Male-biased sex ratio in a small tuatara population

Aim We estimated population size, survival, longevity and sex ratio of tuatara (Sphenodon guntheri Buller 1877) on North Brother Island and determined whether recruitment was occurring, to aid management and identify potential problems for population viability. Location This study was conducted on North Brother Island, Cook Strait, New Zealand. This 4-ha island supports the sole remnant population of S. guntheri. Methods Demographic variables were estimated using capture-recapture methods. Tuatara were individually marked and recaptured during twelve trips, spanning a decade. Population size was estimated for selected trips using the CAPTURE package and finite mixture models, and survival was analysed using the Jolly-Seber (J-S) model. Longevity was estimated using tuatara individually marked in 1957. Results Approximately 350 adult tuatara profile likelihood interval (PLI) (294-427) inhabit North Brother Island, and the sex ratio is strongly biased towards males (1.7M : 1F). Annual adult survivorship is high (0.95) for both sexes and some tuatara live for at least 61 years. Main conclusions The small size and biased sex ratio of this population may make it susceptible to demographic stochasticity, Allee effects, and/or loss of genetic variation. Harvesting for translocation could exacerbate such problems. In addition, tuatara have temperature-dependent sex determination (TSD), with males produced at higher incubation temperatures. Global warming may therefore skew the sex ratio further unless female nest site choice or adaptive shifts in pivotal temperatures compensate for rising temperatures.