Abstract

Poor growth, infertile eggs, shell problems, and loose stools were problems consistently noted in the giant tortoises at the Philadelphia Zoo, and are problems that appear to be associated with captive populations at many other zoos in the United States. Many of these problems are suspected to have an underlying nutritional etiology, so in March 1992 the Philadelphia Zoo began dietary trials on 1.2 Galapagos tortoises (Geochelone nigra) and 5.6 Aldabra tortoises (Geochelone gigantea) to develop a more nutritionally sound feeding program. The giant tortoises used in this study were given a health exam that included documenting weights, curved carapace lengths, and hematological parameters (complete blood count, plasma chemistry profile including vitamin A and E levels) prior to the start of the trial and were sampled periodically throughout the trails. Feed intake and stool quality were assessed by the keepers on a regular basis.

The original diet that the tortoises were fed prior to 14 June 1992 is described in Table 1. The initial trial used an alfalfa hay and biscuit diet (Table 2) similar to one that had been used in a three-month trial at another institution. The biscuit was the Mazuri ADF 16 pellet. This diet was changed incrementally until the new alfalfa hay and biscuit diet was the sole diet fed in June 1992. This alfalfa hay and biscuit diet was offered daily, and a small amount of produce was offered three times/week. Based on estimated feed intake, the diet appeared successful while the animals were maintained in an outdoor enclosure. A better assessment of palatability and overall consumption was obtained when the animals were moved to their indoor enclosures. Palatability of the alfalfa hay and biscuits were poor. Despite various treatments to increase palatability of the alfalfa hay (e.g., soaking hay in water for four hours, chopping hay into 2.5-inch lengths, soaking the chopped hay for two and four hours, soaking chopped hay in orange juice), acceptance continued to be poor. In September 1992, the biscuits were dyed orange in an effort to increase palatability using iron oxide, a dye that is not absorbed by other animals. The efficacy of this treatment was questionable. A side effect of this diet was that the overall hue of the tortoises turned orange, most likely the result of the dye used to make the biscuits orange. In January 1993, since the tortoises did not seem interested in the food on a daily basis, the alfalfa hay and biscuit diet was offered only five times/week and the produce maintained at the same level (Table 3) until the end of March 1993 when the tortoises were returned to the original diet. The orange hue gradually faded when the tortoises returned to the original diet. The keepers perceived a decrease in general activity level and breeding behavior throughout the duration of this dietary trial.

Table 1. Giant tortoise diet used at the Philadelphia Zoo prior to 14 June 1992

^aZoo cake analysis can be provided on request.

Table 2. Tortoise biscuit diet trial, offered 14 June 1992 through 1 January 1993

Table 3. Tortoise biscuit diet trial, offered 2 January 1993 through 29 March 1993

No significant changes in tortoise morphometrics were noted during this trial, nor were perceptible growth lines noted on the carapace scutes. Tortoise stools ranged from loose to soft-formed, but never achieved the firm-formed texture noted in wild tortoises.

Based on this trial, the following conclusions were made: (1) giant tortoises seem to self-adjust their nutrient intake, consuming the same proportion of major nutrients on the original diet and the biscuit based diet; (2) giant tortoises become depressed when fed a high-concentrate/low-produce diet, as indicated by a keeper-perceived decrease in breeding behavior and activity level; (3) giant tortoises seem to metabolize iron oxide differently than mammals; (4) giant tortoises will not consume large amounts of alfalfa hay; (5) collection of feed intake information is not practical when tortoises are maintained outside; and, (6) the consumed diet did not appear to meet the needs of the tortoises due to the lack of significant growth and weight changes in any of the tortoises, and the poor stool quality noted.

The Walkabout Mix diet (Nutrition Support Services, Walkabout Farm, Pembroke, VA) was used for the next dietary trial, and consisted of a produce-based diet to which a premix is added to balance various nutrients. This is a nutritionally complete diet based on dietary analysis of the components and is similar in analysis to the diet known to be consumed by free-ranging giant tortoises. (Tables 4 and 5). Produce rather than hay was chosen as the bulk of the diet to avoid palatability problems. The Walkabout Mix diet was offered starting in March 1994. There were no problems with acceptance of this diet. Significant weight changes occurred in young female Aldabra tortoises while being fed this diet, and growth lines on the carapace scutes were noted even in mature and aged individuals. Stool consistency was firm and similar to that noted in free-ranging tortoises. Although an ethogram for this population has not been produced, keepers believe that more reproductive activity occurs, and the tortoises are more active than in previous years.

Table 4. Walkabout Mix diet Trial, begun 24 March 1994 and offered currently

^aPremix includes certified organically grown clover, certified organically grown dandelion, dry whole egg (culture Salmonella-negative), dried strawberry, dried yellow squash, dried carrot, soybean meal, fresh vitamins, USP-grade calcium carbonate, trace minerals.

Table 5. Nutritional analysis of the Walkabout Mix diet (dry matter basis)^a,b

^aIn addition, B vitamins and amino acids were analyzed but are not included in this table.
^bMoisture of the “as fed” diet is 73%.

Additional items consumed that are not part of either the alfalfa hay and biscuit diet, the original diet, or the Walkabout Mix included grass and browse. The grass of the outdoor pen was 3-way fescue (40% falcon, 40% thunderbird, 20% bonanza) in 1994, but in fall of 1995 the mix was changed to Liberty Mix (44% perennial rye [Lorolium perenne], 34% red fescue [Festuca rubra], and 18% Kentucky blue [Poa pratensis]). Browse is added to the diet in season (e.g., mulberry, Morus sp.; forsythia, Forsythia sp.; poplar, Populus sp.) on a daily to weekly basis.

Based on this dietary trial, the following conclusions were made: (1) giant tortoises prefer a high-produce diet over a hay-based diet and keepers perceive increased breeding behavior and activity level when the nutrients for the produce diet are balanced; (2) the Walkabout Mix is a nutritionally complete diet since the consumed diet did appear to meet the needs of the tortoises based on the significant growth and weight changes in the tortoises, and the good stool quality noted.

Acknowledgments

We thank the staff of the Department of Animal Health for their assistance with this project: Donna Ialeggio, DVM, for her role in obtaining blood samples and Sandy Skeba, AHT, for performing routine hematological analyses. We thank Ellen Dierenfield of the Wildlife Conservation Society for performing vitamin A and E analyses of the plasma samples.