Feeding, digestion and assimilation of a population of giant tortoises (Geochelone gigantea (Sehweigger)) on Aldabra atoll

Abstract

This paper describes an investigation of food processing by the Giant Tortoise (Geochelone gigantea (Schweigger)) on Aldabra atoll in the Indian Ocean. The dry weight assimilation efficiency of the Aldabran tortoise feeding on tortoise turf was found to be 30 per cent, while energy assimilation was calculated as 34·5 per cent. Weight specific consumption varied with body size according to diet and season. Some hollocellulose was broken down during the passage of food through the gut, but it is not clear whether cellulose breakdown is a regular feature of tortoise digestion. Throughput times for passage of food through the gut show a log-normal distribution with median throughput time of 8·8 days on tortoise turf and 12·0 days on the much coarser coastal grass Sporobolus virginicus (L.). Body composition was determined by the dissection of 8 animals. Gut contents accounted for 15 per cent of live weight, and skin, bone and scute for a further 47 per cent, or 68 per cent of dry weight. The average calorific value of tortoise tissue (excluding gut contents) was calculated as 4274 kJ/kg (fresh weight). Results on consumption and assimilation rates and energy contents were combined with data from earlier studies to produce an animal energy budget for the South East Grande Terre population. Discussion focuses on features of the energy budget of this population, which supports a high standing crop biomass on low rates of consumption and assimilation. Both turnover rates (P/B) and production efficiency (P/A) are below 1 per cent and the population appears to be food-limited. Comparisons are made with African wildlife areas with similar rainfall, dominated by large herbivorous mammals, particularly those dominated by elephant, and with lower density tortoise populations on other Aldabra islands. The interactions of food supply, growth rates, reproductive rates and population density are discussed.