Platinum Tortoise Club
- Nov 13, 2016
- Location (City and/or State)
Thank you for your answer and the links that I will read attentively.You are quite correct in that an iodine deficiency or in certain circumstances, excess, can result in a goiter and thyroid dysfunction. In the US, there are areas of the country which have relatively low amounts of iodine in the soil such that pastures and crops are relatively low in iodine. Years ago, when foods were grown locally and before salt was iodinated, there were human and animal goiters caused by iodine deficiency with associated hypothyroidism. Florida soils have adequate iodine. According to the University of Florida, Florida pasture forages have been reported to be low or borderline low in P, Na, Cu, Se and Zn, with Ca, Mg and Co found to be borderline-to-deficient depending on location, season, forage species, and year. Cattle are usually given mineral supplements free choice as a low-cost measure to provide adequate mineral nutrition. Livestock feeds usually have mineral supplementation. This is mentioned as there has been Se deficiency related hypothyroidism reported in mammals. In addition to iodine from forage, our tortoise herd receives limited supplementation. Goitrogenic greens are not regularly fed. Although animals have been received with neck swelling, it has not materialized in animals raised on the farm. As most tortoises’ thyroid is located dorsal to the plastron, the thyroid may be enlarged but is not palpable most of the time. Swelling in a young tortoise’s lower neck may be also be thymic tissue.
Veterinary literature over the past 30 plus years repeatedly mentions an abnormal incidence of goiters in giant Aldabra and Galapagos tortoises. These statements may be antidotal. There are a few case reports of giant tortoise hypothroidism, one of which is linked at the end of this post.
If the mortalities associated with this “Rapid Growth Syndrome” were due to hypothyroidism, it would be easily treated with levothyroxine. Unfortunately, this is not the case.
In the examples of the two tortoises suffering consequences from “Rapid Growth Syndrome” discussed previously, thyroid studies and other chemistries had been drawn before they were ever sent here and as well as after they were here. These animals were euthyroid and chemistries were non-diagnostic. I understand they were raised on a pelleted diet, hay, grass and limited produce and vegetables. Additional dietary details are unknown to me. The tortoise which survived and improved has been on pasture with minimal supplementation and access to soaking areas. The underlying cardiomyopathy and presumed hepatic disease portend an uncertain future.
At the end of this post is a link to an older report regarding Florida raised Galapagos tortoises on grass and Masuri who underwent neck mass biopsies. Note this was the Masuri 5M21 rather than the newer low starch product. Why they had this thymus abnormality is unclear.
There have been many hundreds of Galapagos tortoises hatched in captivity in the US, most of which are no longer with us. Despite what some choose to believe, not all have been raised under improper husbandry conditions. Conditions of rapid growth seems to be the common denomination, often with a pelleted diet exceeding 5% of the diet by dry weight. Several captive bred species of tortoises may grow at 2-4 x the rate compared to that of wild grown animals without apparent problems. Captive bred Giant tortoises given nutritionally complete, easily digestible, relatively low fiber diets with more rapid gut transient times (as compared to grass fed tortoises) may grow as much as 10x as fast as those raised in their native habitat on grasses. No matter how much sun, exercise, humidity, etc., these rapidly growing animals receive, there appears to be some form of poorly understood differential growth with a “metabolic disconnect” suffered by many. A significant number of rapidly grown giant tortoises suffer complications associated with rapid growth including mobility issues, edema, morphological abnormalities, hepatic and cardiac disease.
Logically we would think we should be able to feed the giant tortoises good nutrition to satiety under ideal husbandry conditions and let them grow as fast as they will grow. Unfortunately, this has proven not to be the case due to excessive morbidity and mortality. Slow and steady growth with grass-based diets appear to work without the excess morbidity and mortality of the rapidly grown animals. Why this works is unknown and debatable. How fast can we safely grow the giants? The high fiber diets definitely appear much safer than the easily digestible nutritionally complete lower fiber diets. We know the gut transient times of grass-based diets is about three times slower than that of tortoises fed lettuce and produce. Does crude fiber amount and length play a significant or essential role? Have diets including significant amounts of commercial pellets resulted in a form of liver disease or is it secondary? Will the two, relatively new to market, “low starch” types of commercial tortoise pellets with higher crude fiber prove safer? Are the soy based componets of the pelletted diets appropriate for these animals?
There is not the extensive data on raising Aldabra hatchlings as there is on raising Galapagos hatchlings. Facilities that raise both tend to use the same diets. Growth rates on heavily populated Aldabra are a limited by population density and resources. There is good data on successfully growing Galapagos hatchlings. In excess of 7000 Galapagos hatchings have been raised to release size at the captive breeding centers on the islands with low mortality using a high fiber diet consisting primarily of cut stems and leaves of Xanthosoma sagittifoliumwith very low mortality. There have been more than a thousand hatchlings raised in this country. Many of these have been raised on nutritionally complete, easily digestible, relatively low fiber diets (pellets, produce and some hay or grass) with resultant high growth rates. Unfortunately, many of these hatchlings have subsequently perished or suffered health issues.
Each steward of these amazing giant tortoises will have to review the data, hopefully learn from the mistakes of others, and make their own husbandry decisions.
Gregory J. Fleming, Darryl J. Heard, Elizabeth W. Uhl, and Calvin M. Johnson (2004) Thymic Hyperplasia in Subadult Galapagos Tortoises, Geochelone nigra. Journal of Herpetological Medicine and Surgery: 2004, Vol. 14, No. 1, pp. 24-27.
Terry M. Norton, Elliott R. Jacobson, Randolph Caligiuri and George V. Kollias (1989)
Medical Management of a Galapagos Tortoise (Geochelone elephantopus) with Hypothyroidism
Journal of Zoo and Wildlife MedicineVol. 20, No. 2 (Jun., 1989), pp. 212-216
(May view without charge)
I hopefully can learn enough to make no mistakes with my young aldabra tortoises.