SYMPOSIUM Alligators and Crocodiles Have High Paracellular Absorption of Nutrients, But Differ in Digestive Morphology and Physiology Christopher R. Tracy, 1, * ,†,‡ Todd J. McWhorter, § C. M. Gienger,* ,ô J. Matthias Starck, k Peter Medley, # S. Charlie Manolis,** Grahame J. W. Webb* , ** and Keith A. Christian* *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; § School of Animal and Veterinary Sciences, University of Adelaide, Roseworthy Campus, Adelaide, SA 5371, Australia; ô Department of Biology and Center of Excellence for Field Biology, Austin Peay State University, Clarksville, TN 37044, USA; jj Department of Biology, University of Munich (LMU), Munich, Germany; # Department of the Environment, Environmental Research Institute of the Supervising Scientist, GPO Box 461, Darwin, NT 0801, Australia; **Wildlife Management International Pty. Limited, Berrimah, NT 0828, Australia From the symposium ‘‘Integrated Biology of the Crocodilia’’ presented at the annual meeting of the Society for Integrative and Comparative Biology, January 3–7, 2015 at West Palm Beach, Florida. 1 E-mail: ctracy@fullerton.edu Synopsis 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 inves- tigated 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. mis- sissippiensis 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. Introduction Much of our knowledge regarding crocodilian nutri- tion, growth, and biology has come from propaga- tion and farming of captive individuals for use in the leather industry (Coulson and Hernandez 1983; Garnett and Murray 1986; Webb et al. 1987; Garnett 1988). Attempts to optimize growth rates of individuals, and therefore production, by altering Integrative and Comparative Biology Integrative and Comparative Biology, volume 55, number 6, pp. 986–1004 doi:10.1093/icb/icv060 Society for Integrative and Comparative Biology Advanced Access publication June 9, 2015 ß The Author 2015. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com. Downloaded from https://academic.oup.com/icb/article/55/6/986/2363554 by guest on 17 January 2023