directly into the veins of rats by the method of total parenteral nutrition (TPN). Control rats consuming the TPN solution by mouth ingested glucose at 42 mmol/day and processed it completely, as gauged by negligible appearance of glucose in urine and feces. Experimental rats receiving TPN were able to process infused glucose completely at rates up to 92 mmol/day. At higher infusion rates, they were unable to process further glucose, as gauged by rises in serum and urinary glucose levels and serum osmolality. At the highest infusion rates, they exhibited diuresis, dehydration, and both decreased weight gain and survival. These symptoms closely resemble the human diabetic condition known as nonketotic hypertonicity. Thus, a rat's body has a safety factor of 2.2 (=92/42) for glucose processing: it can process glucose at a rate 2.2 times its voluntary intake. This safety factor represents apparent excess capacity that may have evolved to process other nutrients converted into glucose, to minimize the risk of loads swamping capacities, to handle suddenly increased nutrient requirements, or to effect rapid mobilization of glucose.

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.

Responses of python gastrointestinal regulatory peptides to feeding

In the Burmese python (Python molurus), the rapid up-regulation of gastrointestinal (GI) function and morphology after feeding, and subsequent down-regulation on completing digestion, are expected to be mediated by GI hormones and neuropeptides. Hence, we examined postfeeding changes in plasma and tissue concentrations of 11 GI hormones and neuropeptides in the python. Circulating levels of cholecystokinin (CCK), glucose-dependent insulinotropic peptide (GIP), glucagon, and neurotensin increase by respective factors of 25-, 6-, 6-, and 3.3-fold within 24 h after feeding. In digesting pythons, the regulatory peptides neurotensin, somatostatin, motilin, and vasoactive intestinal peptide occur largely in the stomach, GIP and glucagon in the pancreas, and CCK and substance P in the small intestine. Tissue concentrations of CCK, GIP, and neurotensin decline with feeding. Tissue distributions and molecular forms (as determined by gel-permeation chromatography) of many python GI peptides are similar or identical to those of their mammalian counterparts. The postfeeding release of GI peptides from tissues, and their concurrent rise in plasma concentrations, suggests that they play a role in regulating python-digestive responses. These large postfeeding responses, and similarities of peptide structure with mammals, make pythons an attractive model for studying GI peptides.

Autonomic control of heart rate during forced activity and digestion in the snakeBoa constrictor

Reptiles, particularly snakes, exhibit large and quantitatively similar increments in metabolic rate during muscular exercise and following a meal, when they are apparently inactive. The cardiovascular responses are similar during these two states, but the underlying autonomic control of the heart remains unknown. We describe both adrenergic and cholinergic tonus on the heart during rest, during enforced activity and during digestion (24–36 h after ingestion of 30 % of their body mass) in the snake Boa constrictor. The snakes were equipped with an arterial catheter for measurements of blood pressure and heart rate, and autonomic tonus was determined following infusion of the β-adrenergic antagonist propranolol (3 mg kg–1) and the muscarinic cholinoceptor antagonist atropine (3 mg kg–1).

The mean heart rate of fasting animals at rest was 26.4±1.4 min–1, and this increased to 36.1±1.4 min–1 (means ± s.e.m.; N=8) following double autonomic block (atropine and propranolol). The calculated cholinergic and adrenergic tones were 60.1±9.3 % and 19.8±2.2 %, respectively. Heart rate increased to 61.4±1.5 min–1 during enforced activity, and this response was significantly reduced by propranolol (maximum values of 35.8±1.6 min–1), but unaffected by atropine. The cholinergic and adrenergic tones were 2.6±2.2 and 41.3±1.9 % during activity, respectively. Double autonomic block virtually abolished tachycardia associated with enforced activity (heart rate increased significantly from 36.1±1.4 to 37.6±1.3 min–1), indicating that non-adrenergic, non-cholinergic effectors are not involved in regulating heart rate during activity. Blood pressure also increased during activity.

Digestion was accompanied by an increase in heart rate from 25.6±1.3 to 47.7±2.2 min–1 (N=8). In these animals, heart rate decreased to 44.2±2.7 min–1 following propranolol infusion and increased to 53.9±1.8 min–1 after infusion of atropine, resulting in small cholinergic and adrenergic tones (6.0±3.5 and 11.1±1.1 %, respectively). The heart rate of digesting snakes was 47.0±1.0 min–1 after double autonomic blockade, which is significantly higher than the value of 36.1±1.4 min–1 in double-blocked fasting animals at rest. Therefore, it appears that some other factor exerts a positive chronotropic effect during digestion, and we propose that this factor may be a circulating regulatory peptide, possibly liberated from the gastrointestinal system in response to the presence of food.

Scaling of CO2 production in the timber rattlesnake (Crotalus horridus), with comments on cost of growth in neonates and comparative patterns

To understand the bioenergetic fluxes of free-ranging timber rattlesnakes (Crotalus horridus) better, we measured CO(2) production rate of 83 snakes in response to body mass, body temperature, time of day, sex, and geographic locality (northwest Arkansas and coastal Virginia). Effects of body mass, temperature, time of day, and the temperature-by-time interaction were remarkably similar to effects reported for other rattlesnakes. We noted that C. horridus has relatively high, but precedented, Q(10) (3.71-4.78); however, the adaptive significance of this observation, if any, remains obscure. Once the confounding effect of body mass was statistically adjusted, C. horridus exhibited no sex-specific effects; however, there was a significant locality-by-time effect, which is of equivocal biological significance. In contrast to the findings of a recent review on cost of growth in neonatal reptiles, C. horridus neonates exhibited metabolic rates that were from 200% to 400% greater than expectations from the mass scaling of yearlings and older animals. We interpreted this as evidence for a cost of synthesis in growing neonates. We report regression equations for the estimation of resting CO(2) production rate in C. horridus as a function of body mass, body temperature, and time of day. Our data contribute to a growing, comparative database documenting rattlesnakes as low-energy specialists.

Restoration of the jejunal mucosa in rats refed after prolonged fasting

To investigate the importance of body fuel depletion on gut rehabilitation after food deprivation, we compared the kinetics of jejunal mucosa alteration and restoration in rats that were refed after reaching different stages in body fuel depletion. Rats (P2) were refed while still in the so-called phase II, where body protein utilization is minimized, whereas rats (P3) were refed when they had reached the stage of increasing protein utilization (phase III). There was a significant decrease in total mass of intestine (P2, -30%; P3, -40%) and jejunal mucosa (P2, -52%; P3, -60%), as well in the size of the crypts (P2, -15%; P3, -36%) and villi (P2, -37%; P3, -55%). Structural changes of the mucosa included disappearance of some villi and a reduction in the size and number of crypts. Despite the larger morphological alterations in P3, the restoration of mucosa was as fast and complete after only 3 days of refeeding for both P2 and P3 rats. The respective roles of the mitosis pressure and of the lamina propria dynamics were studied. The rapid reversibility of the gut mucosal alterations due to fasting might constitute an integrative process.

Postprandial exercise: prioritization or additivity of the metabolic responses?

Monitor lizards (Varanus exanthematicus) were used to examine the prioritization or additivity of the metabolic responses associated with exercise and digestion, either of which can elevate metabolic rate independently. Rates of oxygen consumption (V̇.O2) and ventilation (V̇.E) were measured in lizards during fasting exercise, postprandial rest and postprandial exercise. In fasting animals, V̇.O2 increased with walking speed to a maximal value of 15.9mlO2kg−1min−1 at 1.25kmh−1. Postprandial resting metabolic rate was elevated significantly above fasting levels (4.1 versus 2.0mlO2kg−1min−1). During postprandial exercise, V̇.O2 increased to a maximal value of 18.8mlO2kg−1min−1 at 1.25kmh−1. At every level of exercise, V̇.O2 was significantly higher in postprandial animals by a similar increment; the maximal rate of oxygen consumption was significantly increased by 18% in postprandial individuals. Maximal V̇.E did not differ in fasting and postprandial animals and, therefore, the greater V̇.O2max of postprandial animals cannot be attributed to a higher ventilation rate. Air convection requirement (V̇.E/V̇.O2) is significantly lower in postprandial animals at rest and at all levels of exercise, indicating a relative hypoventilation and increased pulmonary oxygen extraction efficiency. We suggest that this increased oxygen extraction may be due to decreased cardiopulmonary shunts and/or to lower mixed venous oxygen content. The data unequivocally support an additivity model rather than prioritization models for the allocation of elevated metabolic rate: the postprandial metabolic increment is not suspended during exercise, but rather is added onto the cost of exercise. It is clear that fasting exercise did not elicit truly maximal levels of cardiopulmonary oxygen transport in these animals, indicating problems for design models that make this assumption.

Effect of feeding on circulating micronutrient concentrations in the Burmese python (Python molurus)

Burmese pythons (Python molurus) regulate digestive performance and metabolism with the ingestion of each meal. To explore the python's postprandial responses, we monitored the concentrations of blood micronutrients and homocysteine during fasting and for 15 days after feeding. Plasma folate concentrations peaked with a 270% increase over fasting levels 3 days after feeding, whereas plasma B-12 peaked with a 66% increase within 1 day. Erythrocyte folate concentrations were highest 15 days after feeding with a 44% increase. The major plasma folate was 5-methyltetrahydrofolate during fasting and was non-5-methyltetrahydrofolate during digestion, whereas erythrocytes contained polyglutamyl forms of non-5-methyltetrahydrofolate. Plasma homocysteine concentrations peaked with a 56% increase 3 days after feeding, and were markedly greater than those of mammals. Plasma zinc and copper did not change significantly. Plasma zinc concentrations were 20 times greater than plasma copper and approximately 30 times higher than those of mammals. Pythons showed a significant postprandial decline of 25% in hematocrit. Plasma pyridoxal 5'-phosphate (coenzyme form of vitamin B-6) was not detected probably due to its tight protein binding. Most micronutrient concentrations appear to plateau 3 days after feeding, suggesting that pythons have relatively rapid homeostasis of micronutrients despite the ingestion of large meals.

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.

Structural flexibility of the intestine of Burmese python in response to feeding

The small intestine of Burmese pythons, Python molurus bivittatus, undergoes a remarkable size increase shortly after feeding. We studied the dynamics, reversibility and repeatability of organ size changes using noninvasive imaging techniques. We employed light and electron microscopy, flow cytometry and immunohistology to study the cytological mechanisms that drive the size changes of the small intestine. Within 2 days of feeding, the size of the small intestine increased to up to three times the fasting value. The size changes were fully reversible and could be elicited repeatedly by feeding. These enormous size changes were possible because the mucosal epithelium of the small intestine is a transitional epithelium that allows for considerable size changes without cell proliferation. Histological evidence suggested that a fluid pressure-pump system (lymphatic, blood pressure) was the driving force that inflated the intestinal villi. The rates of cell proliferation were not elevated immediately after feeding but peaked 1 week later when small intestine size was already declining. In contrast to the current paradigm, we suggest that the small intestine is not part of the previously proposed ‘pay-before-pumping’ model. Instead, the size of the python's small intestine may be upregulated without major metabolic investment. It can occur even if the individual is energetically exhausted. An evolutionary perspective of the transitional epithelium mechanism suggests superior functionality compared with the pay-before-pumping model because it allows for long periods of fasting and depletion of energy reserves, while still enabling the snake to digest prey and absorb nutrients.

Luminal and systemic signals trigger intestinal adaptation in the juvenile python

Juvenile pythons undergo large rapid upregulation of intestinal mass and intestinal transporter activities upon feeding. Because it is also easy to do surgery on pythons and to maintain them in the laboratory, we used a python model to examine signals and agents for intestinal adaptation. We surgically isolated the middle third of the small intestine from enteric continuity, leaving its mesenteric nerve and vascular supply intact. Intestinal continuity was restored by an end-to-end anastomosis between the proximal and distal thirds. Within 24 h of the snake's feeding, the reanastomosed proximal and distal segments (receiving luminal nutrients) had upregulated amino acid and glucose uptakes by up to 15-fold, had doubled intestinal mass, and thereby soon achieved total nutrient uptake capacities equal to those of the normal fed full-length intestine. At this time, however, the isolated middle segment, receiving no luminal nutrients, experienced no changes from the fasted state in either nutrient uptakes or in morphology. By 3 days postfeeding, the isolated middle segment had upregulated nutrient uptakes to the same levels as the reanastomosed proximal and distal segments, but it still lacked any appreciable morphological response. These contrasting results for the reanastomosed intestine and for the isolated middle segment suggest that luminal nutrients and/or pancreatic biliary secretions are the agents triggering rapid upregulation of transporters and of intestinal mass and that systemic nerve or hormonal signals later trigger transporter regulation but no trophic response.

An experimental test of the thermoregulatory hypothesis for the evolution of endothermy

The thermoregulatory hypothesis proposes that endothermy in mammals and birds evolved as a thermoregulatory mechanism per se and that natural selection operated directly to increase body temperature and thermal stability through increments in resting metabolic rate. We experimentally tested this hypothesis by measuring the thermoregulatory consequences of increased metabolic rate in resting lizards (Varanus exanthematicus). A large metabolic increment was induced by feeding the animals and consequent changes in metabolic rate and body temperature were monitored. Although metabolic rate tripled at 32 degrees C and quadrupled at 35 degrees C, body temperature rose only about 0.5 degrees C. The rate of decline of body temperature in a colder environment did not decrease as metabolic rate increased. Thus, increasing the visceral metabolic rate of this ectothermic lizard established neither consequential endothermy nor homeothermy. These results are inconsistent with a thermoregulatory explanation for the evolution of endothermy.

Patterns of cardiovascular and ventilatory response to elevated metabolic states in the lizard Varanus exanthematicus

The principal function of the cardiopulmonary system is the precise matching of O(2) and CO(2) transport to the metabolic requirements of different tissues. In some ecothermic vertebrates (amphibians and reptiles), vdot (O2) increases dramatically following feeding. Factorial increments in vdot (O2) range from 1.7 to 44 times above resting rates, and in some cases vdot (O2) approaches or even exceeds values measured during physical activity. There is virtually no information on the cardiopulmonary response during the postprandial period in these animals or how the pattern of cardiopulmonary support compares with that during activity. In our experiments, pulmonary ventilation (vdot e), heart rate (fh), systemic blood flow (qdot (sys)), rate of oxygen consumption (vdot (O2)) and rate of carbon dioxide production (vdot (CO2)) were measured at 35 degrees C in the lizard Varanus exanthematicus for 24 h prior to the ingestion of meals of various sizes and measured continuously for up to 72 h during the postprandial period. The results of this study were compared with previously published values for treadmill exercise in the same experimental animals. The change in fh and stroke volume (V(S)) for a given increment in vdot (O2) did not differ during exercise and digestion. In contrast, the ventilatory response was very dependent on the nature of the elevated metabolic state. During digestion, an increase in vdot (O2) resulted in a relative hypoventilation in comparison with resting values, whereas hyperventilation characterized the response during activity. During exercise, breathing frequency (f) increased 10- to 40-fold above resting values accompanied by large reductions in tidal volume (V(T)). In contrast, postprandial increases in vdot (O2) resulted in relatively minor changes in f and V(T) almost doubled. These results indicate that, in these lizards, the cardiac response to elevated vdot (O2) is stereotyped, the response being predictable irrespective of the source of the metabolic increment. In contrast, the ventilatory response is flexible and state-dependent, not only in pattern but also in its frequency and volume components.

Ventilatory and cardiovascular responses of a python (Python molurus) to exercise and digestion

To investigate the potential limiting steps of peak metabolic rates, we examined gas exchange rates (vdot (O2), vdot (CO2)), respiratory exchange ratio (RER), breathing frequency, tidal volume, minute ventilation volume (V.e) as well as the heart rate, systemic blood flow and stroke volume of Burmese pythons (Python molurus) while fasting at rest, exercising, digesting and exercising while digesting. All measured variables increased significantly during exercise (crawling at 0.4 km h(−)(1) and at vdot (O2max)), highlighted by a 17-fold increase in vdot (CO2) and a 24-fold increase in V.e. During the digestion of a meal equivalent to 25 % of the snake's body mass, pythons responded with increases in vdot (O2) and heart rate similar to those experienced during exercise, along with a 4.5-fold increase in systemic blood flow. Interestingly, pythons hyperventilated while exercising, whereas they hypoventilated during digestion. The combined demands of exercise and digestion resulted in significantly higher vdot (O2), vdot (CO2), breathing frequency and heart rate than during either exercise or digestion alone. Evidently, the capacities of the ventilatory and cardiovascular systems to transport oxygen to locomotor muscles are not a limiting factor in the attainment of peak metabolic rates during exercise in pythons

Effects of food shortage and oversupply on energy utilization, histology, and function of the gut in nestling song thrushes (Turdus philomelos)

We measured food intake, digestive efficiency, body mass increments, resting metabolic rate (RMR), carcass fat content, size and histological structure of the gut, and the rate of intestinal brush border uptake of l-proline in song thrush (Turdus philomelos) nestlings subjected to food shortage or food surplus under laboratory conditions. We assigned nestlings between 3 and 7 d of age to one of the following treatments: (1) food restriction, which resulted in a slowed growth at the rate found in undernourished, wild nestlings; (2) overfeeding, which totally suppressed begging; and (3) intermediate feeding. Threefold differences in energy consumption caused fivefold differences in body mass increments of the nestlings. Despite this, body mass-corrected RMR and intestinal mass were not affected by the feeding regime. The energy content of fecal output was highest in food-restricted birds, while their carcass fat content was lowest among treatment groups. Intestinal uptake rates of l-proline were low in the overfed and intermediate-fed young but significantly increased in the food-restricted birds, who attempted to maximize their rates of growth and development within the restrictive limits set by feeding regime. We noted a marked decrease of intestinal villi height in overfed birds as compared to intermediate-fed and food-restricted nestlings. We conclude that song thrush nestlings are characterized by a limited plasticity of their developmental program, which prohibits overfed nestlings from significantly up-regulating their gut function to accommodate increased food intake. This suggests that they already grew at a rate close to their physiological maximum. We suggest two interpretations: (1) under natural conditions, song thrush nestlings do not face frequent, unpredictable fluctuations in food abundance that could select for developmental plasticity, or (2) strong selection for uniform adult phenotypes prevents flexible developmental trajectories, which would result in a diversity of adult phenotypes.

Animal Guts as Ideal Chemical Reactors: Maximizing Absorption Rates

I solved equations that describe coupled hydrolysis in and absorption from a continuously stirred tank reactor (CSTR), a plug flow reactor (PFR), and a batch reactor (BR) for the rate of ingestion and/or the throughput time that maximizes the rate of absorption (=gross rate of gain from digestion). Predictions are that foods requiring a single hydrolytic step (e.g., disaccharides) yield ingestion rates that vary inversely with the concentration of food substrate ingested, whereas foods that require multiple hydrolytic and absorptive reactions proceeding in parallel (e.g., proteins) yield maximal ingestion rates at intermediate substrate concentrations. Counterintuitively, then, animals acting to maximize their absorption rates should show compensatory ingestion (more rapid feeding on food of lower concentration), except for the lower range of diet quality for complex diets and except for animals that show purely linear (passive) uptake. At their respective maxima in absorption rates, the PFR and BR yield only modestly higher rates of gain than the CSTR but do so at substantially lower rates of ingestion. All three ideal reactors show milder than linear reduction in rate of absorption when throughput or holding time in the gut is increased (e.g., by scarcity or predation hazard); higher efficiency of hydrolysis and extraction offset lower intake. Hence adding feeding costs and hazards of predation is likely to slow ingestion rates and raise absorption efficiencies substantially over the cost-free optima found here.

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.

Maximum daily energy intake: it takes time to lift the metabolic ceiling

Conventionally, maximum capacities for energy assimilation are presented as daily averages. However, maximum daily energy intake is determined by the maximum metabolizable energy intake rate and the time available for assimilation of food energy. Thrush nightingales (Luscinia luscinia) in migratory disposition were given limited food rations for 3 d to reduce their energy stores. Subsequently, groups of birds were fed ad lib. during fixed time periods varying between 7 and 23 h per day. Metabolizable energy intake rate, averaged over the available feeding time, was 1.9 W and showed no difference between groups on the first day of refueling. Total daily metabolizable energy intake increased linearly with available feeding time, and for the 23-h group, it was well above suggested maximum levels for animals. We conclude that both intake rate and available feeding time must be taken into account when interpreting potential constraints acting on animals' energy budgets. In the 7-h group, energy intake rates increased from 1.9 W on the first day to 3.1 W on the seventh day. This supports the idea that small birds can adaptively increase their energy intake rates on a short timescale.

Phenotypic flexibility of the avian gizzard: rapid, reversible and repeated changes of organ size in response to changes in dietary fibre content

Evolutionary biology presumes that organ capacities match their natural loads. Therefore, in fluctuating conditions, organ systems are expected to show a reversible, repeatable and rapid phenotypic response that is directional and scaled. In this study, phenotypic responses of the gizzard of adult Japanese quail (Coturnix japonica) to experimental mismatches of load and capacity were tested by a series of diet-switching experiments, involving an increased content of non-digestable fibre (NDF) in the diet. The results of all experiments were in accordance with the predictions of the hypothesis that there is matching between loads and capacities. (1) The observed phenotypic responses are directional and scaled to the demands, i.e. increasing NDF elicits an increase in gizzard size. When the proportion of NDF in the diet was raised from 1 % to 45 %, the gizzard was more than twice as large as in the control group. (2) Size responses were reversible, and reduced NDF was followed by a decrease of gizzard size. (3) Phenotypic responses could be elicited repeatedly in three successive trials. (4) Excess capacities were downregulated and insufficient capacities were upregulated. (5) The responses followed changes of loads with almost no time lag, with size changes measurable within 24 h.

Respiratory consequences of feeding in the snake Python molorus

Snakes can ingest large meals and exhibit marked increases in metabolic rate during digestion. Because postprandial oxygen consumption in some snakes may surpass that attained during exercise, studies of digestion offers an alternative avenue to understand the cardio-respiratory responses to elevated metabolic rate in reptiles. The effects of feeding on metabolic rate, arterial oxygen levels, and arterial acid-base status in the snake Python molorus are described. Four snakes (180-250 g) were cannulated in the dorsal aorta and blood samples were obtained during 72 h following ingestion of a meal (rat pups) exceeding 20% of body weight. Oxygen consumption increased from a fasting value of 1.71 +/- 0.08 to 5.54 +/- 0.42 ml kg-1 min-1 at 48 h following feeding, and the respiratory gas exchange ratio increased from 0.67 +/- 0.02 to a maximum of 0.92 +/- 0.03 at 32 h. Plasma lactate was always less than 0.5 mM, so the postprandial increase in metabolic rate was met by aerobic respiration. In fasting animals, arterial PO2 was 66 +/- 4 mmHg and haemoglobin-O2 saturation was 92 +/- 3%; similar values were recorded during digestion, but haematocrit decreased from 15.8 +/- 1.0 to 9.8 +/- 0.8 due to repeated blood sampling. Plasma [HCO3-] increased from a fasting level of 19.3 +/- 0.8 to 25.8 +/- 1.0 mmol l-1 at 24 h after feeding. However, because arterial PCO2 increased from 21.1 +/- 0.5 to 27.9 +/- 1.4 mmHg, there was no significant change in arterial pH from the fasting value of 7.52 +/- 0.01. Acid-base status returned to pre-feeding levels at 72 h following feeding. The increased arterial PCO2 is most likely explained by a reduction in ventilation relative to metabolism, but we predict that lung PO2 does not decrease below 115 mmHg. Although ingestion of large meals is associated with large metabolic changes in pythons, the attendant changes in blood gases are relatively small. In particular, the small changes in plasma [HCO3-] and stable pH show that pythons respond very differently to digestion than alligators where very large alkaline tides have been observed. It is unclear why pythons and alligators differ in the magnitude of their responses, but given these interspecific differences it seems worthwhile to describe arterial blood gases during digestion in other species of ectothermic vertebrates.

Morphological and physiological responses to altitude in deer mice Peromyscus maniculatus

Individuals within a species, living across a wide range of habitats, often display a great deal of phenotypic plasticity for organ mass and function. We investigated the extent to which changes in organ mass are variable, corresponding to environmental demand, across an altitudinal gradient. Are there changes in the mass of oxygen delivery organs (heart and lungs) and other central processing organs (gut, liver, kidney) associated with an increased sustainable metabolic rate that results from decreased ambient temperatures and decreased oxygen availability along an altitudinal gradient? We measured food intake, resting metabolic rate (RMR), and organ mass in captive deer mice (Peromyscus maniculatus bairdii) at three sites from 1,200 to 3,800 m above sea level to determine whether energy demand was correlated with organ mass. We found that food intake, gut mass, and cardiopulmonary organ mass increased in mice living at high altitudes. RMR was not correlated with organ mass differences along the altitudinal gradient. While the conditions in this study were by no means extreme, these results show that mice living at high altitudes have higher levels of energy demand and possess larger cardiopulmonary and digestive organs than mice living at lower altitudes.

Sample collection and clinical pathology of reptiles

Clinical pathology techniques can guide the clinician to diagnose and treat the reptile patient successfully. Text, tables, pictures, and diagrams in this article describe blood and cytology collection techniques and interpretation for the common pet reptile species.

Insulin and proglucagon-derived peptides from the horned frog, Ceratophrys ornata (Anura:Leptodactylidae)

Insulin and peptides derived from the processing of proglucagon have been isolated from an extract of the pancreas of the South American horned frog, Ceratophrys ornata (Leptodactylidae). Ceratophrys insulin is identical to the insulin previously isolated from the toad, Bufo marinus (Bufonidae). Ceratophrys glucagon was isolated in two molecular forms with 29- and 36-amino acid residues in approximately equal amounts. Glucagon-29 is identical to glucagon from B. marinus and from the bullfrog, Rana catesbeiana (Ranidae) and contains only 1 amino acid substitution (Thr29 --> Ser) compared with glucagon from Xenopus laevis (Pipidae). Glucagon-36 comprises glucagon-29 extended from its C-terminus by Lys-Arg-Ser-Gly-Gly-Met-Ser. This extension is structurally dissimilar to the C-terminal octapeptide of mammalian oxyntomodulin and resembles more closely that found in C-terminally extended glucagons isolated from fish pancreata. Ceratophrys glucagon-like peptide-1 (GLP-1) (His-Ala-Asp-Gly-Thr-Tyr-Gln-Asn-Asp-Val10-Gln-Gln-Phe-Leu-Glu- Glu-Lys-Ala-Ala-Lys20-Glu-Phe-Ile-Asp-Trp-Leu-Ile-Lys-Gly- Lys30-Pro-Lys-Lys-Gln-Arg-Leu-Ser) contains 3 amino acid substitutions compared with the corresponding peptide from B. marinus, 8 substitutions compared with GLP-1 from R. catesbeiana, and between 4 and 11 substitutions compared with the three GLP-1 peptides identified in X. laevis proglucagon. GLP-2 was not identified in the extract of Ceratophrys pancreas. The data indicate that, despite its importance in the regulation of glucose metabolism, the primary structure of GLP-1 has been very poorly conserved during evolution, even among a single order such as the Anura.

Rapid changes in the size of different functional organ and muscle groups during refueling in a long-distance migrating shorebird

The adaptive value of size changes in different organ and muscle groups was studied in red knots (Calidris canutus islandica) in relation to their migration. Birds were sampled on five occasions: at arrival in Iceland in May 1994, two times during subsequent refueling, at departure toward, and on return from, the high arctic breeding grounds. During their 24-d stopover in May, body mass increased from 144.3 to 214.5 g. Mass gains were lowest over the first week (0.85 g/d, only fat-free tissue deposited). Over the subsequent 10 d, average mass increased by 5.0 g/d (fat contributing 78%), and over the last week before takeoff, it increased by 2.0 g/d (fat contributing over 100% because of loss of lean components). There were no sex differences in body and fat mass gains. Over the first interval, lean masses of heart, stomach, and liver increased. During the middle 10 d, sizes of leg muscle, intestine, liver, and kidneys increased. Stomach mass decreased over the same interval. In the last interval before takeoff, the stomach atrophied further and the intestine, leg muscles, and liver became smaller too, but pectoral muscles and heart increased in size. Sizes of "exercise organs" such as pectoral muscle and heart were best correlated with body mass, whereas sizes of organs used during foraging (leg muscles) and nutrient extraction (intestine, liver) were best correlated with rate of mass gain. Kidneys changed little before takeoff, which suggests that they are needed as much during flight as during refueling.

Menstrual cycling and breast cancer: an evolutionary perspective

This article attempts to bridge the disciplinary gap between evolutionary biology and clinical studies of women's health. The resulting dialogue is predicted to have useful implications for research aimed at the prevention of women's reproductive cancers. The specific focus is on the relationship between breast cancer and exposure to ovarian hormones during normal menstrual cycling. The clinician's view of normal cycling is radically different from that uncovered by evolutionary studies of noncontracepting populations. This point is illustrated by data on the Dogon of Mali, a traditional West African population with a mean of 8.6 +/- 0.3 live births per woman. The Dogon data include hormonal profiles (urinary estrone-3-glucuronide and pregnanediol-3-glucuronide) of 93 women (sampled twice weekly for 10 weeks) and a census of women's visits to menstrual huts (n = 736 days). Dogon women menstruated regularly only if they were sterile. Otherwise, women aged 20-34 years had a median of only two menses each over the 2-year study period. The median number of menses per lifetime was approximately 100, about a third as many as experienced by an American woman who had three live births. These results contribute to a growing body of evidence that women's bodies were designed by natural selection to spend most of the time in lactational amenorrhea and add support to the view that contraceptives can be made safer if they forego the hormonal swings associated with menstruation. This conclusion is further reinforced by evidence that menstrual bleeding serves no adaptive purpose.