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THE DIGESTIVE SYSTEM OF VERTEBRATES

TOPIC: Digesta transit & retention

Table 7.1.  Effect of body temperature on digesta transit in fish  (From CD Chapter 7)



Table 7.2.  Mean digesta retention time in reptiles   (From CD Chapter 7)

Liquid marker was polyethylene glycol or BaSO4.  Particulate markers were segments of polyethylene tubing.


Table 7.3.  Mean digesta retention time in birds  (From CD Chapter 7)

Digesta transit time of birds tend to be short, and particles are generally retained longer than fluid digesta, but fluid was selectively retained in the ceca of the herbivorous ptarmigan. (From Stevens & Hume 1995)



Figure 7.1a.  Percentage of digesta fluid and particulate markers (+/- SE) recovered from the gastrointestinal tract of the dog at various times following their oral administration during feeding.  Fluid markers consisted of PEG or 51Cr-EDTA.  Plastic markers consisted of polyethylene tubing with an outside diameter of 2 mm, cut into lengths of 2 mm.  S = stomach; SI = small intestine; C = colon; Fe = feces.  Particles were selectively retained by the stomach and the large intestine.  (Modified from Banta et al. 1979.)  (From CD Chapter 7)



Figure 7.1b.  Percentage of digesta fluid and particulate markers (+/- SE) recovered from the gastrointestinal tract of the pig at various times following their oral administration during feeding. Fluid markers consisted of PEG or 51Cr-EDTA.  Plastic markers consisted of polyethylene tubing with an outside diameter of 2 mm, cut into lengths of 2 mm.  S = stomach; SI = small intestine; Ce = cecum; PC = proximal colon; C = colon; TC = terminal colon. Particles were selectively retained by the stomach and the large intestine. (Modified from Clemens et al. 1975a.)   (From CD Chapter 7) 



Figure 7.1c.  Percentage of digesta fluid and particulate markers recovered from the gastrointestinal tract of the rabbit at various times following their oral administration during feeding. Fluid markers consisted of PEG or 51Cr-EDTA.  Plastic markers consisted of polyethylene tubing with an outside diameter of 2 mm, cut into lengths of 2 mm.  S = stomach; SI = small intestine; Ce = cecum; C = colon; Fe = feces.  Particles were selectively retained by the stomach, but fluid was selectively retained by the cecum of rabbits, with a more rapid excretion of particles.  (Modified from Pickard & Stevens 1972.)   (From CD Chapter 7)



Figure 7.2  Gastrointestinal tracts  of reptilian, avian, and mammalian colon fermenters. The principal site(s) of microbial fermentation in these gut drawings are denoted by darker lines. Tortoise, ostrich, and pony from Stevens & Hume 1995; wombat from Harrop & Hume 1980; rhino and elephant from Clemens & Maloiy 1982.  (From CD Chapter 7)


Table 7.4. Mean digesta retention time for herbivorous colon fermenters  (From CD Chapter 7)

Although digesta retention times are affected by differences in the diet, and in the body temperatures of the reptiles, marsupial, and eutherian mammals, colon fermenters retain particulate digesta as long or longer than fluid digesta. The effects of colonic retention of particles can be muted in animals with a relatively large cecum such as the chimpanzee, orangutan and gorilla.  (modified from Stevens & Hume 1995)



Figure 7.3  Gastrointestinal tracts of avian and mammalian cecum fermenters. The principal site(s) of microbial fermentation in these gut drawings are denoted by darker lines. Grouse, rhea, guinea pig, and greater glider from Stevens & Hume 1995; rabbit from Stevens 1977; koala from Harrop & Hume 1980.  (From CD Chapter 7)


Table 7.5.  Mean digesta retention time for herbivorous cecum fermenters  (From CD Chapter 7)

Although digesta retention times are affected by differences in the diet, and in the body temperatures of the bird, marsupials, and eutherian mammals, cecum fermenters retain fluid digesta as long or longer than particulate digesta. Fluid and small digesta particles are selectively retained by the cecum of small mammals with a large cecum, especially in herbivores with a well-developed colonic separation mechanism. The longer digesta retention times of the marsupials are due, partly, to their lower rate of metabolism. (modified from Stevens & Hume 1995)



Figure 7.4  Gastrointestinal tracts of avian and mammalian foregut fermenters. The principal site(s) of microbial fermentation in these gut drawings are denoted by darker lines. Hoatzin from Grajal & Parra 1995; sloth from Stevens 1980; colobus from Stevens 1983; kangaroo from Stevens 1977; sheep from Stevens & Hume 1995.  (From CD Chapter 7)


Table 7.6.  Mean digesta retention time for herbivorous forestomach fermenters  (From CD Chapter 7)

Although digesta retention times are affected by differences in the diet, and in the body temperatures of the bird, sloth and other eutherian mammals, foregut fermenters retain particulate digesta as long or longer than fluid digesta. Most small forestomach fermenters retain fluid and particles for equal lengths of time, but particles are selectively retained by the forestomach of large species and this tends  to increase with an increase in dietary fiber. (modified from Stevens & Hume 1995)



Figure 7.5  Gastrointestinal tracts of herbivores with a combination of digestive strategies. The principal site(s) of microbial fermentation in these gut drawings are denoted by darker lines. Hyrax from Clemens 1977; hamster, dugong, and dik-dik from Stevens & Hume 1995.  (From CD Chapter 7)



Figure 7.6.  Chemical reactor models homologous to fermentation in the digestive tract of vertebrates. They consist of a batch reactor (A), plug flow reactor (B), continuous-flow, stirred-tank reactor (C), and a modified plug flow reactor (D), with pulsed inputs (PI) and outputs ((PO), or continuous inputs (CI) and outputs (CO). (From Stevens & Hume 1995.)  (From CD Chapter 7)



Figure 7.7.  Relationship between cell wall digestibility and mean retention time (MRT) of fiber by foregut and colon fermenters on a grass hay diet. Red circles represent foregut fermenting ruminants and camels; a) barasingha, b) eland, c) nilgae, d) wapiti, e) water buck, f) gaur, g) giraffe, h) gemsbok, i) African buffalo, j) American bison, k) dromedary camel, and l) bactrian camel. Blue circles represent colon fermenting a) Grevy’s zebra, b) mountain zebra, c) plains zebra, d) Asian tapir, e) American tapir, f) Asian wild ass, g) African elephant, h) Asian elephant, i) black rhino, j) Indian rhino, and k) white rhino. R2 = 0.66 for the ruminants and camels and 0.26 for colon fermenters. Yellow triangles represent; (1)  red kangaroos on an alfalfa diet, river hippos on an (2) alfalfa hay or (3) grass diet, and (4) sloths on a diet of Ceropia palmata foliage. Data for ruminants, camels, hippos, and colon fermenters are from Foose (1982). Data on red kangaroos are from Hume (1999) and data on the three-toed sloth are from Foley et al. (1995) and Foley (personal communication.)   (From CD Chapter 7)



Figure 7.8.  Variations in digestive strategy with respect to dietary combinations of refractory carbohydrates (cellulose, hemicellulose, and lignin), and protein (A) or fermentable solutes (B). The size and shape of the boxes represent the range of diets within which each digestive strategy is postulated to be effective. (From Cork et al. 1999.)  (From CD Chapter 7)



Figure 7.9.  Relationships between digestive strategies and the body mass of reptilian, avian and mammalian herbivores. Dark, shaded areas include cecum and forestomach fermenters that feed on less fibrous aquatic plants (From Stevens 1998)  (From CD Chapter 7)



Figure 7.10.  Relationship between digestive retention time and the body mass of grazing ruminants. Retention time is plotted against body mass for forages of different degrees of digestibility. Shaded area indicates the retention time necessary for digestion of most of the energy by rumen bacteria. A 120 kg animal at a body temperature of 38 'C would achieve maximum efficiency of microbial fermentation for forage of 50% digestibility within 45 hours, regardless of the site or mechanism of digesta retention. (From Demment & Van Soest 1985.)  (From CD Chapter 7)


Table 7.7.  Characteristics of digestive strategies  (From CD Chapter 7)

Plus symbols represent the presence of a characteristic. Arrows represent an increase or decrease in retention time. Minus symbols represent either the absence of a characteristic or no change in the retention time. (modified from Stevens & Hume 1995)


Table 7.8.  Adaptations of digestive strategies to environment  (From CD Chapter 7)

(Stevens 1998)


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