Feeding and Nutrition of Reptiles


Reptile Nutrition: From Mouth to Vent
Avian species typically have shorter mean retention time of digesta than do similar sized nonflying mammalian species Common pet snakes such as rat snakes, milk snakes, boas and pythons typically feed on rodents but some pet and many exhibition snakes require special diets. The gastrointestinal tract of the snake is simple and relatively short compared to other reptiles. Exposure to unfiltered natural sunlight, depending on latitude, during warmer months and use of UVB bulbs during the rest of the year usually eliminate the risk of bone disease caused by insufficient absorption of calcium due to a vitamin D deficiency. For example, female Aedes aegypti mosquitoes feed on both sugar-rich nectar and protein-rich vertebrate blood. Peptides are broken into smaller peptides, and peptidases reduce the enzymes to amino acids.

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Figure 4A adapted, with permission, from reference B Small intestine nominal smoothbore tube surface area in omnivorous birds and mammals same symbols and lines as in A. When the lines were fit to the common slope of 0.

Figure 4B adapted from reference The difference in paracellular absorption between birds and nonflying mammals is not simply explained by mediated absorption in birds of the carbohydrate probes that are presumed to be absorbed passively.

Nor is the difference in paracellular absorption between birds and nonflying mammals explained by longer retention of digesta in the gut of the former relative to the latter. Avian species typically have shorter mean retention time of digesta than do similar sized nonflying mammalian species Because birds typically achieve higher paracellular absorption with less intestinal length and surface area than do similar sized nonflying mammals, there apparently are differences in intestinal permeability per unit intestinal tissue.

This was confirmed in a comparison of pigeons and laboratory rats. The difference in paracellular solute absorption between mammals and birds cannot be linked to differences in solvent drag because it is so difficult to distinguish between water absorbed by the paracellular route versus aquaporins, which occur in intestine of both mammals and birds Enhanced paracellular absorption may have evolved as a compensation for smaller intestinal size in birds compared with nonflying mammals Fig.

The difference in intestinal surface area between birds and nonflying mammals did not depend on diet in the analysis. Diet did have a significant effect on gut size, but the effect was on cecal and large intestine size. Another advantage of paracellular absorption is that it is an energetically cheap way to match absorption rate to substrate concentration in the diet and lumen. If there has indeed been natural selection for smaller intestinal size in fliers, and increased paracellular absorption as a compensation, then one might expect to find the same patterns found in flying birds versus nonflying mammals in a comparison within mammals between fliers i.

Preliminary evidence suggests that this is the case 75 , but more extensive sampling is necessary. Generally, in vertebrates, the more carnivorous the species, the lower its rate of intestinal mediated glucose absorption This pattern, first described in a survey of more than 40 species drawn from the major vertebrate classes , is apparent also in comparative studies within fish 51 and birds Based on phlorizin-binding studies in a limited number of species, it appeared that species differences in tissue-specific glucose uptake may largely reflect species differences in the number of copies of the main apical membrane glucose transporter SGLT1, although it is possible that differences in turnover time of the transporter can also contribute There was no marked pattern of higher intestinal transport activity for amino acids among the more carnivorous vertebrate species , This is perhaps expected because all animals, regardless of diet, need protein and so there should not be strong selection for very low protein processing capability in animals.

In addition, it has been argued that it would be advantageous for herbivores with relatively rapid gut throughput to have compensatorily higher biochemical capacity to process proteins and recover them rather than excrete them.

There is overwhelming evidence that the digestive and absorptive function of the GI tract of animals can vary with diet composition. This flexibility is exhibited at two levels: The biochemical flexibility is generally considered to maximize the acquisition of carbon for energy production and essential nutrients for maintenance and growth, while protecting against the acquisition of excessive, potentially toxic, amounts of certain dietary constituents e.

Any nutritional imbalance that might arise from this strategy is widely considered to be corrected postabsorption, so that the retention and use of certain nutrients are optimized, while surplus metabolites can be eliminated , In this section, the relationship between diet composition and digestive enzyme activity is addressed first, followed by consideration of transporters in the GI tract.

Many studies on vertebrates have demonstrated that the production of digestive enzymes increases with availability of substrate in the gut lumen. For example, this effect has been confirmed in rodents for all of the major pancreatic enzymes amylase, lipase, and proteases and enzymes of the intestinal brush border sucrase-isomaltase, maltase-glucoamylase, and aminopeptidase-N Other data relate to a variety of mammals, birds, reptiles and fish, as well as a number of invertebrates [reviewed in reference ].

This mode of regulation both maximizes the digestibility of substrates and minimizes the cost of synthesizing excess enzyme when the substrate is at low levels. The mechanistic basis of the impact of diet on digestive enzyme activity has not been investigated in most species but, where studied, there is persuasive evidence that differential enzyme activity is underpinned by changes in gene expression.

For example, the elevated expression of intestinal sucrase-isomaltase gene in the intestine of rats and mice fed on high-carbohydrate diets is controlled by the transcription factors Cdx-2 and HNF-1 36 ; and the recruitment of these transcription factors to the promoter region is correlated with the acetylation of histones H3 and H4 associated with this gene Adaptive variation in digestive enzyme activity with diet composition is crucial to the lifestyle of many animals.

For example, female Aedes aegypti mosquitoes feed on both sugar-rich nectar and protein-rich vertebrate blood. The gut protease activity is undetectable in individuals feeding on a sugar meal but, within hours of taking a bloodmeal, the digestive protease activity in the midgut increases rapidly, reaching a maximum after about 2 days. The synthesis of two trypsins, known as the late trypsins, is regulated by dietary protein content.

Initial production within 3 h of feeding is from a preformed mRNA, in response to protein in the blood; and subsequent production 8—10 h after feeding comes from de novo trypsin gene expression, induced by amino acid products of trypsin-mediated digestion of blood proteins The other midgut trypsin, called early trypsin, is synthesized constitutively. Nevertheless, some studies have found that the secretion of digestive enzymes does not vary in a simple fashion with substrate concentration.

For some insects feeding on a nutritionally unbalanced diet, such that one dietary component is in excess, the enzymes mediating the degradation of that dietary component can be downregulated.

These data suggest that an insect has the capacity to regulate digestive enzymes homeostatically, such that enzymes yielding nutrients in excess are secreted at lower rates than enzymes that generate nutrients in deficit. The production of some digestive enzymes appears to be regulated by integrated sensing of both the nutrients available in the gut and the nutritional requirements of the animal.

This complexity may not be revealed in the nutritionally sufficient diets that are commonly used for laboratory maintenance of animals, but could be important for animals in the field with access diets of variable and often suboptimal composition.

The enzyme activities were downregulated in insects on diets containing an excess of the substrate. Current understanding of the matching of transporter function to diet composition derives largely from the classic work of Diamond and colleagues , conducted on isolated intestine preparations of mice. The transport of nutrients that are metabolized for energy production increase with increasing dietary supply, while those mediating the uptake of essential but non energy-yielding nutrients tend to decrease with increasing dietary supply.

Thus, transporter activity for sugars e. Interestingly, the uptake of dietary essential amino acids, such as histidine, lysine, leucine, and methionine, tends to increase slightly at low dietary levels the reverse of the response to nonessential amino acids , indicating the central role of dietary essential amino acids for protein synthesis and use of nonessential amino acids as a respiratory substrate.

How this differential response to essential versus nonessential amino acids is achieved despite the overlapping substrate specificities of the various amino acid transporters Table 3 is not fully understood. Kinetic analyses of nutrient uptake indicate that the diet-dependent variation in sugar and amino acids transporter activity is mediated predominantly by changes in the density of transporters on the apical membrane Two processes can mediate increased transporter function: Most research has focused on expression response to dietary nutrients.

For example, in response to high dietary supply of sugars, the expression of genes encoding the transporters SGLT1 for glucose and GLUT5 for fructose is increased. The effect of dietary soluble carbohydrate on the transcript abundance of the glucose transporter gene SGLT1 in A the mid-intestine of day-old piglets and B the duodenum of horses fed sequentially on different diets including hay essentially starch-free and grain containing 0. The activity of the Pept-1 peptide transporter in the intestine is elevated by high dietary protein.

The expression of various transporter genes is regulated in anticipation of food. Adult rats exhibit diurnal variation in expression of sugar transporters in the intestine, with induction of GLUT2 glucose transporter , GLUT5 fructose transporter , and Pept-1 expression 3 to 4 h before the onset of peak feeding by the animal , , In addition, preexisting pools of transporter proteins, probably localized in the cytosol, are likely localized to the membrane; this can achieve more rapid changes in transporter activity than changes in gene expression.

The central role of transporters in the modulation of absorption with diet raises important questions about the capacity of an animal to regulate uptake of nutrients with significant levels of passive absorption. For these nutrients, uptake is predicted to increase monotonically with concentration in the gut lumen.

The uptake of the vitamins pantothenic acid, ascorbic acid, and choline conforms to this expectation. Absorption of these vitamins is predominantly passive and, unlike other essential nutrients, it is not upregulated in response to low dietary supply Nutrients that are taken up by the paracellular route are also predicted not to be tightly regulated.

The biochemical flexibility of the GI tract in a given animal is the product of its evolutionary history, with taxa that have diets of variable composition predicted to display greater phenotypic flexibility than those with relatively uniform diets.

This issue has been explored particularly in relation to variation in the capacity of animal species with different diets to modulate their transporter activity. For example, glucose transporter function in vertebrates tends to be higher and more flexible to diet in herbivores and omnivores than in carnivores These differences reflect evolutionary adaption to diet, with a lower and more uniform carbohydrate: Two specific comparisons illustrate the relationship between diet and the phenotypic flexibility in the biochemistry of nutrient acquisition in the GI tract.

The first comparison relates to sugar transport in domestic dogs and cats. The glucose transporter SGLT1 is expressed in the intestine of both the domestic dog and cat, but its expression level is twofold greater and is more responsive to dietary carbohydrate in the dog than the cat 18 , Despite the poor capacity of the domestic cat to utilize diets with significant levels of carbohydrate, many commercial cat diets contain relatively high levels of carbohydrate.

For cats maintained on these diets, it is likely that rodents, small birds, etc. The second example of interspecies differences in nutritional flexibility concerns two passerine birds, the house sparrow P. In the field, the initial diet of nestling house sparrows is dominated by insects, but switches subsequently to seeds.

In nestling sparrows fed on a diet containing starch, the gut maltase activity of the birds increased by more than twofold Fig. Furthermore, this effect was correlated with changes in transcript abundance of the maltase gene, indicating the central role of gene expression in regulating digestive function , In contrast to the house sparrow, the intestinal maltase activity of zebra finch was not responsive to variation in dietary starch content As the comparison of house sparrow and zebra finch illustrates, interspecific difference in dietary flexibility is underpinned by a parallel difference in biochemical and genetic flexibility.

Diet-induced changes in the activity of digestive enzymes in day-old nestling house sparrows 2—3 days before fledging. B Amino-peptidase N activity [Data from Fig. Major changes in GI enzymes and transporters occur during development in many animals.

In some groups such as ruminant mammals, insects, amphibians, and fish, these are also accompanied also by dramatic changes in GI structure. The reviews by Buddington and colleagues in the early s 49 , 50 , 54 summarized results for about 12 vertebrate species, and additional work in the past 15 years has resulted in many more studies of developmental changes in digestion and features of digestive physiology, as well as an expanded list of species including more than a dozen fish species see below , six amphibian species, a turtle 35 , five avian species, and a dozen mammals.

The latter class has been most intensively studied, and reviews of work in that group , , , provide some major themes that apply as well to other groups.

Henning provides a good overview of GI development in mammals, especially in the laboratory rat, the most studied of about a dozen mammalian species that have been surveyed to date 3 , 17 , 49 , 56 , 57 , 65 , , , , , , , , , , , , , , , , , , During the gestational phase, organs undergo morphological maturation [see also reference ] and many proteins required for digestion and absorption of components of milk are expressed e.

The second major phase of changes occurs at the onset of weaning day 15 in the rat , when the GI tract acquires proteins required for digestion and absorption of solid food that contains substrates not contained in milk, such as fructose and starch. Until weaning, the stomach of the neonate is not acidic and substantial amounts of gastric and pancreatic proteases are not expressed.

Accordingly, the small intestine has a high capacity for pinocytotic absorption of intact protein and intracellular breakdown by lysosomal proteinases.

The pinocytotic uptake capacity declines at weaning, although molecular details of this have not been elucidated. Large changes occur in proteins important in processing of carbohydrate, which is the diet component that changes most dramatically e. Activity increases markedly for sucrase-isomaltase, maltase-glucoamylase, trehalase, and GLUT-5, the fructose transporter, in most cases accompanied by increases in the expression of their genes.

Activity of lactase-phlorizin hydrolase declines at this time, associated with changes in transcription, translation, and protein turnover see discussion about lactase, above. If a young mammal is allowed to prolong suckling, or is fed on a lactose-containing diet after weaning, the onset of the decline in lactase is delayed, but only slightly.

Many of these patterns are apparent in at least a dozen other species of mammals that have been studied, although in species such as carnivorous marine mammals and ruminants sucrase activity remains low , and in ruminants dramatic changes occur in GI tract structure postnatally [i.

Also, in some species e. Studies using rat, mouse, and human fetal intestine grafted into adult hosts, or using altered diets, have shown that many of these changes occur in the absence of specific ontogenetic signals from either the lumen or circulation. In addition to this intrinsic timing, circulating levels of hormones such as glucocorticoid and epidermal growth factor are involved in maturation and growth. An interesting illustration of some of the variability in patterns of development comes from a comparison of patterns for two major sugar transporters, SGLT1 and GLUT5 This is not necessarily the case for increased glucose-transport activity, which may occur without a coinciding large increase in SGLT1 mRNA in rats and in lamb intestine [though see reference ].

In neonate rats, luminal or dietary carbohydrate does not induce glucose transport which is contrary to the situation in adult rats , whereas in lambs dietary glucose is required for induction of glucose transport activity. A final notable difference is that luminal fructose is specifically required for induction of GLUT5, whereas glucose transport activity can be induced with glucose and a number of other sugars and even nonmetabolizable sugar analogues. As growth continues after weaning, tissue-specific intestinal enzyme activities and transport rates tend to be relatively constant or decrease, but total capacity increases due to the increase in intestinal mass 50 , 53 , 55 , 56 , , , , , Studies in cats and rats yielded some evidence for particular changes in transporter-specific activity or intestinal mass coinciding with whole-organism growth rate peaks 53 , Developmental changes in GI function during the pre- and postnatal periods also occur in birds, as chicks accommodate the transition from a lipid-rich yolk diet inside the egg to a carbohydrate- and protein-based diet post hatch.

At least six avian species have been studied: Based on expression profiling and measures of activity, species in both groups have at hatch the full suite of enzymatic, pancreatic, and intestinal activities to digest fat, carbohydrate, and protein [e. Sklan and colleagues — , , and Planas and colleagues 16 , have studied the molecular basis for ontogenetic changes in carbohydrate digestion and absorption in chickens during the week before and after hatching. Increases in both SI activity and glucose transport occurred 2 days before hatch and at hatch day.

Some regulation of glucose transport activity by posttranscriptional mechanisms is suggested by the fact that transport did not change significantly during the week posthatch , , whereas SGLT1 mRNA significantly increased Likewise, when hexose transport in jejunal brush border membrane vesicles declined with age in older chicks, the site density of SGLT1 declined in parallel but SGLT1 mRNA did not change significantly Ontogenetic changes related to carbohydrate digestion and absorption in chicks.

A Changes related to glucose absorption: B Changes related to carbohydrate breakdown: SI mRNA from reference C Changes related to homeobox gene of the caudal family cdxA: In both young chickens and house sparrows, the posthatch increases in maltase activity are controlled by intrinsic regulatory mechanisms, but maltase activity can also be doubled by increased dietary carbohydrate 33 , 43 , and this is correlated with a doubling in maltase-glucoamylase mRNA transcription in the house sparrows Large changes occur posthatch in intestine size and digestive capacity as birds grow.

For example, in altricial house sparrows digestive biochemistry was dynamic over their 2-week period from hatching to fledging from the nest.

Tissue-specific activities of some intestinal enzymes increased by more than 10 times e. In three precocial species [chickens 33 , , wild jungle fowl , ducks ] tissue-specific enzyme, and transport rates were constant or declined with age but overall digestive and absorptive capacity increased, along with intestine mass, in direct proportion to metabolic body mass, which was the pattern described for mammals.

A large number of studies of GI development in at least a dozen fish species have been published in the past decade 59 , 67 , 96 , , , , , , , , , , , , , , — , , , , due to their importance in aquaculture, and many studies include newer molecular and gene expression approaches , As in birds, a major ontogenetic change in fish is that the source of nutrients and energy necessary to continue larval development changes from the yolk reserves to the ingested food, which is mainly protein and fat in carnivores but higher in carbohydrates in omnivores and herbivores.

Initially, a functional gastric region may be absent [e. The major pancreatic neutral lipase is bile activated lipase, and cod also have a nonfunctional pancreatic lipase related protein, but the expression of only the former increases during development. The activity of neutral lipase did not increase in parallel to gene expression. The mismatch between activity and gene expression measurements was partly explained by a nonspecific analytical method, because the whole body is analyzed the gut of very small larvae is not isolated and some fish tissues outside the GI tract could have lipase activity.

Also, in cod and some other fish neutral lipase activity in prey i. Interestingly, the Atlantic cod genome does not seem to contain colipase that typically is essential for pancreatic lipase activity. Fish amylases and glucose transporters appear to be molecularly closely related to those in mammals and to have comparable characteristics , Prickleback fishes, which include species that shift during development from carnivory to herbivory as well as species that remain carnivores, have provided examples of intrinsic vs.

The diet shifter C. Some new proteolytic enzymes are produced, such as pancreatic trypsin and stomach pepsin and chitinase s , which increase the capacity to digest animal matter.

Surprisingly, the ratio of intestinal glucose uptake to proline uptake, which is an index for the relative capacity for glucose and proline absorption, did not change between bullfrog tadpoles and adults and was characteristic of vertebrate carnivores Shifts during development in feeding versus nonfeeding or in dietary habits occur in diverse invertebrates, including lobsters and insects , and digestive enzyme levels may change in correlation with changes in the major dietary substrates.

Insects provide some of the best researched examples of developmental changes in digestive biochemistry. Common cutworms Spodoptera litura ; Lepidoptera , a highly polyphagous pest of subtropical and tropical crops, can be used to illustrate a pattern that is probably common As in many insects, chymotrypsin-like SPs are major midgut digestive enzymes.

Two have been identified in cutworms, Slctlp 1 and 2 , and expression of the latter gene was analyzed in sixth instar larvae following molting from the fifth instar until pupation a week later Fig.

Slctlp 2 was expressed on feeding days and downregulated on nonfeeding days and stages such as pupa Fig. No transcripts were found at the adult stage, perhaps because the adult moths do not feed on protein. Apparent transcription control of SP activity was also demonstrated in the scarabaeid beetle Costelytra zealandica Food appears to act as a proximate signal for expression, based on up-and-down expression in cutworm larvae according to feeding regime Fig. It seems reasonable that digestive SPswould be downregulated during nonfeeding stages or during fasting within a stage given the energy required to produce these proteins and to ensure that pupating larva are protected from spurious self digestion Expression of serine protease Slctlp2 in common cutworm larvae S.

A mRNA from midguts of sixth instar larvae at days 0 to 7. Day 0 is the day the larvae just molted. At days 6 and 7 of the sixth larval stadium, the larvae stopped feeding and entered the prepupal stage. Data are transcript abundance normalized to actin transcript.

Each bar represents the mean of three independent repeats of the experiment. Both figures based on data from reference Other interesting comparisons are provided by social insects, where the division of labor may include individuals in castes that collect and digestively process plant and animal foods and then feed other material to individuals in the colony. In the wood eating termite Reticulitermes speratus , for example, intrinsic cellulase gene expression is much reduced in reproductives compared with workers , and protease levels are much reduced in colony members of ants, wasps, and honeybees that are fed amino-acid-rich excretions of other colony members , Their functions include communication, attraction, or in defense against herbivores, predators, pathogens, and competitors SMs are so pervasive that it is almost a certainty that any thorough analysis of a plant food, and maybe even many animal foods, will identify some SMs.

Some are thought to play an important role in human health, variously acting as antioxidants or antimicrobials, modifying hormone titers, and interfering with DNA synthesis. Other SMs directly damage GIT mucosa, such as lectins , proanthocyanidins 2 , and hydrolysable tannins Protease inhibitors can permeabilize the peritrophic membrane of caterpillars In the following sections, we highlight numerous examples of key digestive processes being influenced by compounds from many of the major groups of SMs Table 4.

But, also, considering the structural and functional diversity of digestive tracts among animals, it should not surprise that impacts of SMs are not necessarily general but depend on digestive features and perhaps even adaptive counterresponses of consumers. There is a long history of use by humans of natural products as laxatives Some SMs that alter digesta transit in humans and wild animals are listed in Table 4. An important consequence of rapid digesta transit can be malabsorption, as occurs even for animals with rapid transit time ingesting passively absorbed compounds.

For example, digestion time and glucose absorption was reduced when sunbirds ingested nectar from tobacco plants that contain particular alkaloids The few examples in Table 4 show how the compounds that influence transit time are chemically heterogeneous, and they also could act through a variety of mechanisms. These might include osmotically based mechanisms, which might draw water into the lumen by acting as introduced osmolytes or by receptor-mediated increase in secretion of ions, or by a nonosmotic mechanism such as direct action on motility patterns via receptor-mediated changes in neuromuscular activity [e.

Chemicals from many of the major groupings of SMs e. Mechanisms vary, including competitive and noncompetitive enzyme inhibition as well as disruptions of the emulsification process important in digestion of fat One of the best studied chemical groups are protease or proteinase inhibitors PIs , which bind to digestive proteins and reduce digestive efficiency and hence growth rate , Many insects, mammals, and birds respond by increasing secretion of proteolytic enzymes and, in the vertebrates, by increasing the size of the pancreas, which synthesizes many of the enzymes, often with the net effect of restoring digestive efficiency and growth rate.

Research suggests antagonistic coevolution between plants and herbivores in which the plants produce a variety of PIs with specific action against different kinds of proteases and the animals produce digestive enzyme variants that are fairly insensitive to the PIs Trypsin inhibitor in castor bean leaf extract inhibited trypsin-like activity in the coffee leaf miner Leucoptera coffeella ; Table 4 but not bovine trypsin Helicoverpa larvae have been identified whose chymotrypsin activity is resistant to a serine PI from Nicotiana alata , whereas other Helicoverpa larvae have an enzyme variant that is susceptible Their respective cDNAs were isolated and critical residues that conferred resistance were identified.

Helicoverpa larvae were also found to produce midgut proteases 85 or trypsin isoforms that were either sensitive or insensitive to inhibition by soybean trypsin inhibitor STI. The STI-senstive trypsim isoforms were produced constitutively, but production of the induced STI-insensitive forms was regulated transcriptionally following ingestion of STI A somewhat analogous scenario is emerging from studies of inhibitors of carbohydrases.

Mulberry leaves produce sugar-mimic alkaloids that inhibit sucrase and trehalase activity Table 4. However, activities in domesticated silkworms Bombyx mori , which are mulberry specialists, are not affected whereas activities in Eri silkworms Samia ricini , which are generalist insect herbivores, were inhibited by very low concentrations of the alkaloids In another example, when larvae of bean weavils Zabrotes subfasciatus were fed seeds of Phaseolus vulgaris they secreted inducible isoforms of alpha-amylases that were insensitive to the alpha-amylase inhibitor that is found in the plant, whereas their constitutively produced alpha-amylase was inhibited by SMs in the plant [reference 29 ; see also references 29 , ].

The entire topic of coevolution of digestive enzymes and plants SMs is not only interesting but also very important, because plant biologists are now experimentally manipulating in crop plants the genes that regulate inhibitory SMs to enhance resistance to crop pests.

Tannins are water-soluble polyphenolic compounds with a molecular weight between and Da, and have the putative function as possible digestibility reducers They can interact with proteins and other macromolecules in vitro through hydrogen bonding and hydrophobic bonds, and thus bind enzymes and their nutrient substrates.

Several studies document their inhibition of many enzyme activities in vitro: These data lead to an expectation that they will reduce diet digestibility Even if digestive enzymes are inhibited in vitro , the effects can, in principle, be prevented or reversed in vivo by change in pH or by surfactants detergents such as bile acids or other tannin-binding material in the gut such as mucus The complexed tannins may escape both enzymatic and microbial degradation, and may be excreted in the feces, thus protecting the animal from either damage to the gut epithelium, true digestibility reduction, or toxicity But, this response leads to increased fecal loss of the energy and nitrogen in the tannin-protein complex and thus to a decline in apparent digestive efficiency, though not true digestive efficiency per se In some species, the relationship between dietary tannin content and reduction in apparent digestibility can be used to increase the accuracy of predictive equations of food digestibility based on food chemical composition Thus, with tannins, the effects on animals are not general but depend on the particular tannin structure, concentration, and on particularities of the consumer.

Because of this, it has been argued that they are not typically disruptors of intrinsic breakdown processes in either insects 26 or monogastric mammals Intestinal enzymes can activate certain toxins. Some SMs are synthesized and stored in plants as glycosides, that is, essentially bound to a glucose molecule, which can provide the plant a measure of self-protection from the more toxic aglycone These SMs are thus stored in an inactive form until activated by a glycohydrolase enzyme e.

It is not known whether such genetic or phenotypic adaptive response to dietary glycosides occurs in a vertebrate species. Although birds may have a homolog of the lactase gene , it is uncertain whether birds are capable of hydrolyzing plant glycosides, which might make them relatively immune to these plant toxins compared to other animals.

Levels of lactase activity are trace or immeasurably low in chickens 84 and in house sparrows P. Also, in a study with cedar waxwings Bombycilla cedrorum , the birds were not affected by the toxic glycoside, amygdalin, when administered orally, excreting it intact SMs from major groups such as phenolics and terpenoids are known to have antimicrobial activity Terpenoid compounds, including essential oils and saponins glycosides of terpenes and steroids , appear to have the largest negative effects, based on a meta-analysis of treatments in ruminants in 36 studies Scores of specific essential oils have been tested and found to be inhibitory against many bacterial genera 2 , and in the meta-analysis, they and saponins also appeared to inhibit protozoal growth Besides inhibiting fermentation, essential oils can decrease the rate of bacterial deamination of protein in the lumen 2.

The complexing ability of proanthocyanidins and other tannins makes them reactive with bacterial cell walls and extracellular enzymes , This could be the basis for how they can reduce microbial fermentation 39 , , , and growth, alter microflora populations, and reduce attachment of fungi and bacteria to substrates 2. Another phenolic SM, usnic acid found in some lichens, had a potent antimicrobial effect against 25 of 26 anaerobic rumen bacterial isolates from reindeer Rangifer tarandus , but one isolate was resistant.

The usnic acid-resistant microbe is one of at least three fairly well-documented examples of ruminal microorganisms that can apparently tolerate some SMs. Their findings help explain earlier findings that rumenal microbiota from reindeer performed better at in vitro digestion when usnic acid was added, whereas addition of usnic acid to sheep rumenal microbiota depressed digestion Another famous example is the bacterium Synergistes jonesii , which is capable of degrading mimosine metabolites and imparts mimosine resistance in the host ruminant, allowing it to eat Leucaena spp.

Finally, some GI microorganisms can apparently tolerate high concentrations of tannins, and tannin-tolerant or tannin-degrading bacterial species , have been isolated from a variety of wild mammals worldwide, especially those that consume diets high in tannin content Reptiles are ectothermic creatures. Ectothermic, for those of you more familiar with warm-blooded species, means that the animal controls its body temperature by outside means. The sun or warm surfaces are the most common ways for reptiles to warm themselves, but in certain species continuous or rapid movement can also play a role.

Reptiles as an ectothermic species have a variable metabolic rate based on their core temperature, speeding up when at their optimal temperature and slowing down at night or in winter when cold.

Reptiles even can go from having an aerobic metabolism to an anaerobic metabolism depending on the situation, like strenuous activity or diving. Inter species and size effect metabolic rates like that seem in mammals. Smaller reptiles often have a quicker metabolic rate compared to larger species. Reptile metabolic rates will also depend on the species diet. Are they a predator or an herbivore?

Predatory species that seek out there food daily, like smaller insectivores, have a quicker metabolism to keep up appropriate gut function and activity levels. Ambush species like pythons that sit and wait for long periods of time actually can shut down their GI tract altogether to conserve energy for months at a time.

Species that hibernate or go through periods of estivation also can slow their metabolic rates to extreme levels to conserve energy and rely on fat stored in the liver. When the ambush species finally eats or the hibernation ends a metabolic rate can increase times to allow for digestion or food seeking behavior.

Herbivore species gain a lot less energy from their food sources, but spend considerably less energy acquiring their nutritional demands.

An advantage to being an ectothermic species is they do not waste energy maintaining their body temperature. If we were to compare the caloric demands of two species, take an insectivore species of birds for instance, the caloric intake for the bird in one day would last a reptile of similar size 35 days! This conservatory and efficient use of calories has allowed reptiles to survive in particularly harsh environments successfully for millions of years. The gastrointestinal tract in reptiles, like other animals, starts at the mouth.

Turtle and tortoise species have a beak, compared to snakes and lizards that have teeth. Apprehension and mastication of food is variable among species since forelimbs are rigid or non-existent in some reptile species.

The tongue in certain lizards can be quite muscular and used for food apprehension. Mastication is minimal, with large chunks of food being swallowed after aggressive shaking or tearing. Snakes, on the other hand, swallow their prey whole. To safely swallow such large portions, reptiles have specially evolved oral secretory glands that function to lubricate the food moving down the esophagus. Some snake species have even more evolved secretory glands that inject venom into their prey to immobilize them before swallowing.

This is crucial to prevent internal trauma from prey that can cause damage while passing down the esophagus. The esophagus is a thick-walled muscular tube located behind the windpipe that extends through the neck and chest to the stomach. The bolus of food moves through the esophagus by peristalsis: The contractions are assisted by the pull of gravity. The esophagus joins the stomach at a point just below the diaphragm. A valvelike ring of muscle called the cardiac sphincter surrounds the opening to the stomach.

The sphincter relaxes as the bolus passes through and then quickly closes. The stomach is an expandable pouch located high in the abdominal cavity. Layers of stomach muscle contract and churn the bolus of food with gastric juices to form a soupy liquid called chyme.

The stomach stores food and prepares it for further digestion. In addition, the stomach plays a role in protein digestion. Gastric glands called chief cells secrete pepsinogen.

Pepsinogen is converted to the enzyme pepsin in the presence of hydrochloric acid. Hydrochloric acid is secreted by parietal cells in the stomach lining.

The pepsin then digests large proteins into smaller proteins called peptides. To protect the stomach lining from the acid, a third type of cell secretes mucus that lines the stomach cavity. An overabundance of acid due to mucus failure may lead to an ulcer. The soupy mixture called chyme spurts from the stomach through a sphincter into the small intestine. The inner surface of the small intestine contains numerous fingerlike projections called villi the singular is villus.

Each villus has projections of cells called microvilli to increase the surface area. Most chemical digestion takes place in the duodenum. In this region, enzymes digest nutrients into simpler forms that can be absorbed. Intestinal enzymes are supplemented by enzymes from the pancreas, a large, glandular organ near the stomach.

Sense organs