MOSER, P; STARCK, J.M.; Univ. of Munich, Germany; Univ. of Munich, Germany: Flexible Responses to Feeding and Fasting in Green Iguana (Iguana iguana)

We studied the flexible changes of morphology and physiology of to gastrointestinal tract of green iguana (Iguana iguana) in response to feeding and fasting. The green iguana is a strictly herbivorous lizard which feeds continuously. Under natural and experimental conditions, it easily tolerates fasting periods of two weeks. We studied 10 half grown iguanas during feeding and during a two weeks fasting periods. The size of the small intestine and the liver was measured daily using noninvasive transcutaneous ultrasonography. Light- and transmission electron microscopy was used to study responses on the level of cells and tissues. The length of intestinal villi, the thickness of the mucosa and the length of the microvilli was measured from micrographs. We used open system respirometry to record resting metabolic rates. During the fasting period, the size of the liver and the small intestine declined significantly. During a 10-day fasting period, the size of the small intestine declined to 65% of feeding size. When animals were fed again, the gut size returned to normal size within three days. Liver size also declined during fasting and was reestablished within 2 days after re-feeding. On the level of tissues and cells we observed similar changes as described previously for snakes, frogs and crocodiles. Resting metabolic rate declined significantly during the fasting period. Within 24h after re-feeding the animals resumed normal resting metabolic rates. In conclusion, iguanas show a remarkable flexibility of their gastrointestinal system to respond to periods of feeding and fasting. A comparison of the herbivorous iguana with previously studied carnivorous reptiles and amphibians suggests that flexibility of the gut and liver is an evolutionary old feature and may be traced down to the root of tetrapod life.