title: Alligators and Crocodiles Have High Paracellular Absorption of Nutrients, But Differ in Digestive Morphology and Physiology.
abstract: Much of what is known about crocodilian nutrition and growth has come from animals propagated in captivity, but captive animals from the families Crocodilidae and Alligatoridae respond differently to similar diets. Since there are few comparative studies of crocodilian digestive physiology to help explain these differences, we investigated young Alligator mississippiensis and Crocodylus porosus in terms of (1) gross and microscopic morphology of the intestine, (2) activity of the membrane-bound digestive enzymes aminopeptidase-N, maltase, and sucrase, and (3) nutrient absorption by carrier-mediated and paracellular pathways. We also measured gut morphology of animals over a larger range of body sizes. The two species showed different allometry of length and mass of the gut, with A. mississippiensis having a steeper increase in intestinal mass with body size, and C. porosus having a steeper increase in intestinal length with body size. Both species showed similar patterns of magnification of the intestinal surface area, with decreasing magnification from the proximal to distal ends of the intestine. Although A. mississippiensis had significantly greater surface-area magnification overall, a compensating significant difference in gut length between species meant that total surface area of the intestine was not significantly different from that of C. porosus. The species differed in enzyme activities, with A. mississippiensis having significantly greater ability to digest carbohydrates relative to protein than did C. porosus. These differences in enzyme activity may help explain the differences in performance between the crocodilian families when on artificial diets. Both A. mississippiensis and C. porosus showed high absorption of 3-O methyl d-glucose (absorbed via both carrier-mediated and paracellular transport), as expected. Both species also showed surprisingly high levels of l-glucose-uptake (absorbed paracellularly), with fractional absorptions as high as those previously seen only in small birds and bats. Analyses of absorption rates suggested a relatively high proportional contribution of paracellular (i.e., non-mediated) uptake to total uptake of nutrients in both species. Because we measured juveniles, and most paracellular studies to date have been on adults, it is unclear whether high paracellular absorption is generally high within crocodilians or whether these high values are specific to juveniles.
authors.affiliation: *Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT 0909, Australia; Department of Zoology, University of Melbourne, Parkville, VIC 3010, Australia; Department of Biological Science, California State University Fullerton, Fullerton, CA 92831, USA; ctracy@fullerton.edu.
authors.forename: Christopher R
authors.initials: CR
authors.lastname: Tracy
authors.affiliation: School of Animal and Veterinary Sciences, University of Adelaide, Roseworthy Campus, Adelaide, SA 5371, Australia;
authors.forename: Todd J
authors.initials: TJ
authors.lastname: McWhorter
authors.affiliation: *Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT 0909, Australia; Department of Biology and Center of Excellence for Field Biology, Austin Peay State University, Clarksville, TN 37044, USA;
authors.forename: C M
authors.initials: CM
authors.lastname: Gienger
authors.affiliation: Department of Biology, University of Munich (LMU), Munich, Germany;
authors.forename: J Matthias
authors.initials: JM
authors.lastname: Starck
authors.affiliation: Department of the Environment, Environmental Research Institute of the Supervising Scientist, GPO Box 461, Darwin, NT 0801, Australia;
authors.forename: Peter
authors.initials: P
authors.lastname: Medley
authors.affiliation: **Wildlife Management International Pty. Limited, Berrimah, NT 0828, Australia.
authors.forename: S Charlie
authors.initials: SC
authors.lastname: Manolis
authors.affiliation: *Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT 0909, Australia; **Wildlife Management International Pty. Limited, Berrimah, NT 0828, Australia.
authors.forename: Grahame J W
authors.initials: GJ
authors.lastname: Webb
authors.affiliation: *Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT 0909, Australia;
authors.forename: Keith A
authors.initials: KA
authors.lastname: Christian
mesh_terms: D000489:Alligators and Crocodiles / Q000378:metabolism*; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D001692:Biological Transport; D050260:Carbohydrate Metabolism; D004063:Digestion / Q000502:physiology*; D007408:Intestinal Absorption; D013045:Species Specificity