ONTOGENIC CHANGES IN FOOD-INTAKE AND DIGESTION RATE OF THE HERBIVOROUS MARINE IGUANA (AMBLYRHYNCHUS-CRISTATUS, BELL)

Young reptiles have higher relative energy demands than adults, but th e proposed ontogenetic changes in diet to fulfil these demands were no t found in the algae-eating Galapagos marine iguanas on Santa Fe. Feed ing and digestion rates were investigated to analyse how young achieve higher energy intake. Daily food intake of free ranging marine iguana hatchlings (6-11 months old) was about one third that of adults, but relative intake (g dry mass . g-1 wet mass . day-1) was four times hig her in the hatchlings. During feeding experiments, relative daily food intake of hatchling marine iguanas was approximately three times high er than that of adults (0.042 vs 0.013 g dry mass . g-0.8 wet mass . d ay-1), and mean gut passage time was two times shorter (5 vs 1 0 days) . The hatchlings also maintained high body temperatures (36.7-degrees- C) even under relatively cool day-time air temperatures of 32-degrees- C. Apparent digestibility of algal food measured both during feeding t rials and by Mn2+ AAS (atomic absorption spectrometry) for free-rangin g iguanas was 70%, independent of body size and temperature. The red a lgae prevailing in the diet were high in protein (30% dry mass) and en ergy (12. 1 kJ/g dry mass). Diving iguanas had higher rates of energy intake than intertidal foragers, but daily intake was less. Maintenanc e of high body temperature enabled hatchlings to achieve high digestio n rates and, combined with high relative intake, thus achieve sufficie nt energy intake for rapid growth despite higher mass specific metabol ic rates. Estimates of biomass of marine iguanas and their algal food are given for a section of coastline on Santa Fe.