ORIGINAL PAPER I.D. Hume C. Beiglbo¨ck T. Ruf F. Frey-Roos U. Bruns W. Arnold Seasonal changes in morphology and function of the gastrointestinal tract of free-living alpine marmots (Marmota marmota) Accepted: 14 September 2001 / Published online: 23 January 2002 Ó Springer-Verlag 2002 Abstract The gastrointestinal tracts of 76 free-living al- pine marmots (Marmota marmota) shot during a pop- ulation control program in Switzerland were collected and analysed for patterns of change in morphology and function over the period from emergence from hiber- nation in April to just before re-entry into hibernation in September. Between first emergence and mid-summer (July) the fresh tissue mass of the stomach increased by 105%, the small intestine by 259% (among the largest recorded for a mammal), caecum by 185%, proximal colon by 138%, and distal colon by 144%. Mitotic ac- tivity was greatest in the small intestine; the mitotic in- dex was high (40%) compared with indexes in the stomach and hindgut (approximately 4%) even at emergence, and increased to approximately 60% by mid- summer. Microbial activity in the caecum was also sig- nificant at emergence. The stomach (length) and caecum (length and fresh mass) increased in response to ingested food earlier than did the small intestine. Between mid- summer and September there were decreases in small intestinal tissue mass and mitotic activity. It is concluded that the gastrointestinal tract of alpine marmots prob- ably continues to function throughout hibernation at a low level, with a mid-winter trough as part of an en- dogenous circannual rhythm. However, after emergence in spring, increases in size and activity of the tract ap- pear to be a response to ingested food rather than to an endogenous signal. The early signs of down-regulation of the small intestine before re-entry into hibernation, together with its delayed up-regulation in response to food in spring, are consistent with the high costs of maintaining this section of the digestive system. Keywords Marmot Gastrointestinal tract Hibernation Mitotic activity Microbial activity Abbreviations BrdU bromodeoxyuridine PCNA perichromonucleolin antigen PUFA poly-unsaturated fatty acid SCFA short-chain fatty acid Introduction The gastrointestinal tract is one of the most metaboli- cally intense organs of the vertebrate body in terms of energy utilisation and protein synthesis, so it is expensive to maintain (Stevens and Hume 1995). Although Speakman and Johnson (2000) concluded that no par- ticular tissue of lactating mice could be singled out as costly to maintain on the basis of mass, estimates of the gastrointestinal tract’s contribution to the total oxygen consumption of the animal range from 12% in rats to 25% in pigs (McBride and Kelly 1990). Thus it might be expected that, if the organ is not being used, it should be reduced in size and/or activity. Many studies have shown that the gastrointestinal tract is remarkably flexible in size (Piersma and Lindstro¨m 1997). Some of the largest changes in gastrointestinal tract mass have been demonstrated in sit-and-wait foraging Burmese pythons (Python molurus), which consume large meals at long and unpredictable intervals (Secor and Diamond 1997), and in long-distance migratory garden warblers (Sylvia borin), which fatten prior to migration but may not feed during migratory flights of several thousand kilometres (Hume and Biebach 1996). When not feeding, gut mass in both species is substantially reduced, and the energy not used in maintaining expensive intestinal tis- sue maximises the intervals that pythons can survive between meals and the distance that garden warblers can J Comp Physiol B (2002) 172: 197–207 DOI 10.1007/s00360-001-0240-1 Communicated by G. Heldmaier I.D. Hume (&) C. Beiglbo¨ck T. Ruf F. Frey-Roos U. Bruns W. Arnold Research Institute of Wildlife Ecology, University of Veterinary Medicine, Savoyenstrasse 1, 1160 Vienna, Austria E-mail: ianhume@bio.usyd.edu.au Fax: +61-2-93514119 Present address: I.D. Hume School of Biological Sciences A08, University of Sydney, NSW 2006, Australia