Abstract

Hypocalcemia and hypovitaminosis D are commonly encountered medical problems in captive animals that do not have access to sunlight, especially in herbivorous reptiles.^7,9-11 Vitamin D₃ (cholecalciferol) is produced in the skin during exposure to sunlight, specifically ultraviolet light in the range of 285–315 nm (UVB), in a temperature-dependent isomerization.⁷ Circulating Vitamin D₃ is hydroxylated in the liver into 25-hydroxyvitamin D [25(OH)D].^6,11 This is typically very plentiful, and has a half-life of weeks to months in humans. 25(OH)D is hydroxylated in the kidney to 1,25-hydroxyvitamin D [1,25(OH)D], and this step is typically very tightly regulated.^6,11 In humans the half-life is measured in hours.¹¹ Because of the tight regulation and short half-life of 1,25(OH)D, 25 (OH)D is generally used in determining vitamin D deficiencies in animals.¹¹ Some animals are known to maintain very high circulating levels of 1,25(OH)D due to presumed target organ resistance (higher levels of 1,25(OH)D required at the level of intestinal mucosa, renal tubules, etc.), so knowledge of 1,25(OH)D levels may also be helpful.⁴ Studies of UVB and dietary calcium/vitamin D in several reptile species have shown that there is considerable variation in the requirement for dietary vitamin D and for UVB.^2,7,9-11

While many animals can utilize dietary cholecalciferol or ergocalciferol (vitamin D₂), there are some birds, reptiles and New World primates for whom dietary ergocalciferol cannot be utilized.^4,8,11 Artificial UVB has been shown to increase circulating vitamin D levels in several animals, but there are many variables in the effectiveness of artificial lights, including distance, temperature, and time spent basking.^2-4,6,10,11 Ultimately sunlight is the best way to ensure adequate levels of vitamin D in most animals. Unfortunately, there are very few reported baseline vitamin D levels for free-ranging animals in their native habitat. Here we report baseline levels for wild Ricord’s iguanas (Cyclura ricordi) and rhinoceros iguanas (Cyclura cornuta).

West Indian iguanas of the genus Cyclura are the largest land vertebrates endemic to the Caribbean islands and, as a group, are considered to be the most endangered lizards in the world.¹The island of Hispaniola is home to the only two sympatric species of Cyclura, the critically endangered Ricord’s iguana, and the rhinoceros iguana, which is considered threatened in its natural environment. Little is published regarding the general biology or physiology of these species in the wild or captivity. Comprehensive baseline data, including vitamin D status, will also assist in clinical diagnosis and in improving captive breeding environments for these animals.

Fourteen wild Ricord’s iguanas and seven wild rhinoceros iguanas were captured on Isla Cabritos in Lago Enriquillo National Park, Dominican Republic. Animals were captured using either a pole snare or by live trapping with baited Tomahawk traps, and were manually restrained for all procedures. A complete physical examination was performed. Blood was collected from the ventral tail vein into lithium heparin and serum separator blood tubes. Blood tubes were kept cool until they were centrifuged, and the plasma/serum was then frozen. Animals were then released at the capture site. Thirteen captive rhinoceros iguanas from Parque Zoológico Nacional, Dominican Republic, were also sampled. These animals are housed outside, and fed a diet of vegetables and fruit. Plasma was also collected from five Jamaican iguanas (Cyclura collei), two Grand Cayman iguanas (Cyclura lewisi), and two rhinoceros iguanas at the Indianapolis Zoo before and after being placed in outdoor pens for 4–6 wk during the summer. The diet for these animals consisted of daily salad, supplemented with calcium carbonate three times weekly, and cholecalciferol twice monthly. Plasma [25(OH)D] and 1,25-hydroxyvitamin D [1,25(OH)D] levels for all samples were measured at Boston University using previously described methods.⁵

Table 1 shows the average 25(OH)D and 1,25(OH)D levels for wild Ricord’s iguanas, wild rhinoceros iguanas and captive Rhinoceros iguanas in the Dominican Republic. Table 2 shows the average 25(OH)D and 1,25 (OH)D levels for the Indianapolis Zoo animals. While sample size is quite small in the Indianapolis Zoo study, the results do suggest that sunlight remains an important aspect of vitamin D mobilization in these animals.

Table 1. Vitamin D levels in Dominican Republic iguanas

Table 2. Vitamin D levels in Indianapolis Zoo iguanas before and after exposure to summer sunlight

Acknowledgments

The authors gratefully acknowledge the Indianapolis Zoo Conservation and Research Department, The Toledo Zoo Conservation and Research Department, the Riverbanks Zoo Conservation Support Fund, and the Indianapolis Chapter of the American Association of Zoo Keepers for financial support of this work.

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