Research on the digestion of the giant tortoisesResearch on the digestion of the Galápagos Giant tortoises

Zoo Research as contribution to the species protection

Ricarda Gisler, Jean-Michel Hatt; Department for Zoo, domestic and wild animals, University Zürich and Zoo Zürich

Since scientific understanding of their digestion processes and nutritional requirements was until recently rudimentary, errors have often been made in the feeding of plant-eating tortoises. These inadequate diets often led to skeletal and urogenous problems. Many threatened tortoise species – including the Galapagos giant tortoises – have now been included in a "species protection" programme, and, using recent knowledge of their physiology and nutritional requirements it is proposed to develop new strategies to save these animals from extinction and to re-establish their populations. The object of the present study in thesis form was to obtain new basic information concerning the digestion and nutritional preferences of the Galapagos giant tortoises and so contribute to their successful maintenance and breeding.
The research was carried out on the giant tortoises in the Zurich zoological gardens and was made possible by generous donations from the Swiss Association of Friends of the Galapagos Islands to whom we here express our gratitude.

The tests were conducted using so-called food-markers. These are substances which are swallowed by the animals, pass through their digestive tract without being digested or in any way changed and finally can be detected in the faeces. For these studies a relatively new type of marker, so-called alkanes, were used. These are carbohydrates which occur naturally in the wax layers of many plants. They have the advantage that they are present in the animals’ normal food, so in no way affect it, and in addition are easy to detect in food and faeces. Alkanes have already been successfully used in digestion studies with fish, birds, and mammals but this is the first time with reptiles.

In a first study the length of time the food took to pass through the digestive tract of juvenile and adult Galapagos giant tortoises was determined. For this eight animals received the food-markers in a single dose and their faeces were collected over the following month. By means of the alkane the mean retention time (i.e. the average time the food took to pass through the digestive tract of the animal) was calculated. This retention time is much longer for reptiles – many days- as compared to warm-blooded animals in which the food passage lasts only a few hours. The food-marker used, alkane C36, performed extremely well here. For the juvenile animals the mean retention time was 9 days, for the adults 12 days probably because the adult animals swallowed larger mouthfulls which slows down the passage in the intestine. However it was interesting to note that liquids were retained for equal times, 9 days, in both age groups, despite the 7 times greater weight of the adults. Therefore there does not seem to be a significant dependence of retention time on weight or age for Galapagos giant tortoises.

In a second study nine giant tortoises were fed for a month on the same food mixture of hay, apples, and alkane-marked tortoise pellets, to maintain a balance of nutritional components in their digestive tract. Consequently by the alkane method the composition of the food consumed individually by each animal could be calculated. In this test as well the use of alkanes was fully successful; the results calculated from the alkane content of the food and faeces corresponded well with the observed values. It could also be shown that the juvenile giant tortoises in the 2nd group, which were fed alongside the adult tortoises, selectively consumed more of the tasty apple pieces than the adult tortoises. This shows the potential of this marker for future studies with Galapagos giant tortoises in the wild ; for example with its help qualitative and quantitative analyses of consumed food could be made from samples of faeces, and so possible food preferences or seasonal food variations estimated. A suitable component for such studies would be, for example, opuntia cactus, which is one of the most important foods for Galapagos giant tortoises, since it has a high alkane content.

Furthermore, tests were made on how well the Galapagos giant tortoises digest various foods. These showed that the Galapagos giant tortoises are very well adapted to the coarse food of their homeland, since they can very well digest the fibre-rich plants.
The results of this study show clearly that alkanes have a very high potential as food-markers for reptiles in studies of the physiology of their digestion, so allowing much useful information concerning these animals to be collected.