ORIGINAL PAPER Monica Alterskjær Olsen ® Reidar Myklebust Turid Kaino ® Vibeke Sødring Elbrønd Svein Disch Mathiesen The gastrointestinal tract of Ade ´ lie penguins – morphology and function Accepted: 13 April 2002 / Published online: 19 June 2002 Ó Springer-Verlag 2002 Abstract The aim of this study was to provide data on the morphology of the gastrointestinal tract of Ade´lie penguins (Pygoscelis adeliae). It was found to consist of a long oesophagus, a two-chambered stomach, a small intestine measuring only 5.2·body length, two rudi- mentary caeca and a short colon, typical of carnivorous birds. The stomach comprised a glandular proventri- culus and a muscular gizzard that frequently contained grit. An acidic pH was recorded in both chambers. Ultrastructural studies of the small intestinal mucosal membrane revealed epithelial cells with elongated, ir- regular microvilli and high affinity for toluidine blue, absorptive intestinal epithelial cells and goblet cells. Numerous large lymphocyte-like cells were observed close to the brush border of the epithelium, and empty spaces on the epithelial surface reflected normal cell loss in the small intestine. The rudimentary caeca and colon provide relatively little volume and time for symbiotic bacteria to aid the digestion of crustacean chitin. Introduction Ade´lie penguins (Pygoscelis adeliae) breed in large populations at the coast of the Antarctic continent, foraging on krill (Euphausia sp.) and larval fish (Emison 1968; Watanuki et al. 1994). The feeding ecology and nutrient composition of the diet are known to be closely related to both the length and the volume of the various parts of the avian alimentary tract. In general, herbiv- orous birds have heavier gizzards, longer small intestines and heavier caeca than carnivorous species (e.g. Barnes and Thomas 1987). Lipids and proteins are likely to be important as energy resources to Ade´lie penguins. But they also ingest significant amounts of structural car- bohydrates, since up to 10% of the krill dry matter consists of chitin (Clarke 1980). In fact, Ade´lie penguins are known to have endogenous enzymes that may con- tribute to the host’s degradation of chitin (Stemmler et al. 1984). Only few and incomplete data have so far been published on the gastrointestinal morphology of seabirds such as penguins and the closely related Pro- cellariiformes (Table 1). This study was carried out to provide a morphological description of the alimentary tract of Ade´lie penguins, since so little is known about how they digest and utilise their prey. Materials and methods Fifteen Ade´lie penguins were collected during the Norwegian Antarctic Research Expedition 1996/1997 under licence issued by the Norwegian Government. Ade´lie penguins normally lay their eggs in November, hatching occurs in December and the chicks are reared in the brief summer and released in February. The fledging weight of the chicks ranges between 2.8 and 3.2 kg (Salihoglu et al. 2001). Our birds, nine males and six females, were taken in the Weddell and Haakon VII Seas during the austral summer (between 15 January and 15 February; Table 2). They were not breeding at the time of capture; some of them may even have been too young for this, considering their weight (Table 2). The birds seemed to be fasting. Little food, occasionally mixed with penguin feathers, was found in their stomach (Table 3), possibly due to moulting. The penguinsweresacrificedbyalethaldoseofpentobarbital(50 mg/ml) Polar Biol (2002) 25: 641–649 DOI 10.1007/s00300-002-0395-6 M.A. Olsen (&) Department of Arctic Biology and Institute of Medical Biology, University of Tromsø, 9037 Tromsø, Norway E-mail: monicao@fagmed.uit.no R. Myklebust Departments of Electron Microscopy and Morphology, Faculty of Medicine, University of Tromsø, 9037 Tromsø, Norway T. Kaino ® S.D. Mathiesen Department of Arctic Veterinary Medicine, The Norwegian School of Veterinary Science, 9292 Tromsø, Norway V.S. Elbrønd Department of Anatomy and Physiology, The Royal Veterinary and Agricultural University, Grønnega˚rdsvej 7, 1870, Frederiksberg C, Denmark