ARTICLE Comparison of gut fill in sheep (Ovis aries) measured by intake, digestibility, and digesta retention compared with measurements at harvest Adam Munn, Mathew Stewart, Elizabeth Price, Alice Peilon, Tom Savage, Irene Van Ekris, and Marcus Clauss Abstract: Gut capacity is an important factor in digestive physiology and is often measured as dry matter fill (DMF) following dissection, which prevents repeated measures in the same animal. It was proposed to calculate DMF from food intake, digest- ibility, and gut mean retention time (MRT), but empirical tests of this are few. We calculated DMF from intake, digestibility, and the MRT of small-particle (1 mm) and large-particle (20 mm) markers in 20 sheep (Ovis aries L., 1758) fed at different intake levels and compared results with DMF at dissection at the end of the feeding trial. MRT for smaller particles was significantly shorter than for larger particles (34.4 ± 6.1 vs. 42.5 ± 7.6 h, respectively). Correspondingly, DMF calculated from smaller particles (0.98 ± 0.27 kg) was significantly lower than DMF calculated from larger particles (1.20 ± 0.30 kg). The latter was not significantly different from DMF measured at dissection (1.18 ± 0.34 kg). These results suggest that DMF can be estimated from measures of digestive physiology. The choice of particle marker to determine MRT is crucial for the accuracy of the proxy. In ruminants, where small particles are consistently eliminated faster than larger particles, considerations of marker particle size are partic- ularly important. Key words: herbivore, ruminant, particle size, passage, gut capacity, sheep, Ovis aries. Résumé : La capacité du tube digestif, un facteur important en physiologie digestive, est souvent mesurée comme étant le volume de matière sèche (DMF) au moment de la dissection, ce qui empêche des mesures répétées pour le même animal. S’il a été proposé de calculer le DMF a ` partir de l’ingestion d’aliments, de la digestibilité et du temps de rétention moyen (MRT) dans le tube digestif, il existe peu de tests empiriques. Nous avons calculé le DMF a ` partir de l’ingestion d’aliments, de la digestibilité et du MRT d’un petit (1 mm) et d’un grand (20 mm) marqueurs particulaires chez 20 moutons (Ovis aries L., 1758) nourris selon différents degrés d’ingestion, et comparé les résultats avec les DMF mesurés après la dissection a ` la fin de l’essai d’alimentation. Le MRT pour les petites particules était significativement plus court que pour les grandes particules (34,4 ± 6,1 contre 42,5 ± 7,6 h, respectivement). Ainsi, le DMF calculé a ` partir des petites particules (0,98 ±0,27 kg) était significativement plus faible que le DMF calculé pour les particules plus grandes (1,20 ± 0,30 kg), ce dernier n’étant pas significativement différent du DMF mesuré après la dissection (1,18 ± 0,34 kg). Ces résultats donnent a ` penser que le DMF peut être estimé a ` partir de mesures de la physiologie digestive. Le choix du marqueur particulaire utilisé pour déterminer le MRT est d’importance cruciale pour l’exactitude de l’estimation. Pour les ruminants, chez lesquels les petites particules sont uniformément éliminées plus rapidement que les particules plus grandes, les considérations touchant a ` la taille du marqueur particulaire sont particulièrement importantes. [Traduit par la Rédaction] Mots-clés : herbivore, ruminant, taille des particules, passage, capacité du tube digestif, mouton, Ovis aries. Introduction In the interplay of various anatomical and physiological ad- aptations that form the digestive strategies of herbivores, gut capacity is an important factor that influences all other mea- sures (Hume 2005; Clauss et al. 2013). Typically, gut capacity is measured as the mass of gut contents of dissected animals or as volumetric measurements of gut sections (e.g., Demment 1982; Weckerly 2010). The disadvantages of this approach are that ani- mals must be killed, and as such, repeated measurements are not possible. Repeatable measures of gut fill in the same animals un- der different circumstances are, however, especially useful for investigating how animals might use changes in gut fill to manage changes in diet, or to examine differences between life history and reproductive stages, in parallel to investigating changes in intake and digestibility (Munn et al. 2012). As an alternative to harvest and dissection for measuring gut fill, it is possible to estimate gut fill (dry matter; DM) using the measures of food intake (DM), the mean retention time (MRT) of the digesta, and the apparent digestibility of DM (aDDM) (Blaxter et al. 1956). Most notably, Holleman and White (1989) constructed Received 28 November 2014. Accepted 19 June 2015. A. Munn. Centre for Sustainable Ecosystem Solutions, School of Biological Sciences, The University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia; Faculty of Veterinary Science, University of Sydney, Sydney, NSW 2006, Australia. M. Stewart, E. Price, and A. Peilon. Centre for Sustainable Ecosystem Solutions, School of Biological Sciences, The University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia. T. Savage. School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia. I. Van Ekris. Faculty of Veterinary Science, University of Sydney, Sydney, NSW 2006, Australia. M. Clauss. Clinic for Zoo Animals, Exotic Pets and Wildlife, Vetsuisse Faculty, University of Zurich, Winterthurerstrasse 260, 8057 Zurich, Switzerland. Corresponding author: Adam Munn (e-mail: amunn@uow.edu.au). 747 Can. J. Zool. 93: 747–753 (2015) dx.doi.org/10.1139/cjz-2014-0314 Published at www.nrcresearchpress.com/cjz on 23 July 2015. Can. J. Zool. Downloaded from www.nrcresearchpress.com by Geographisches Institut der Univ. Zuerich on 10/03/15 For personal use only.