Genevieve Jorgensen of the Royal Danish Veterinary School has been studying Giant Tortoise diet.
Giant tortoises are fascinating creatures and are familiar to many people. Some people are lucky enough to have seen them in the wild, but they are also commonly kept in zoos. Although they seem to adapt well to captivity, there are gaps in our knowledge about tortoise health and nutrition. To help address this, myself and my colleague Beth Fledelius from the Royal Danish Veterinary School have been carrying out a study of tortoises on Aldabra. We have taken 45 blood samples from tortoises on one of the islands, and analysed these using portable equipment. We also collected food plants for analysis in Denmark, to calculate calcium content.
Our study also involved tortoises from the island of Curieuse. Twenty years ago, tortoises were translocated to Curieuse from Aldabra. We wanted to investigate whether any differences in blood chemistry are already apparent, and took 23 blood samples here. It is already well known that Aldabra tortoises have a limited food supply, and this may have made a difference.
Our analysis has since shown that the differences in the two environments and the tortoise diets are reflected in blood chemistry – a poorer diet and dehydration are evident from the Aldabra samples, but blood calcium is at a healthy level. Calcium deficiency is often a problem for captive tortoises.
The results of our research will be published and should help zoo vets to look after Aldabra and other tortoises in captivity, to improve their health and reproductive success. The study was made possible by SIF and SCMRT-MPA, and we would like to express our gratitude to these organisations.
Although it has been observed that Giant Tortoises on Aldabra can feed on over 80 species of flowering plants, the preferred diet of a large part of the population, which is at Grand Terre, is ‘Tortoise Turf’. This is a unique mosaic of about 10 genera of grasses, sedges and other low-growing plants. These appear to be genetic dwarfs that have evolved to be able to withstand grazing pressure by the tortoises.
- There are 28 Giant Tortoises on Cousin island.
- They drink water through their noses.
- They lay around 10 eggs, and urinate as they do so, to soften the soil which then ‘bakes’ hard around the eggs to protect them.
- Aldabra has the largest population of them – around 100,000 in 1997.
- A census in 2000 showed there had been a significant decline.
- Tortoises are vulnerable to drought – the 1990s were the driest decade on record.
- Over-grazing by goats can deprive tortoises of food and shade from the intense heat of the sun.