Abstract

Introduction

Tuatara are the last extant members of the suborder Sphenodontia. They are now limited to a few offshore islands in New Zealand where they live in burrows in coastal forests or scrub. They are most active between the temperatures of 15°C and 18°C. While they have been seen foraging at 7°C, they can cease feeding, retreat below ground and become torpid. In the wild, diet consists of mixed invertebrate and small vertebrate prey items.¹

Tuatara become sexually active between approximately 11–13 years of age. Females of this species lay clutches of an average of 11 eggs roughly every 4 years. Vitellogenesis takes many months; mating occurs in autumn, and egg laying occurs about 2 months following emergence from torpor in the spring.¹

While appearing morphologically similar to lizards, comparatively little is known about their physiology. This study was undertaken in response to two clinical cases of hypocalcemia in gravid tuatara housed at Chester Zoo, UK.

Case Report

A group of young tuatara was donated to Chester Zoo in 1994 (five female and three male). From 1994 to 2004, they were housed in an indoor enclosure and were exposed to natural daylight through a UV opaque roof. Diet consisted of invertebrate prey, gut-loaded and dusted with a proprietary calcium supplement and occasional whole rodent prey items. No ultraviolet (UV) lights or vitamin D supplementation were used.

Eggs were found in the enclosure for the first time in summer 2004, indicating that at least one of the females had become sexually mature. Age range of females at this time was 17–20 years.

In early winter 2004, one female exhibited poor muscle tone and appeared to be moribund. Radiography revealed shelled eggs and mild to moderate osteomalacia. Blood calcium was low: 0.69 mmol/L (reference range 2.18–5.79 mmol/L¹). Demeanor and muscle tone improved rapidly with parenteral administration of calcium borogluconate, and a presumptive diagnosis of hypocalcemia precipitated by egg production was made.

The remaining animals were also radiographed and blood sampled. Sample volume was limited; therefore, levels of total blood calcium and the vitamin D₃ metabolite 25 hydroxycholecalciferol (25(OH)D₃) were prioritized (see Table 1). All but one of the females were found to be gravid, and one showed signs of weakness and muscle fasciculations that resolved on parenteral administration of calcium borogluconate.

Table 1. Total serum calcium and 25(OH)D₃ level for each tuatara, both for the winter and summer

^aDenotes animals showing clinical signs. Range for serum calcium of wild females in summer 2.18–5.69 mmol/L (n=10)

The asymptomatic animals were given a prophylactic dose of calcium glucobionate (200 mg/kg) orally and returned to their burrows to continue hibernation/torpor. The remainder were kept warm and given oral calcium and vitamin D supplementation for 2 months before being allowed to return to torpor.

All four gravid females laid their eggs without problems between May and June 2005. Radiographic evaluation and blood sampling was repeated during summer 2005 (Table 1). Calcium levels were within normal range for tuatara,¹ and to date all the animals remain in good health.

Discussion

Hypocalcemia associated with egg production is well documented in reptiles. The affected animals usually have marginal access to calcium, and the demands of vitellogenesis can precipitate a hypocalcemic crisis.³

Tuatara nutrition at Chester Zoo was reviewed. Using requirements for other reptiles as a guide, dietary calcium and phosphorus levels were felt to be adequate, and vitamin D deficiency was suspected.

The serum obtained in winter 2004 and summer 2005 were analyzed for 25(OH)D₃ levels at Manchester Children’s Hospital. Unfortunately, this laboratory was unable to handle very small sample sizes, and results were obtained for only half of the animals (Table 1).

In the absence of published values from wild tuatara, these data were difficult to interpret. Mean serum 25(OH)D₃ values for reptiles kept in their natural environments can vary widely between suborders ranging from 8.12 ng/ml in the desert tortoise (Gopherus agassizii) to 150 ng/ml in green iguanas (Iguana iguana).⁴

Serum from six adult male and six adult female animals sampled in the field during the summer months were obtained from Victoria University, Wellington, New Zealand. These were analyzed at Steroid and Immunobiochemistry Unit, Canterbury Health Laboratories, New Zealand, and data are presented in Table 2.

Table 2. 25 hydroxycholecalciferol (25(OH)D₃) serum levels in wild tuatara, taken in summer

There was a trend toward slightly lower vitamin D levels in our captive tuatara in summer when compared to their wild counterparts at the same time of year; however, this difference was not significant. By contrast, the captive winter values were significantly lower than both wild and captive summer levels (p<0.05). In the absence of wild winter values, the serum levels <1.5 ng/ml seen in the captive animals in winter are difficult to interpret but could be consistent with a vitamin D deficiency.

The marked increase in 25(OH)D₃ levels seen between winter 2004 and summer 2005 in the captive animals must have been the result of vitamin D obtained from the whole vertebrate prey, as no UV light or artificial vitamin D supplementation was given, and invertebrate prey is deficient in this vitamin. Our data thus provides evidence that tuatara can use oral sources of vitamin D. Tuatara are known also to be capable of maintaining adequate calcium homeostasis without access to oral sources of vitamin D. UV and calcium supplementation alone is the treatment of choice for young tuatara with nutritional secondary hyperparathyroidism that are being fed an exclusively invertebrate diet.²

Conclusions

The first data on 25(OH)D₃ levels in wild tuatara are presented here. It is expected that levels will vary with season, as is seen in captive tuatara, and hence further studies including data from torpid tuatara are needed. While these would be difficult to achieve in truly wild populations, data from animals kept in naturalistic enclosures in New Zealand would be welcomed.

Comparisons of winter (torpid) and summer (active and feeding) vitamin D levels in this study, and successful treatments of nutritional secondary hyperparathyroidism using calcium supplementation and UV light alone, demonstrate that tuatara can use both sources to achieve calcium homeostasis.

Egg production, however, can produce a significant threat to calcium homeostasis. Clinical signs were apparent at blood calcium levels below 2.0 mmol/L. In two out of five study animals, the dietary D₃ obtained by feeding occasional vertebrate prey was insufficient to allow for reproductive demands.

A combination of increased dietary D₃ and access to UV light should prevent this occurring in the future.

Acknowledgments

Thanks to Prof. Charles Daugherty, Nicola Nelson and Sue Keall of Victoria University (Wellington) New Zealand for providing the serum from the wild tuatara, Brett Gartrell, wildlife veterinarian at Massey University (Palmerston North), and Janine Shaw, Section Leader, New Zealand Veterinary Pathology Laboratory, (Palmerston North) for arranging and performing the analysis. Isolde McGeorge, team leader reptile section Chester Zoo for care and information on the Chester group, and Steve Unwin, veterinary officer Chester Zoo, and Richard Jakob-Hoff, senior veterinarian Auckland Zoo, for their help and advice.

Literature Cited

1.  Blanchard, B., 2002. Tuatara captive management plan and husbandry manual. Threatened Species Occasional Publication No. 21, Department of Conservation, Wellington, New Zealand.

2.  Boardman, W.S.J., B. Blanchard. 2006. Biology, captive management and medical care of tuatara. In: D.R. Mader (ed.). Reptile Medicine and Surgery 2nd ed., Saunders Elsevier, St Louis, Missouri. Pp. 1008–1012.

3.  Denardo, D. 2006. Reproductive biology. In: D.R. Mader (ed.). Reptile Medicine and Surgery 2nd ed. Saunders Elsevier, St Louis, Missouri. Pp. 376–390.

4.  Ullrey, D.E., J.B. Bernard. 1999. Vitamin D: metabolism, sources, unique problems in zoo animals, meeting needs. In: M.E. Fowler, R.E. Miller (eds.). Zoo and Wild Animal Medicine Current Therapy 4. W.B. Saunders, Philadelphia, Pp. 63–78.