Stable carbon isotope ratios as indicators of marine versus terrestrial inputs to the diets of wild and captive tuatara ( Sphenodon punctatus )

Stable carbon isotope analysis was used to examine feeding relationships of wild tuatara on Stephens Island and captive tuatara in New Zealand institutions. We first measured δ 13 C in three food items of wild tuatara. Pectoral muscle of fairy prions (a seabird eaten seasonally by tuatara) was significantly enriched in 13 C compared with whole bodies of wild insects (darkling beetles and tree weta). Values for 8 13 C in blood cells varied significantly among wild tuatara of different life‐history stages. Male tuatara were more enriched in 13 C than were females or juveniles, suggesting that males prey more heavily on seabirds. Insect foods of captive tuatara varied dramatically in δ 13 C; this is attributed to differential consumption of plant material derived from the C 3 and C 4 photosynthetic pathways. Blood cells from four different groups of captive tuatara differed significantly in δ 13 C. This was perhaps related to assimilation of insects with different δ 13 C values, and cannot be attributed to differences in seabird predation as captive tuatara do not have access to seabirds. For wild tuatara on Stephens Island, stable carbon isotope analysis provides support for the dietary information available from behavioural observations, gut analyses and measurements of plasma composition.