Egg production by tuatara on Lady Alice and Stephens Island, New Zealand

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Abstract
Radiography was used to determine the proportion of female tuatara, Sphenodon punctatus, that carry eggs each year, over 6 years on Stephens Island and 5 years on Lady Alice Island. The smallest female found to be gravid, located on Stephens Island, was 170 mm snout‐vent length. Between 1983 and 1987 the gravidity rate for Lady Alice Island females averaged 28.8% (range = 22.2–45.5%), while between 1982 and 1987 the gravidity rate for Stephens Island females averaged 21.9% (range = 8.2–29.6%). Excluding data from 1986, when only 11 females from Lady Alice Island were sampled, annual gravidity rates did not differ significantly between islands. There was significant annual variation in the proportion of females carrying eggs on Stephens Island, but not on Lady Alice Island. On Stephens Island, gravidity rate was especially low in 1982 (8.2%). There were significant positive, but different, correlations between clutch size and female body size on both islands. Mean clutch size for Lady Alice Island females (7.9 eggs; range = 5–13 eggs) was significantly smaller than that of Stephens Island females (9.4 eggs; range = 1–18 eggs) but Lady Alice Island females were on average larger. This difference in clutch size may be due to depleted food resources on Lady Alice Island. The introduced kiore, Rattus exulans, is present there, but not on Stephens Island. Females on both islands produce clutches on average just once every 4 years, but some females produce eggs 2 years apart. We suggest that the risks females take in migrating to, and remaining several weeks at, their nesting sites are such that it may be advantageous for them to reproduce infrequently. Females of at least 55–60 years age can still be reproductively active.