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Absorption rate of Infrequently eating animals?

Absorption rate of Infrequently eating animals?



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Clearly, creatures such as us humans, after vastly increasing the entropy of our food, expel most of the mass that we consume.

Some creatures, however, do NOT get the opportunity to eat nearly as often, even though these creatures are if anything more competitive (and perhaps cannibalistic!) than we are - snakes come to mind here, as do spiders.

Such a lifestyle implies that growing large - or at least to adult form - quickly has a huge advantage evolutionary.

So the awkward question that arises is thus: do such animals have a higher "absorption rate", if you will, then humans and other frequent feeders?

In other words, do such animals maximize their diet by incorporating more of the mass of their food and expelling less of it, or is there some physics/chemistry which makes this difficult?

Thanks!


I'll take a stab at this.

What fraction of your food is absorbed and what fraction is expelled is a function of what kind and how much indigestible material there is in the food, and not related to how often the animal eats.

How often an animal eats is a function of its metabolic demands(how many calories it needs) and how many calories it can get in one meal. Reptiles, as ectotherms, have a lower metabolic demand than endotherms. In addition, snakes can swallow huge amounts(relative to body weight) of high-calorie food in one sitting. As a result, snakes eat infrequently. Pet snakes are fed weekly or biweekly, depending on a number of factors. A reticulated python can swallow and digest something up to its own body weight. Ten weeks of digestion gives 1.4% of its own body weight daily, even without fasting.

Elephants are a migratory large mammal and eat about 4% of their body weight each day, more or less. This takes about 12-18 hours to collect, according to these people.

Elephants eat high-fiber, low-calorie foods, and as a result about 20% of their food mass is turned directly into poop. They feed often, and don't absorb much. Hummingbirds eat constantly, consuming their own body weight in food each day, but poop almost not at all. Their feces contains the indigestible parts of their food, just as the elephant feces does. The different is that large parts of elephant diet is indigestible fiber, while a tiny part of the hummingbird diet is indigestible insect exoskeletons.

All creatures absorb as much nutrition from their food as they can, and expel the remainder. How much remainder there is depends on what kind of food they eat.

Growth rates versus digestion rates and mass intake is a totally different and much more complicated question. Generally speaking, the growth-limiting factor is DNA replication or an essential acid or mineral deficiency and not a raw mass problem.


  • Author : Jonathan Safran Foer
  • Publisher : Penguin UK
  • Release Date : 2010-03-04
  • Genre: Social Science
  • Pages : 352
  • ISBN 10 : 9780141932651

To reduce risk of pandemics for ourselves, our gaze needs to turn to the health of animals. From the bestselling author of the essential new 2019 book on animal agriculture and climate crisis: We are the Weather Discover Jonathan Safran Foer's eye-opening and life-changing account of the meat we eat 'Should be compulsory reading. A genuine masterwork. Read this book. It will change you' Time Out Eating Animals is the most original and urgent book on the subject of food written this century. It will change the way you think, and change the way you eat. For good. Whether you're flirting with veganuary, trying to cut back on animal consumption, or a lifelong meat-eater, you need to read this book. 'Shocking, incandescent, brilliant' The Times 'Everyone who eats flesh should read this book' Hugh Fearnley-Whittingstall 'Universally compelling. Jonathan Safran Foer's book changed me' Natalie Portman 'Gripping [and] original. A brilliant synthesis of argument, science and storytelling. One of the finest books ever written on the subject of eating animals' Times Literary Supplement 'If you eat meat and fish, you should read this book. Even if you don't, you should. It might bring the beginning of a change of heart about all living things' Joanna Lumley


Gymnosperms

The gymnosperms are characterized by "naked" seeds in cones, or red or blue "berries," and usually evergreen, needle-like or scale-like leaves.

Taxaceae - Yew Family

Several species are cultivated as ornamentals in North Carolina, but T. canadensis Marsh. is found naturally in North Carolina only in the extreme northwestern counties. These are evergreen shrubs with alternate, linear leaves and scarlet "berries" only the outer red coat (aril) is edible.

Group number: 3. (Dangerous but uncommon)

Poisonous principle: Alkaloid taxine ephedrine and HCN.

Parts of plant: Leaves bark, seeds. Fresh or dry.

Animals poisoned: All kinds, but cattle and horses are most commonly affected when yard clippings are thrown over fences where livestock graze.

Symptoms: Nervousness, trembling, ataxia, collapse, and dyspnea. Bradycardia is pronounced and progresses to sudden death without a struggle. A subacute poisoning may occur 1-2 days after ingestion acute poisoning is accompanied by gastroenteritis.

Necropsy: Acute: no lesions. Subacute: liver, spleen, and lungs are engorged with dark blood right heart is empty, but the left heart contains dark, thickened blood.

Pinaceae and Cupressaceae - Pine and Cedar Family

These conifers are seldom eaten, but may be harmful if eaten in large quanities, or when eaten exclusively when other forage is not available.


Materials and methods

Animals and their maintenance

The five species of this study span the geographic range and morphological diversity of the genus Python(Fig. 1). Python brongersmai Stull 1938, the blood python, inhabits eastern Sumatra and neighboring portions of Malaysia (Keogh et al., 2001). They are an extremely heavy-bodied snake [body mass to total length ratio of 8.97±0.23 (mean ± 1 s.e.m.) Fig. 1] with a body mass reaching 22 kg and a body length up to 2.5 m(Shine et al., 1999 Keogh et al., 2001). Python molurus L. is a large snake, up to 8 m in length and 100 kg in mass that ranges from India east into Thailand(Murphy and Henderson, 1997). Python regius Shaw 1802, the ball python, inhabits west-central Africa and is the smallest of the Python species (2 m) and is stout in body shape (Obst et al.,1984). Python reticulatus Schneider 1801, the reticulated python, ranges throughout southeastern Asia and Indonesia(Pope, 1961). Considered the longest snake in the world (reported lengths of 10 m), P. reticulatushas the most slender body shape (body mass to total length ratio of 4.53±0.18 Fig. 1) of the Python species used in this study. Python sebae Gmelin 1789, the northern African python, occurs throughout much of the northern portion of sub-Saharan Africa and is also a large python (8 m in length and 100 kg in mass) with a body shape similar to that of P. molurus(Broadley, 1984). In general, Python species are considered to be sit-and-wait foragers that feed relatively infrequently in the wild (Pope,1961 Murphy and Henderson,1997). Sit-and-wait foraging snakes lie in wait in a camouflaged location from which they can ambush passing prey(Pope, 1961 Slip and Shine, 1988 Greene, 1997).

The pythons used in this study were born in captivity and purchased commercially. We housed snakes individually in 20 l plastic boxes and maintained them at 28-32°C under a photoperiod of 14 h:10 h L:D. Snakes were fed laboratory rats once every 2 weeks and had continuous access to water. To reduce potential body-size effects, we used snakes of similar mass resulting in no significant difference among the five Python species in body mass for either the metabolic or intestinal experiments. Prior to the start of experimentation, we withheld food from snakes for a minimum of 30 days to ensure that they were postabsorptive. Python molurus has been found to complete digestion within 10-14 days after feeding(Secor and Diamond, 1995). All individual snakes used in this study were between 18 and 24 months old, with body masses of studied P. brongersmai, P. molurus, P. regius, P. reticulatus and P. sebae averaging 806±51 (N=9),760±47 (N=7), 707±71 (N=10), 757±49(N=10) and 759±47 (N=10) g, respectively. Animal care and experimentation were conducted under protocols approved by the University of Alabama Institutional Animal Care and Use Committee.

Photographs and relative body shape (body mass, Mb/total length, TL) of the five Pythonspecies used in this study. (A) P. brongersmai, (B) P. regius, (C) P. sebae, (D) P. molurus, (E) P. reticulatus. Note the significant variation in body shape from the short and stout P. brongersmai to the long and slender P. reticulatus. In the histogram, letters above bars that are different denote significant (P<0.05) differences between means, as determined from post hoc pairwise comparisons.

Photographs and relative body shape (body mass, Mb/total length, TL) of the five Pythonspecies used in this study. (A) P. brongersmai, (B) P. regius, (C) P. sebae, (D) P. molurus, (E) P. reticulatus. Note the significant variation in body shape from the short and stout P. brongersmai to the long and slender P. reticulatus. In the histogram, letters above bars that are different denote significant (P<0.05) differences between means, as determined from post hoc pairwise comparisons.

Measurements of postprandial metabolic response

We quantified the postprandial metabolic response of each species by measuring rates of oxygen consumption(O2) from snakes fasted for 30 days and following feeding. Measurements were made using closed-system respirometry as described(Secor and Diamond, 1997 Secor, 2003). Each metabolic trial began by measuring O2 of fasted snakes twice a day (morning and evening) for up to 6 days and assigning the lowest measured O2 of each snake over that time period as its standard metabolic rate (SMR). Snakes were then fed a meal consisting of one to three rats equaling 25.0±0.0% of their body mass and metabolic measurements were resumed at 12-h intervals for 3 days and at 24-h intervals thereafter for 11 more days. At 5-day intervals during metabolic measurements, snakes were removed from their chambers, weighed,provided with water, and then returned back to their chambers.

We characterized the postprandial metabolic response of meal break down,absorption and assimilation of each snake by quantifying the following six variables as described by Secor and Faulkner(Secor and Faulkner, 2002):(1) SMR, the lowest measured O2 prior to feeding (2) peak O2, the highest recorded O2following feeding (3) factorial scope of peak O2, calculated as peak O2divided by SMR (4) duration, the time after feeding that O2 was significantly elevated above SMR (5) SDA, specific dynamic action: the total energy expenditure above SMR over the duration of significantly elevated O2 and (6) SDA coefficient, SDA quantified as a percentage of meal energy. We quantified SDA(kJ) by summing the extra O2 consumed above SMR during the period of significantly elevated O2 and multiplying that value by 19.8 J ml -1 O2 consumed assuming that the dry matter of the catabolized rodent meal is 70% protein,25% fat and 5% carbohydrates, and generates a respiratory quotient (RQ) of 0.73 (Gessaman and Nagy, 1988). The energy content of rodent meals was calculated by multiplying the rodent wet mass by its energy equivalent (kJ g -1 wet mass) determined by bomb calorimetry. Five individual rats,each of three different size classes, were weighed (wet mass), dried,reweighed (dry mass), ground to a fine powder, and pressed into pellets. Three pellets from each individual rat were ignited in a bomb calorimeter (1266,Parr Instruments Co., Moline, IL, USA) to determine energy content (kJ g -1 ). For each rat, we determined wet-mass energy equivalent as the product of dry mass energy content and rodent's dry mass percentage. The three rodent size classes we used weighed on average 45±0.2, 65±5.0 and 150±5.0 g and had an energy equivalent of 6.5±0.3,7.0±0.4 and 7.6±0.3 kJ g -1 wet mass,respectively.

Tissue collection

For each species, we killed (by severing the spinal cord immediately posterior to the head) three individuals that had been fasted for 30 days and three individuals 2 days following the consumption of rodent meals equaling 25% of the snake's body mass. Following death, a mid-ventral incision was made to expose the GI tract and other internal organs, which were each removed and weighed. We emptied the contents of the stomach, small intestine and large intestine of fed snakes and reweighed each organ. The difference between full and empty weight of each organ was noted as the mass of the organ's content. Organ content mass was divided by meal mass to illustrate for each species the relative extent of digestion at 2 days postfeeding.

Intestinal nutrient uptake

In fasted and digesting snakes we measured nutrient transport rates across the intestinal brush border membrane using the everted sleeve technique as developed by Karasov and Diamond (Karasov and Diamond, 1983) and modified for snakes by Secor et al.(Secor et al., 1994) and Secor and Diamond (Secor and Diamond,2000). The empty small intestine was everted (turned inside out),divided into equal-length thirds each third was weighed and sectioned into 1-cm segments. Segments were mounted on metal rods, preincubated in reptile Ringer's solution at 30°C for 5 min, and then incubated for 2 min at 30°C in reptile Ringer's solution containing an unlabeled and radiolabeled nutrient and a radiolabeled adherent fluid marker ( l -glucose or polyethylene glycol). We measured, from individual intestinal segments, total uptake (passive and carrier-mediated) of the amino acids l -leucine and l -proline and active carrier-mediated uptake of d -glucose. Because of the similarities between uptake rates of the proximal and middle intestinal regions, we report the average uptake rates of those two segments (noted hereafter as the anterior intestine) and those of the distal segment.

A pair of studies has shown the everted sleeve technique to severely damage the intestinal mucosa of birds, and thus question the method's ability to accurately quantify intestinal performance for those species (Starck et al.,2000 Stein and Williams,2003). To determine whether the method has any damaging effects on python intestine, we compared sets of intestinal segments removed from the proximal region of the small intestine of fed P. molurus, P. reticulatus and P. sebae at two stages of the everted sleeve protocol prior to eversion and after everted tissues were incubated at 30°C in unstirred reptile Ringers for 5 min and in stirred reptile Ringers for 2 min. We prepared each intestinal segment for light microscopy (described below) and examined cross sections of the intestine for damage to the mucosal layer.

For each of these three pythons, everting, mounting and incubating intestinal segments did not damage the mucosal layer. Between the two stages of the procedure, we observed no significant difference (all P>0.47) in villus length (N=20 per stage of procedure)for these three species. In contrast to some birds, the everted sleeve can be performed without damaging the intestinal mucosa of pythons, as well as the mucosa of lizards and anurans (Secor,2005b Tracy and Diamond,2005).

Brush border enzyme activity

From each intestinal third we measured the activity of the brush border-bound hydrolase, aminopeptidase-N (EC 3.4.11.2) following the procedure of Wojnarowska and Gray (Wojnarowska and Gray, 1975). Aminopeptidase-N cleaves NH2-terminal amino acid residues from luminal oligopeptides to produce dipeptides and amino acids that then can be absorbed by the small intestine(Ahnen et al., 1982). From 1-cm segments, scraped mucosa was homogenized in PBS (1:250 dilutions) on ice. Activity of aminopeptidase-N was measured using leucyl-β-naphthylamide(LNA) as the substrate and p-hydroxymercuribenzoic acid to inhibit nonspecific cytosol peptidases. Absorbance of the product resulting from the hydrolysis of LNA was measured spectrophometrically (DU 530, Beckman Coulter,Inc., Fullerton, CA, USA) at 560 nm and compared to a standard curve developed with β-naphthylamine. Enzyme activities were quantified as μmol of substrate hydrolyzed per minute per gram of protein. Protein content of the homogenate was determined using the Bio-Rad Protein Assay kit based on the method of Bradford (Bradford,1976).

Intestinal morphology and organ masses

We quantified the effects of feeding on small intestinal morphology by measuring intestinal mass, intestinal length, mucosa and muscularis/serosa thickness and enterocyte dimensions from fasted and fed snakes. Immediately following the removal and flushing of the small intestine, we measured its wet mass and length. From the middle region of the small intestine, a 1-cm segment was fixed in 10% neutral-buffered formalin solution, embedded in paraffin and cross sectioned (6 μm). Several cross sections were placed on a glass slide and stained with Hematoxylin and Eosin. We measured mucosa and muscularis/serosa thickness and enterocyte dimensions from individual cross sections using a light microscope and video camera linked to a computer and image-analysis software (Motic Image Plus, Richmond, British Columbia,Canada). We calculated the average thickness of the mucosa and muscularis/serosa from ten measurements taken at different positions of the cross section. Likewise, we averaged the height and width of ten enterocytes measured at different positions of the cross section and calculated their volume based on the formula for a cube (enterocyte width 2 ×height). To assess postprandial effects on the mass of other organs, we weighed the wet mass of the heart, lungs, liver, empty stomach, pancreas,empty large intestine and kidneys immediately upon their removal from snakes. Each organ was dried at 60°C for 2 weeks and then reweighed for dry mass.

Small intestinal capacity

For each nutrient we quantified the intestine's total uptake capacity(reported as μmole min -1 ) by summing together the product of segment mass (mg) and mass-specific rates of nutrient uptake (nmole min -1 mg -1 ) for the proximal, middle and distal segments. Likewise, we quantified total small intestinal capacity for aminopeptidase-N activity by summing the products of mucosa segment mass (mg)times segment aminopeptidase-N activity, calculated as μmol of substrate hydrolyzed per minute per mg of mucosa. Mucosa mass was calculated from the mass of scraped mucosa from a 1-cm segment of intestine and multiplying that mass by segment length.

Statistical analyses

For each metabolic trial we used repeated-measures design analysis of variance (ANOVA) to test for significant effects of time (before and after feeding) on O2. Additionally, we used post hoc pairwise mean comparisons(Tukey-Kramer procedure) to determine when post feeding O2 was no longer significantly different from SMR, and to identify significant differences in O2 between sampling times. To test for species effects on metabolic variables, we used ANOVA for mass-specific rates and analysis of covariance (ANCOVA), with body mass as the covariate, for whole-animal measurements. Significant ANOVA and ANCOVA results were followed by post hoc comparisons to identify significant differences between species.

A repeated-measures design ANOVA and post hoc comparisons were employed to test for positional effects (proximal, middle and distal regions of the small intestine) on nutrient uptake rates and aminopeptidase-N activities. We used ANOVA to determine the postfeeding effects on nutrient uptake rates and aminopeptidase-N activity, and ANCOVA (body mass as the covariate) to test for postfeeding changes in total small intestinal capacity for nutrient uptake and aminopeptidase-N activity. Likewise, we used ANCOVA(body mass as the covariate) to test for postfeeding effects on intestinal mass, length and morphology, and the wet and dry masses of other organs. Species differences in intestinal morphology were also explored by ANCOVA and post hoc comparisons. We designate the level of significance as P<0.05 and report mean values as means ± 1 s.e.m.


STARVING SNAKES

All animals face the risk of periods of food deprivation, which can lead to starvation and ultimately death. Most animals, especially mammals, are not well adapted to withstand food deprivation and extended periods of starvation. But some animals, such as penguins and ground squirrels, have developed strategies that enable them to survive multiple months without food. Snakes,however, are in a league of their own in their ability to deal with food limitation and can endure multiple years of starvation. Although this has been known for a long time, very little is known about the underlying biological mechanisms. To investigate this stunning phenomenon, Marshall D. McCue from the University of Arkansas, USA, examined changes in physiology,morphology and body composition in response to 168 days of starvation in three species of snakes: the ball python (Python regius), the ratsnake(Elaphe obsolete) and the western diamondback rattlesnake(Crotalus atrox).

It is not a simple task to define when fasting turns into starvation,especially in infrequently eating animals. In this study, McCue defined the starvation period as starting when animals were deprived of a meal they would otherwise voluntarily have eaten, which is around 2 weeks after a meal. With this in mind, the 62 snakes were subdivided into four groups: fasting, and 56,112 and 168 days of starvation. All animals had access to fresh water throughout the experiment. McCue then measured the effects of starvation on body composition, mass and length, and resting metabolic rate over a 24 h period.

Following 168 days of starvation, all snakes had lost a percentage of their initial body mass: ratsnakes 9.3%, pythons 18.3% and rattlesnakes 24.4%. Despite this serious weight loss, and in contrast to previous investigations on reptiles and fish, all three species increased in length by around 4%. This indicates that there is a rather high selection pressure on length in these sub-adult snakes – size apparently does matter. Starvation also induced a highly significant decrease in resting metabolic rate in all three species,especially in rattlesnakes, which had a metabolic depression of an astounding 72%. This is surprising, since snakes have a very low resting rate even before the onset of starvation, and it was not expected that they could reduce this much further.

To find out how starvation affected body composition, McCue measured the water content of dead snakes by freeze-drying and subsequently measured the amount of lipid, carbohydrate and protein in their bodies. Because the snakes had access to water during the experiment, relative water content increased in all the species by an average of 6%, despite their weight loss. The relative protein content increased in all species during starvation, whereas lipid and carbohydrate content decreased. This shows that all snakes preferably use fat stores over protein as an energy source during starvation. Comparing body composition between the species, McCue found that ratsnakes began to break down proteins faster than pythons and rattlesnakes. This is probably because ratsnakes generally have an abundant food supply in their natural habitat and are maybe not as adapted to starvation as the other species.

The results show that starving snakes reduce their resting metabolic rate and change to metabolising lipids while sparing their protein stores. This was done to a degree where all snakes were able to increase in length despite a significant weight loss. Further investigations are needed to determine whether the observed metabolic depression is achieved through reductions in protein synthesis, reducing nerve activity or by something else entirely. Nevertheless, this paper very elegantly demonstrates one of the reasons why snakes are such an evolutionary success – they are well adapted to survive in areas with a low density of prey.


MATERIALS AND METHODS

Animals

The little brown bat (Myotis lucifugus LeConte) is insectivorous (Fenton and Barclay, 1980). Both rodents of this study (white-footed mouse, Peromyscus leucopus Rafinesque and northern grasshopper mouse, Onychomys leucogaster Wied-Neuwied) are somewhat omnivorous, but specialize on protein-rich diets more than many rodents grasshopper mice in particular are unique amongst North American rodents in having a diet composed primarily of arthropods and vertebrates (Martin and Nelson, 1951 McCarty, 1978 Lackey et al., 1985). Myotis lucifugus were captured in Dane County, Wisconsin, near night roosts using mistnets and were used in experiments within 12 h of capture (Table 1). Onychomys leucogaster were trapped in Finney County, Kansas. Peromyscus leucopus were caught in Dane County, Wisconsin, and species identity was verified by molecular techniques (see ‘Gene expression’, below). Rodents were maintained in captivity at the University of Wisconsin-Madison, where they were housed singly and were supplied ad libitum with water and food (Purina 5010 Rodent Diet 23.9% protein, 5% fat, by mass) until experiments (no longer than 2 months). To add more protein to the diet, O. leucogaster was supplemented with 2 g Purina Cat Chow Complete (34% protein, 13% fat) daily, which they were observed to consume entirely. All experiments were conducted at night to coincide with the active periods of the animals, as all three species are nocturnal. Experimental protocols were approved by the University of Wisconsin-Madison Animal Care and Use Committee (protocol no. A1441). Permits for trapping were obtained from the appropriate state authorities (Wisconsin Department of Natural Resources permits E/T 704 and SCP-SOD-03-2011 Kansas Department of Wildlife, Parks and Tourism permit SC-126-2012).

Assessment of paracellular absorption in intact animals

In rodents, we assessed paracellular absorption of probes by urine recovery (Fasulo et al., 2013a). We prefer urine collection over blood collection because it eliminates the need for repeated blood sampling in small animals and allowed us to reduce the number of animals used. In separate trials, individuals were dosed via intraperitoneal injection (in saline) or oral gavage (in saline with 50 mmol l −1 d -glucose). After dosing, rodents were put in wire-bottomed metabolic cages with access to water (with 50 mmol l −1 glucose) but no food. Addition of glucose to the water adds some nutrition and encourages the animals to drink and urinate more frequently. Cages were checked approximately every hour and urine was collected and weighed. Subsamples were counted in 5 ml Ecolume scintillation cocktail in 8 ml glass vials using a Wallac 1414 liquid scintillation counter (PerkinElmer, Waltham, MA, USA), and we determined total recovery of the probe after 24 h. Fractional absorption of the probes was determined by dividing the recovery (percentage of the dose recovered) after gavage by the recovery after injection.

Animals were dosed with the following probes: [ 3 H]-3-O-methyl- d -glucose (3OMD-glucose), [ 14 C]- l -arabinose, [ 3 H]-lactulose and [ 14 C]-creatinine. 3OMD-glucose (Mr 194) is a glucose analog that is not metabolized but is absorbed both via glucose transporters and via tight junctions. To estimate the paracellular absorption of an ‘amino acid-like’ molecule, we chose creatinine (Mr 113), which is similar in size to proline (Mr 115) and contains nitrogen. There are reports of some proteins that are capable of transporting creatinine in the kidney, notably OAT2 (Lepist et al., 2014). However, expression of OAT2 is either low or absent in the intestine (Maubon et al., 2007 Meier et al., 2007 Estudante et al., 2013), and creatinine absorption in the intestine has generally been ascribed to the paracellular pathway (Dominguez and Pomerene, 1945 Pappenheimer, 1990 Turner et al., 2000). In addition, we tested for saturation kinetics of intestinal creatinine absorption (see ‘Validation of paracellular probes’, below). Arabinose (Mr 150) and lactulose (Mr 342) are carbohydrates that have no affinity for intestinal transporters and were chosen for their similarity and dissimilarity, respectively, to the size of glucose (Mr 180). Rodents were tested in multiple trials that were separated by at least 3 days.

Bats urinate infrequently and it can be difficult to separate their urine from their feces. For these reasons, we determined fractional recovery by serial blood sampling (for details, see Caviedes-Vidal et al., 2008 Karasov et al., 2012). Individual bats were dosed with an oral gavage (in saline and 50 mmol l −1 glucose) or intraperitoneal injection (in saline), and a blood sample was taken from the antebrachial or uropatagial vein after

5, 15, 30, 45, 60 and 90 min. From this, we constructed a curve of dose-corrected plasma concentration versus time. We determined the area under the curve via the trapezoidal rule. To determine the area under the curve after the final point, we plotted ln(concentration) versus time and determined the elimination constant (Kel) as the negative slope of the line joining the final two points in the curve. We averaged the Kel for all injection trials and then applied this slope to the gavage trials. The area under the curve after the final point was calculated as the final concentration divided by Kel. For each probe, fractional absorption was determined by dividing the average area under the curve in gavage trials by the average area under the curve in injection trials. Individual bats could only be used in single trials and were then euthanized.

In situ luminal perfusions

We performed perfusions of the intestinal lumen as described previously (Price et al., 2013a). M. lucifugus individuals were naïve, whereas the rodents had been previously used (at least 3 days before) in the intact animal absorption study. Animals were maintained at 37°C with a Deltaphase isothermal heating pad (Braintree Scientific, Braintree, MA, USA) while under isoflurane anesthesia. The abdomen was opened and we cannulated the small intestine close to the stomach. An exit cannula was placed 9.1±0.45 cm distally, and pre-warmed saline was pumped through the intestine to flush out any gut contents. We then began the experimental perfusion using a peristaltic pump to circulate a buffer (10 mmol l −1 d -glucose, 10 mmol l −1 l -proline, 1 mmol l −1 l -arabinose, 1 mmol l −1 creatinine, 1.2 mmol l −1 NaHPO4, 110 mmol l −1 NaCl, 5 mmol l −1 KCl, 1 mmol l −1 MgSO4, 2 mmol l −1 CaCl2, 20 mmol l −1 NaHCO3, pH 7.4) at a flow of 1 ml min −1 . In M. lucifugus and P. leucopus, we measured absorption of the probes by adding tracer amounts of [ 3 H]-proline, [ 3 H]-3OMD-glucose, [ 14 C]-creatinine and [ 14 C]-arabinose. Due to a paucity of animals, we only made measurements of O. leucogaster using labeled [ 3 H]-proline and [ 14 C]- l -arabinose with liquid scintillation counting, and measured glucose concentration using a kit (Sigma-Aldrich, St Louis, MO, USA). After exiting the intestine, the perfusate returned to a reservoir (which was kept at 37°C in a water bath) and recirculated for 104±4 min and was then collected. Animals remained alive throughout the perfusion.

We weighed the perfusate prior to and following the perfusion. Subsamples (50 μl) collected before and after the perfusion were counted. After each experiment, the perfused length of the intestine was measured using calipers. The intestine was then cut longitudinally and laid flat to measure the circumference using the average of three measurements along its length. We calculated the nominal surface area (which is the surface area of a smooth bore tube and does not account for villous magnification) as the product of length×circumference. Absorption of each probe was calculated from the loss of total radioactivity during the perfusion experiment and was normalized among experiments by dividing by contact time on the intestine (min) and nominal surface area (cm 2 ). Readers can re-express absorption per milligram of wet intestine using the following conversion factors for M. lucifugus and P. leucopus, respectively: 78.3 mg cm −2 and 63.1 mg cm −2 (not measured for O. leucogaster). For arabinose and creatinine, we also calculated clearance (μl min −1 cm −2 ) by dividing absorption by (CinitialCfinal)/(Cinitial/Cfinal), where C is concentration (Sadowski and Meddings, 1993). Because they are not transporter mediated (and therefore do not exhibit saturation kinetics), absorption of arabinose and creatinine vary linearly with concentration. Dividing absorption by the concentration to calculate clearance thus provides a value that can be compared with other studies that employ other concentrations.

To estimate the proportion of glucose absorption that was paracellular, we used arabinose, and to estimate paracellular proline absorption we used creatinine. l -Arabinose and creatinine absorptions were measured at 1 mmol l −1 , whereas glucose and proline absorptions were measured at 10 mmol l −1 . Because the absorption of arabinose and creatinine is not carrier mediated (see ‘Validation of paracellular probes’, below), their absorption rates are directly proportional to their luminal concentrations. Therefore, we multiplied arabinose or creatinine absorption by 10, divided by total glucose or proline absorption at 10 mmol l −1 , and multiplied this quotient by 100%. For this calculation, we assumed that absorption of 3OMD-glucose was representative of glucose absorption, although the larger size of 3OMD-glucose and lower affinity for the glucose transporter may make this an underestimation. The effects of probe choice for estimating paracellular nutrient absorption are considered further in the Discussion.

Validation of paracellular probes

To verify that our l -arabinose and creatinine probes have no affinity for intestinal transporters, we used an everted sleeve technique to test for saturation kinetics (Karasov and Diamond, 1983 Lavin et al., 2007). Seven naïve M. lucifugus and P. leucopus were euthanized with CO2 and the entire small intestine was immediately removed and perfused with ice cold Ringer solution (125 mmol l −1 NaCl, 4.7 mmol l −1 KCl, 2.5 mmol l −1 CaCl2, 1.2 mmol l −1 KH2PO4, 1.2 mmol l −1 MgSO4 and 20 mmol l −1 NaHCO3, pH 7.3–7.4). We everted the intestine and cut it into sections that were then secured to the tip of a metal rod (2 or 3 mm diameter). Six 1-cm sections were mounted using surgical thread with the aid of grooves at 1 and 11 mm from the rods' ends excess tissue was cut away. Throughout preparation, tissues were kept cold in Ringer solution gassed with 95% O2 and 5% CO2.

We prepared experimental incubation solutions modified from the Ringer solution using isosmotic replacement of NaCl. We tested for saturation of intestinal transporters using pairs of adjacent sections of intestine one section was incubated with 100 mmol l −1 mannitol (with only tracer amounts of the probe of interest,

1 μmol l −1 ) while the other section was incubated with a probe at high concentration (100 mmol l −1 d -glucose, 100 mmol l −1 l -arabinose or 100 mmol l −1 creatinine). Each pair of solutions was labeled with an impermeant marker to account for adherent fluid ([ 3 H]polyethylene glycol, Mr 4000) as well as the appropriate probe ([ 14 C] d -glucose, [ 14 C] l -arabinose or [ 14 C]creatinine).

For 5 min prior to testing, mounted intestinal tissue was pre-incubated in the Ringer solution gassed with 95% O2 and 5% CO2 at 37°C. Tissues were then transferred to the experimental solution where they were incubated for 2 min (glucose) or 4 min (arabinose and creatinine), during which the solution was gassed at 37°C and mixed at high speed with a spin bar. The tissue was then removed, blotted to remove excess solution, cut from the mounting rod with a scalpel, and placed in a tared scintillation vial and weighed. We added 1 ml Soluene-350 tissue solubilizer (PerkinElmer) and incubated the tissue overnight at 55°C, before adding 5 ml scintillation cocktail for counting. Samples of the incubation solutions were taken prior to incubation and prepared similarly for counting.


Materials and methods

Animals and housing

We used 18 adult Gila monsters Heloderma suspectum Cope 1869 (11 males, mean mass 468 g, range 401–632 g, and 7 females, mean mass 490 g,range 332–605 g), acquired from the Arizona Game and Fish Department captive collection of non-releasable animals. Animals were transported by air to the University of Alabama and maintained prior to experimentation in large fiberglass cages (67 cm×30 cm×18 cm), 4–5 Gila monsters per cage, at a temperature of 25–28°C. Within the cages, Gila monsters were provided with refugia and water. Prior to study, Gila monsters were fasted for a minimum of 30 days to ensure that they were postabsorptive.

Experimental procedures

Gila monsters experience significant elevations in plasma exendin-4 concentrations after biting or feeding on rodent prey however, if force-fed rodents while under anesthesia, they do not increase plasma exendin-4 concentrations (Christel and DeNardo,2006). Likewise, when feeding upon chicken egg white and yolk they do not experience an elevation in plasma exendin-4(Christel and DeNardo, 2006). To examine the metabolic responses of Gila monster to different meals and to circulating plasma exendin-4 concentrations, we used the following three meal treatments: (1) pre-killed neonate rats (∼20 g) fed voluntarily, (2)pre-killed neonate rats fed under anesthesia, and (3) chicken egg white and yolk fed voluntarily. For the second meal treatment, which was designed to prevent endogenous exendin-4 release, we lightly anesthetized each Gila monster by placing it in a chamber containing isoflurane until it was unresponsive to touch. We then used a bird speculum to keep the mouth open and a pair of hemostats to push the pre-killed neonate rat into the Gila monster's stomach. For each meal treatment we used six Gila monsters and meal mass was equal to 10.02±0.01% of Gila monster body mass.

To explore the postprandial responses of Gila monsters for intestinal function and morphology, and the potential regulatory role of exendin-4 for those responses, we compared intestinal structure, nutrient uptake and hydrolase activity among four feeding treatments. Twelve Gila monsters were divided equally among four treatment groups so that there would be no significant difference in mean body mass among treatments. The four treatment groups were: (1) after a 30-day fast with no expected circulating exendin-4(Fasted) (2) 1 day following the voluntary consumption of a pre-killed neonate rat meal with expected elevated plasma exendin-4 levels (1DPF) (3) 1 day following the force-feeding under anesthesia (described above) of a pre-killed neonate rat with no expected release of exendin-4 (1DPF-anes) and(4) 3 days following the voluntary consumption of pre-killed neonate rat meal with expected elevated plasma exendin-4 levels (3DPF). For the three feeding treatments, meal mass was equivalent to 10.01±0.004% of Gila monster body mass. After treatment, Gila monsters were killed by severing their spinal cord, immediately posterior to the head. A mid-ventral incision was made to expose the internal organs, which were removed and weighed. For fed animals,the stomach and small intestine were emptied of their contents and reweighed. The difference between organ full mass and empty mass was recorded as the wet mass of organ contents.

Metabolic rate and specific dynamic action (SDA)

We quantified pre- and postprandial metabolism of Gila monsters by measuring rates of oxygen consumption(O2) and carbon dioxide production(CO2) using closed-system respirometry as described by Secor(Secor, 2003). Gila monsters were placed individually into opaque respirometry chambers (volume 9 l) and maintained at 30°C within an environmental chamber. Each respirometry chamber was fitted with incurrent and excurrent air ports, each attached to a three-way stopcock. With the exception of sampling periods, air was continuously pumped into chambers through the incurrent air port.

For each measurement of gas exchange, we withdrew a 50 ml air sample from the excurrent air port, and closed both ports to seal the chamber. 0.5–1 h later, the excurrent air port was opened and a second 50 ml air sample was withdrawn. Air samples were pumped (125 ml min –1 ) through a column of water absorbent material (Drierite™ W. A. Hammond Drierite Co., Xenia, OH, USA) and CO2 absorbent material (Ascarite IIThomas Scientific, Swedesboro, NJ, USA) into an O2 analyzer(S-3A/II AEI Technologies, Pittsburgh, PA, USA) and through a column of water absorbent material into a CO2 analyzer (CD-3A AEI Technologies). We calculated whole-animal (ml h –1 ) and mass-specific (ml g –1 h –1 ) rates of O2 and CO2, corrected for standard pressure and temperature as described previously(Vleck, 1987).

We began the metabolic trial by measuring rates of gas exchanges of each Gila monster twice a day (at ∼08:00 h and 20:00 h) for 4 days. We assigned for each Gila monster its standard metabolic rate (SMR) as the lowest O2 and accompanied CO2measured over those days. Following SMR determination, each Gila monster was fed, returned to its respirometry chamber, and measurements resumed at 12 h intervals (∼08:00 h and 20:00 h) for 3 days and thereafter at 1-day intervals (∼08:00 h) for 7 more days.

We characterized the postprandial metabolic response to meal digestion,absorption and assimilation for each animal by quantifying the following seven variables as described by Secor and Faulkner(Secor and Faulkner, 2002):(1) SMR, the lowest measured O2 prior to feeding (2) peak O2, the highest recorded O2following feeding (3) factorial scope of peak O2, calculated as peak O2divided by SMR (4) respiratory exchange ratio (RER) calculated as CO2/O2(5) duration, the time after feeding that O2 was significantly elevated above SMR (6) SDA (specific dynamic action), the total energy expenditure above SMR over the duration of significantly elevated O2 and (7) SDA coefficient, SDA quantified as a percentage of meal energy. We quantified SDA(kJ) by summing the extra O2 consumed above SMR during the period of significantly elevated O2 and multiplying that value by 19.8 J ml O2 consumed, assuming that the dry matter of the catabolized meal is 70% protein, 25% fat and 5%carbohydrates, and generates a respiratory quotient (RQ) of 0.73(Gessaman and Nagy, 1988). The energy content of the rodent and egg meals was calculated by multiplying meal mass by its specific energy equivalent (kJ g –1 wet mass)determined by bomb calorimetry. Five neonate rat and five eggs (minus the shell) were weighed (wet mass), dried, reweighed (dry mass), ground to a fine powder, and pressed into pellets. Three pellets from each individual rat or egg were ignited in a bomb calorimeter (1266, Parr Instruments Co., Moline,IL, USA) to determine energy content (kJ g –1 ). For each meal,we determined wet-mass energy equivalent as the product of dry mass energy content and dry mass percentage. The neonate rats had a dry mass percentage of 26.0±0.3% and an energy equivalent of 6.82±0.13 kJ g –1 wet mass, whereas the shell-less eggs had a dry mass percentage of 24.6±0.3% and an energy equivalent of 7.14±0.17 kJ g –1 wet mass.

Intestinal morphology and organ masses

We examined the effects of feeding treatment on small intestinal morphology by measuring intestinal mass, intestinal length, mucosa and muscularis/serosa thickness, and enterocyte dimensions from fasted and fed Gila monsters. For each Gila monster, we weighed the emptied small intestine and measured its length. A 1 cm segment from the middle region was fixed in 10%neutral-buffered formalin solution, embedded in paraffin, and cross-sectioned(6 μm slices). Several cross-sections were placed on a glass slide and stained with Hematoxylin and Eosin. The thickness of the mucosa and muscularis/serosa layers and the height and width of ten enterocytes were measured at ten sites on each cross-section using a light microscope and video camera linked to a computer and image-analysis software (Motic Image Plus,British Columbia, Canada). For each Gila monster we report the average thickness of the mucosa and muscularis/serosa layers, the average height and width of enterocytes, and the average enterocyte volume, calculated using the formula for a cube (enterocyte width 2 ×height). To determine treatment effects on the mass of other organs, we determined the wet mass of the heart, lungs, liver, empty stomach, pancreas, empty large intestine and kidneys immediately upon their removal, dried each organ at 60°C for 2 weeks, and reweighed each to obtain dry mass.

Intestinal aminopeptidase-N activity

For fasted and fed Gila monsters, we measured the activity of the brush border bound hydrolase, aminopeptidase-N (APN EC 3.4.11.2) from the proximal third of the small intestine, following the procedure of Wojnarowska and Gray(Wojnarowska and Gray, 1975)and used previously on pythons (Ott and Secor, 2007a). Aminopeptidase-N cleaves NH2-terminal amino acid residues from luminal oligopeptides to produce dipeptides and amino acids that then can be absorbed by the small intestine(Ahnen et al., 1982). Scraped mucosa from intestinal segments was homogenized in PBS (1:250 dilutions) on ice. We used leucyl-β-naphthylamide (LNA) as the substrate and p-hydroxymercuribenzoic acid to inhibit nonspecific cytosol peptidases to quantify APN activity. Absorbance of the product resulting from the hydrolysis of LNA was measured spectrophometrically (DU 530, Beckman Coulter, Fullerton, CA, USA) at 560 nm and compared to a standard curve developed with β-naphthylamine. We quantified APN activity as μmol substrate hydrolyzed min –1 g –1 mucosal protein. Protein concentration of the homogenate was determined using Bio-Rad(Hercules, CA, USA) Protein Assay kit based on the Bradford method(Bradford, 1976).

Intestinal nutrient uptake

We measured nutrient transport rates across the intestinal brush border membrane of fasted and fed Gila monsters using the everted sleeve technique as described by Karasov and Diamond (Karasov and Diamond, 1983) and Secor and Diamond(Secor and Diamond, 2000). This method can be performed on the intestines of lizards and snakes without damaging the intestinal mucosal (Ott and Secor, 2007a Tracy and Diamond, 2005). The small intestine was removed, cleared of its contents, everted and divided into equal-length thirds (proximal, middle and distal). Each third was weighed and then sectioned into 1 cm segments. Segments were mounted on metal rods, preincubated in reptile Ringer's solution at 30°C for 5 min, and then incubated for 2 min at 30°C in reptile Ringer's solution containing an unlabeled and radiolabeled nutrient and a radiolabeled adherent fluid marker ( l -glucose or polyethylene glycol) (Secor et al., 1994). From intestinal segments we measured the total uptake (passive and carrier-mediated) of the amino acids l -leucine and l -proline, as well as the active, carrier-mediated uptake of d -glucose. Nutrient uptake rates were quantified as nanomol nutrient transported min –1 incubation mg –1 segment wet mass. In addition, we quantified the intestine's total uptake capacity (reported as μmol min –1 ) for each nutrient by summing together the product of segment mass (mg) and mass-specific rates of nutrient uptake (nmol min –1 mg –1 ) for the proximal, middle and distal segments.

Statistical analysis

For each of the three SDA trials, we used repeated-measures analysis of variance (RM-ANOVA) to test for significant effects of time (before and after feeding) on O2. Concurrently, we used post hoc pairwise mean comparisons (Tukey test)to determine when postfeeding O2 was no longer significantly different from SMR, and to identify significant differences in O2 between sampling times. To test for meal treatment effects on metabolic variables, we used ANOVA for mass-specific rates and analysis of covariance (ANCOVA), with body mass as the covariate, for whole-animal measurements. To identify positional effects on nutrient uptake rates, we employed RM-ANOVA for each treatment (fasted and fed). Among treatments we used ANOVA to compare nutrient uptake rates for each intestinal position and APN activity for the middle segment, and ANCOVA to compare intestinal nutrient uptake capacities. We followed significant ANOVA and ANCOVA results with post hoc tests to identify significant differences between meal treatments. We calculated each statistical analysis using SAS and designated the level of statistical significance as P=0.05. Mean values are reported as mean ± 1 s.e.m.


Crikey! How Crocs Digest Animals Whole

Crocodiles are ferocious creatures that will eat snakes, buffalo, cattle and even people. New research explains crocodiles' spectacular method of digesting large meals that lets them eat 23 percent of their body weight at once, bones and all.

If people could gorge like crocodiles, a 130-pound woman could down a 30-pound hamburger in one sitting.

The secret behind this champion eating is a heart valve that crocs control neurologically, which lets blood bypass the lungs and flow through a special aorta straight to the stomach, enabling them to secrete gastric acid at rates 10 times faster than those measured in any other animal.

Crocodiles, alligators and other crocodilians all share this ability, said biologist C. G. Farmer at the University of Utah, who discovered the connection between the heart valve and digestion in research that will be detailed in the March-April issue of the journal Physiological and Biochemical Zoology.

"It's been known for many years that reptiles can shunt blood past the lungs, but the function has not been understood," Farmer told LiveScience. Many possible explanations for the purpose of the heart valve have been proposed, including the suggestion that the process is important for diving underwater for long periods of time, although no data has yet been found to support this hypothesis.

"Some people in the field are pretty sure this is explained by diving, so I think they're going to be surprised," Farmer said.

Farmer and her colleagues surgically altered some crocodiles so that they could not use the valve to send blood past the lungs. The biologists then measured how quickly the crocs could secrete stomach acid and found that those with the valve intact produced acid at a much higher rate.

When blood bypasses the lungs, it holds on to the carbon dioxide that would have normally been released into the gases in the lungs. Carbon dioxide is a chemical ingredient of gastric acid, so the more CO2 in the blood when it reaches the stomach, the more acid can be produced. This is essential for digesting large amounts of food.

"If any animal eats a meal that size, they can't process it immediately," Farmer said. "As the meal is being broken down, the stomach holds on to the bulk of the food and sends little bits on to the intestine. If they weren&rsquot able to secrete a lot of acid in their stomachs, the food there would putrefy due to the overgrowth of bacteria. Eating big meals infrequently has selected for this ability."

The excess of stomach acid is also helpful in dissolving the bones of prey crocodiles swallow whole.

While the neurologically-controlled valve is found only in crocodilians, all reptiles have some kind of shunting system for moving blood past the lungs.

Farmer said it would be interesting to see if Burmese pythons also use the system for digestion, because they can eat meals that weigh more than 100 percent of their body mass.

"They do have a shunt system, and I'll bet you they're using it," she said. "It's just hard to study for technical reasons. But I bet you money this is going to apply to all reptiles."


Predation and Food Webs

Walter K. Dodds , Matt R. Whiles , in Freshwater Ecology (Second Edition) , 2010

Adaptations of Predators

Predation can be characterized logically by a sequence of events. Prey must be encountered and detected, then attacked and captured, and finally ingested ( Brönmark and Hansson, 1998 ). Adaptations are evident at all stages this section is organized by the natural sequence of events.

The behavioral strategy used by organisms to obtain prey can vary from remaining immobile and allowing prey to approach to active foraging. The specific strategy that has evolved presumably allows for the most efficient harvesting of food given morphological, abiotic, and community constraints. The simplest encounter strategy is to “sit and wait” for prey. For example, pike (Esox) lay hidden, waiting for prey to come close. Pike have large tail (caudal) fins and pull their bodies into “S” shapes from which they can accelerate explosively to catch prey. Odonate dragonfly nymphs also wait for prey. They have a hinged labial jaw that ejects very rapidly and snatches prey. Hydra capture larval bluegill (Lepomis macrochirus) that contact its stinging nematocysts, a form of sit-and-wait predation that can have considerable impact on the populations of the larval fish (Elliot et al., 1997).

Carnivorous plants in wetlands are notable sit-and-wait predators on insects. Most species tolerate or require saturated soils and are wetland species ( Juniper et al., 1989 ). Passive trapping strategies are used, which include the use of adhesive traps such as in sun dew (Drosera), chambers that can be entered but not left as in the pitcher plants (e.g., Sarracenia ), snap traps such as the Venus flytrap (Dionaea), and triggered chambers as in the bladderworts (Utricularia). These plants also need to attract insects, and adaptations include visual stimuli such as UV patterns visible to insects. Olfactory substances can be produced, including those with nectar scent or the smell of putrefaction, and nectar rewards may be used to lure insects. Tactile stimuli are also important in some cases, such as that of the bladderwort, Utricularia, which has filamentous extensions on submerged gas-filled bladders that mimic filamentous algae and attract epiphyte feeders when the invertebrate contacts the extensions, the bladder fills rapidly with water and sucks in the prey ( Juniper et al., 1989 ). Carnivorous plants are generally found in low-nutrient environments and are photosynthetic, so they use their prey as a source of nitrogen and phosphorus. Carnivorous plants are an excellent example of convergent evolution leading to solution of a problem (nutrient limitation) from divergent plant lineages using a wide variety of capture and attraction mechanisms.

Sensing prey can be accomplished by a variety of adaptations, depending on the organisms and their prey. Invertebrates use visual (in a few organisms with well-developed eyes), mechanical, tactile, and chemical cues ( Peckarsky, 1982 ). In an ingenious demonstration of the importance of the use of mechanical cues, Peckarsky and Wilcox (1989) recorded hydrodynamic pressure wave patterns associated with escaping Baetis nymphs. Predatory stonefly nymphs (Kogotus modestus) attacked Baetis models in greater frequency when the wave patterns were played back than when they were not.

Fishes can sense prey visually, chemically, electrically, or hydrodynamically. Electrical sensory systems in fishes are highly developed the paddlefish (Polyodon spathula) can sense the electrical activity of a swarm of Daphnia 5 cm away ( Russell et al., 1999 ).

The idea that evolution through selection leads to maximization of net energy gained per unit time feeding, led to the concept of optimal foraging ( Pyke et al., 1977 Schoener, 1987). This simple concept can be applied to all phases of predation (encounter, detection, attack, capture, and ingestion). We have already discussed the relative merits of sitting and waiting for prey as opposed to active searching. Food quality, quantity, and spatial distribution are often additional considerations for optimal foraging. Optimal foraging has been well established for several fish species (Mittelbach and Osenberg, 1994). A food item may not be preferred if it is high quality but very rare relative to a lower quality food source. All items, even remotely suitable items, may be taken when food is limiting, but only the most profitable may be taken when more is available. For example, bluegill will capture all sizes of zooplankton in equal amounts when the food is at low density but will take large Daphnia preferentially at higher zooplankton concentrations ( Fig. 20.6 ).

Figure 20.6 . Selectivity of bluegill (Lepomis macrochirus) on size of prey (Daphnia magna) at two prey densities. At low densities, all sizes of Daphnia are equally represented in the bluegill guts. At high densities of prey, the larger zooplankton are preferred (selectivity of 1 means consumption is the same as expected relative to random encounter rates).

(Data from Werner and Hall, 1974 ).

Predictions can also be made regarding how long a predator will remain in a patch of food. If a patch is of low quality, then moving to another patch may be more beneficial. However, if the cost of moving to another patch is high, it can be beneficial to extract more food from the current patch. Obviously, temporal and spatial scale are important considerations when making predictions using optimal foraging models. Similar considerations form the basis of the field of landscape ecology, discussed in Chapter 22 .

A problem with using optimal foraging theory to make predictions about behavior of predators is that an investigator may not be able to identify the most important selective forces (Gatz, 1983). For example, we could predict that large prey items are preferable to smaller items because they contain more energy. However, large prey may be encountered infrequently and may take more energy to capture. Taking many small prey items may be more energy efficient and seems to be the strategy used by many predatory freshwater fishes (Juanes, 1994).

The number of prey eaten per unit prey density is referred to as the functional response. The number of predators per unit prey density is referred to as the numerical response. A functional response curve can take one of three general forms ( Fig. 20.7 ).

Figure 20.7 . Holling's three types of functional response curves and the proportion of prey consumed assuming constant predator numbers.

In a type I functional response, predation is linearly related to prey density until some saturation is reached this is the simplest predation model. In a type II response, there is a hyperbolic response to prey density. The form of this response is similar to that seen in the Michaelis–Menten uptake kinetics described in Chapter 17 . This form of response is typical of predators that do not have a complex behavioral response to acquiring prey. Just as an enzyme can react with only so many molecules per unit time, a predator can only take a maximum number of prey items per unit time.

The type III response is seen in more advanced predators. At very low prey densities, few or no prey items are taken (optimal foraging theory predicts that there is not enough energy gain to profitably take prey). At intermediate prey densities the rate of capture increases, and eventually the predator is saturated, as in types I and II. Numerical response generally occurs over longer time periods because it requires changes in the predator's populations (from birth, death, immigration, and emigration), unlike functional responses that are primarily limited by behavioral and physiological aspects of the predator and distribution of prey in the environment.

Several strategies are used to consume prey. Many predators need to consume prey whole. These are referred to as gape-limited predators, and the size of prey they can consume is limited by the size of their mouth or gape ( Zaret, 1980 ). The dominant vertebrate predators in freshwaters are mostly of this type. Generally, gape-limited predators are highly dependent on the size of their prey, whereas others may be dependent to various degrees.


Zusammenfassung

Eine Fütterungsmaschine in Serienanordnung mit 36 Käfigen wird beschrieben, mit der die Mahlzeitenhäufigkeit und die zeitliche Aufeinanderfolge für bis zu 6 Gruppen von Ratten, Hamstern oder Mäusen unabhängig voneinander programmiert werden kann. Die Apparatur kann für 2 prinzipiell verschiedene Zwecke benützt werden: erstens zur Ausschaltung unspezifischer Wirkungen von Nahrungsbestandteilen auf die Futteraufnahme, und zweitens zum Erzwingen extrem variierender Mahlzeitenfolgen mit dem Zweck, den Einfluss verschieden häufiger Futteraufnahme auf die Kariesaktivität zu untersuchen. Es wird ein Experiment an Ratten beschrieben, in dem programmierte Fütterung 12 ×, 18 ×, 24 × oder 30 × täglich zu einer hoch signifikanten positiven Korrelation zwischen Fütterungshäufigkeit und Kariesbefall führte.