An investigation of vitamin D metabolism in kiwi (Apteryx mantelli), tuatara (Sphenodon punctatus) and New Zealand sea lion (Phocarctos hookeri) and the relationship of vitamin D metabolism with their life history characteristics : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies at Massey University, Turitea, Palmerston North, New Zealand
Vitamin D, a fat-soluble vitamin, has a wide range of functions in vertebrates. The aim of the study was to determine if the evolutionary history of different animal species affects their predominant route of vitamin D metabolism. The species chosen in this study were Brown kiwi (Apteryx mantelli) for their nocturnalism, tuatara (Sphenodon punctatus) for their diurnal sun basking nature and New Zealand sea lion (Phocarctos hookeri), as a marine mammal species.
A survey of plasma or serum concentrations of 25-hydroxyvitamin D2 (25(OH)D2) and 25-hydroxyvitamin D3 (25(OH)D3) in kiwi, tuatara and New Zealand sea lion and analysed the ability of skin to produce vitamin D3 in response to UV exposure from post mortem samples of these three species. Assessment of morepork (Ninox novaseelandiae) skin was also carried out as an additional example of a nocturnal species.
Wild kiwi had lower plasma 25(OH)D3 concentrations than captive kiwi and this variation was most likely of dietary origin. The low concentrations of plasma 25(OH)D3 in wild kiwi in their natural habitat, suggest that these minimal levels are sufficient to fulfill their vitamin D requirements in the body or they utilise calcium independent of vitamin D. Captive diets for kiwi may be over-supplemented with vitamin D. In contrast to this finding, the skin of both kiwi and morepork was able to produce small but measureable amounts of vitamin D3 in response to UV exposure. This result was unexpected, considering their nocturnal nature and the overall pattern of vitamin D metabolism in the kiwi is still unclear.
Vitamin D metabolism in tuatara suggests that both dietary and dermal pathways are important. The survey of plasma 25(OH)D3 concentrations in captive tuatara showed variation between the five zoological institutions, which was correlated to the variation in the dietary vitamin D provided between captive institutions.
However, analysis of tuatara skin showed that tuatara had a strong ability to synthesise vitamin D dermally, indicating that it is an important route of vitamin D metabolism in tuatara.
New Zealand sea lion showed overall higher serum 25(OH)D3 concentrations than kiwi and tuatara, which might be attributed to the high UV-B radiation exposure they receive in their natural habitat. New Zealand sea lion skin also had comparatively higher vitamin D concentrations both prior to and in response to UV exposure, which shows that dermal route of vitamin D is an important route of metabolism in these marine mammals.