Diets of wild tuatara (Sphenodon punctatus) on Stephens Island

Abstract

Seasonal variation in the diets of live wild adult male, adult female (snout-vent length; SVL ≥ 180mm) and juvenile (SVL = 100-170mm) tuatara (Sphenodon punctatus) from Keeper's Bush (Stephens Island, New Zealand) were described following analyses of faecal and stomach contents. In addition, the diets of post-hatchling tuatara (SVL = 50-100mm) were described from faecal analysis. The diets of tuatara of unknown life history stage were described following the collection and analyses of field-collected faeces (scats).

Stomach contents and faeces were examined and the contents were categorised as being: "darkling beetle", "tree weta", "giant weta", "other invertebrates", "seabird remains", "seabird egg", "reptile", "plant/dirt" and "unidentified material". The frequency of occurrence (%) and proportion by volume (%) of each food item in the stomach contents and scats was recorded and compared among seasons and tuatara life history stages.

The results confirm that adult and juvenile tuatara are opportunistic feeders, feeding on a range of invertebrates, especially beetles. Seabirds are found in the diet of tuatara, being found almost only during summer. "Other invertebrates" (70-77%) and "darkling beetles" (14-71 %) were the most frequently occurring food items in stomach contents and scats. "Plant/dirt" was also frequently occurring found in between 58-80% of scats and stomach contents . The representation of each food item in the diet of tuatara is primarily dependent on the availability of the food item, with prey size and mobility also being important.

There was little difference between the diets of adult and juvenile tuatara, whereas the diet of post-hatchling tuatara was significantly different from the larger tuatara. Post-hatchling tuatara feed exclusively on small invertebrates including snails.

Seabirds, specifically fairy prions (Pachyptila turtur), breed on Stephens Island in large numbers during summer; it was only during this time that they were found in the diets of tuatara of known life history stage. Adult and juvenile tuatara were found to have eaten fairy prions. Growth curves were constructed for five fairy prion chicks from hatching to fledging. Using these growth curves and the presence/absence of down in the "seabird remains" from tuatara stomach contents and scats, I was able to determine that most predation events involved chicks and fledglings. I was unable to determine whether any part of the seabird body was selectively eaten. Episodes of juvenile and adult tuatara scavenging on fairy prion carcasses were observed.

There were differences in the proportion by volume and frequency of occurrence of food items represented in stomach contents and scats. Soft-bodied invertebrates constituted a greater proportion of stomach contents volume, whereas the remains of larger and indigestible food items were more frequently occurring in scats.

The proportion of tuatara with food in their stomachs at the time of stomach-pumping shows that there is no seasonal difference or difference among life history stages. There was also no difference in the volume of food recovered among seasons or life history stages. Experiments examining the rate of gastric evacuation in adult males show that there is seasonal variation in the rate of evacuation, with food remaining in the stomach for at least 48h during November, but only 20h during January. Further studies examining the energy content of food eaten by wild tuatara are required.

It is hoped that the results of this study will be used in conjunction with previous diet studies to improve the health of captive tuatara.