Do TSD, sex ratios, and nest characteristics influence the vulnerability of tuatara to global warming? Nicola J. Nelson a, * , Michael B. Thompson b , Shirley Pledger c , Susan N. Keall a , Charles H. Daugherty a a School of Biological Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand b Integrative Physiology Research Group, School of Biological Sciences (A08), University of Sydney, Australia c School of Mathematical and Computing Sciences, Victoria University of Wellington, New Zealand Abstract. Tuatara (Sphenodon punctatus ) are threatened New Zealand reptiles with temperature- dependent sex determination (TSD). Higher incubation temperatures produce males, and less than 1 8C separates production of males and females. We investigated variability in nesting ecology to assess whether global warming is likely to result in increasingly male-biased populations. We examined nesting seasons during 1998/1999 and 2002/2003 in New Zealand’s largest tuatara population on Stephens Island, and collected hourly temperature recordings and physical descriptions from 70 nests. Nest depths were not significantly different between years, and ranged from 10 to 230 mm from the soil surface to the top egg. Incubation temperatures in successful nests throughout the year-long incubation period ranged from 1.6 to 38.4 8C. Sex ratios of nests were correlated with incubation temperature: 64% males were produced in 1998/1999, a relatively warm season, but we predict an equal sex ratio was produced in 2002/2003. Although temperatures varied over the 2002/2003 season with respect to monthly long-term averages, 2002 was the second warmest year on record. Stephens Island supports a wide range of nesting habitat, a relatively large population of tuatara, and nest characteristics are highly variable. As such, this population is likely to be resilient to global warming in the short term because an equal sex ratio was predicted from a relatively warm season. However, most other islands where tuatara occur are smaller, have smaller populations, and have fewer open areas for nesting and/or shallower soils. These conditions are more likely to produce a male bias in hatchlings because female 0531-5131/ D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2004.08.093 * Corresponding author. Tel.: +64 4 463 7443; fax: +64 4 463 5331. E-mail address: nicola.nelson@vuw.ac.nz (N.J. Nelson). International Congress Series 1275 (2004) 250 – 257 www.ics-elsevier.com