Abstract

Thyroid hormone levels were measured in wild-caught and captive-raised alligators from the Everglades National Park. Serum chemistry, CBC (complete blood count), and body condition score of the animals were also determined to evaluate possible causes of poor body condition score. Preliminary data has shown a significant decrease in T3, T4, and rT3 levels in wild Everglades National Park alligators compared to captive-raised but genetically similar animals.

Introduction

American alligators (Alligator mississippiensis) have been grown commercially as a farm animal for their meat and hides in Florida since 1975. Eggs are collected from nests at various lakes in Florida every year and distributed among the farmers who then incubate and hatch them. Our laboratory has collected eggs from various lakes in Florida, the Everglades National Park, and the Rockefeller Refuge in Southwestern Louisiana. The eggs are taken to the University of Florida, College of Veterinary Medicine for incubation and hatching¹. The hatchlings are taken to Gatorland Zoo and placed in the hatchling house where they are maintained at a relatively constant air temperature of 32 degrees Celsius and fed a mixture of horsemeat blended with minerals, vitamins, and antibiotics. The hatchlings are moved to larger pens at just less than one year of age. They usually are more than 65 centimeters in length at this time.

Blood has been routinely collected from captive-raised alligators and analyzed for serum chemistry, CBC (complete blood count), and serum protein electrophoresis. The routine analysis of the alligator blood has produced a set of "normal values" for the animals that can be used as reference for alligators raised under these captive conditions. The values vary slightly between groups of alligators hatched from eggs from the various lakes of Florida, the Everglades, and from the Rockefeller Refuge. This is assumed to be normal variation between animal groups based upon geographic distribution. Wild alligators from the Everglades National Park have been observed to be of low weight for their length and have low body fat based upon body condition score. It is assumed at this time that the cause of the poor condition of these alligators is one or more of the following: "resource limitation" ³ (low prey quality and/or availability); parasitism; sub-clinical disease. These wild animals are genetically similar to the captive alligators which were hatched and raised from eggs collected from the Everglades National Park.

Materials & Methods

Blood has been collected from wild alligators in the Everglades National Park and shipped to the University of Florida, College of Veterinary Medicine for serum chemistry, CBC, and serum protein electrophoresis analysis. The analytical methods used are the same as those used on the captive animals held at Gatorland Zoo. This was done to reduce differences or errors due to analytical methods between laboratories.

An aliquot of serum was sent to Michigan State University for the analysis of thyroid hormones T4, T3, and rT3. A preliminary analysis of thyroid hormone levels was performed on blood collected from 40 two-year-old captive Everglades alligators raised and held at Gatorland Zoo and 24 wild caught alligators of unknown age from the Everglades National Park. All data sets were analyzed using SAS statistical analysis software with an IBM style computer.

Results

Statistically significant differences were observed between wild and captive-raised Everglades animals for all thyroid hormones. Mean T4 levels were double in the captive alligators compared to the wild animals. Mean T3 levels were 25 times higher in captive alligators than wild Everglades alligators. Mean rT3 levels were 10 times higher in captive animals compared to the wild alligators (Table 1).

Several serum chemistry values were significantly different between wild and captive-raised Everglades alligators (Table 1). Mean cholesterol level for captive-raised animals (121 mg/dl), were significantly different from mean values for the wild animals (66 mg/dl). Creatinine levels for captive farm raised animals were nearly double the wild alligators (Table 1).

Serum protein electrophoresis had marked differences in captive-raised compared to wild animals. The area of the electrophoretogram in the beta 1 to the gamma 1 regions for domestic mammals showed a large significant elevation for the wild animals¹ (Table 1).

Discussion

The levels of T3 and T4 were significantly lower for the wild Everglades alligators compared to captive-raised animals. During periods of starvation, mammals normally show an inversion of T3 and rT3 levels; thus, the level of T3 hormone will decrease while rT3 will increase. This exact phenomena was not observed in our alligators based on the data. All thyroid hormone levels for wild alligators were lower than those for captive-raised alligators. Poikilothermous animals may reduce thyroid hormone production naturally in an attempt to conserve energy during periods of starvation. Therefore, this may be a normal physiologic response. It could also be the animal's response to a lowered temperature during torpor or hibernation, or response to a disease state. While disease has not been ruled out in the wild animals that were tested, the only physical abnormality noted was poor body condition correlating with low caloric intake. The blood was collected during a time when ambient air temperature was high enough that the animals should not have been in torpor. Water temperature during the experimental period was assumed to be high enough to keep the animals from entering torpor. There is no data available on "normal" values for thyroid hormone levels in the alligator. Serum thyroid hormone levels in the wild alligators correlate with malnutrition and/or starvation⁴. Several more samples need to be tested so that a statistical standard range for thyroid hormones is available and reliable for Everglades alligators. The low levels of cholesterol present in the serum chemistry evaluation could be consistent with starvation or liver failure in domestic animals². Low Creatinine levels is an indicator of lower muscle protein metabolism and could also be an indicator of starvation².

Table 1. Comparative clinical values for captive-raised and wild everglades alligators.

Acknowledgments

The author wishes to thank the Everglades National Park for their help in collection and delivery of the wild alligator blood samples to the University of Florida, College of Veterinary Medicine. Thanks to the clinical pathology department at the University of Florida, College of Veterinary Medicine for analysis of serum chemistry, CBC, and protein electrophoresis. We also thank Dr. Refsal at Michigan State University for thyroid hormone analysis on the alligator blood.

References

1.  Barnett JD, Cardeilhac PT, McGuire PM, Fleming DM, Bass OL, Barr B, Wolff W. 1997. Chronic Inflammation in American Alligators (Alligator mississippiensis) in the Everglades. Proceedings IAAAM 28th annual conference.

2.  Duncan RJ, Prasse KW, Mahaffey EA. 1994. Veterinary Laboratory Medicine: Clinical Pathology. Iowa State University Press, Ames, Iowa. Pp. 37-62.

3.  Kushlan JA, Jacobson J. 1990. Environmental Variability and the Reproductive Success of Everglades Alligators. J. Herp., 24. (2); 176-184.

4.  Griffin JE, Ojeda SR. 1996. Textbook of Endocrine Physiology. Oxford University Press, New York, New York. P. 267.