Modelling the incubation microclimate to predict offspring sex ratios and hatching phenology in tuatara (Sphenodon punctatus)
Authors:   , Carter Anna L. 
Published: November 2021

<p>Successful conservation of terrestrial biodiversity requires understanding and predicting the impacts of rapid climate warming on the suitability of both current and potential future habitats. Most predictions of range shifts and other population-scale effects of climate change rely to some extent on statistical links between a species' known geographical distribution and the suite of environmental conditions experienced within that space. However, species' responses to climate change are likely to be more complex than can be represented by the projection of current species-environment relationships into unknown environments. An important goal in biodiversity conservation is the development of quantitative tools with which to assess habitat suitability independently of distributions.  In populations of oviparous species, climate change and habitat modification may have distinct effects on different life stages. Temperatures that are well within the thermal tolerance range of adults, for example, may affect embryonic development rates, hatching phenology, or offspring survival and phenotype. I examined how environmental variation may affect the thermal suitability of habitat for facilitating embryonic development and maintaining balanced sex ratios in tuatara (Sphenodon punctatus), an endemic New Zealand reptile with temperature-dependent sex determination (TSD). Once widespread throughout New Zealand, populations are now restricted to offshore islands and fenced mainland sanctuaries, though establishment of additional populations via translocation is ongoing. Due to intensive conservation efforts, tuatara are not classified as an endangered species, but, like other species in which hatchling sex is determined by the incubation environment, populations are potentially at risk from the detrimental effects of sex-ratio bias.  I conducted two seasons of field work on the island of Takapourewa to quantify the relationship between rapid vegetation succession and selection of nesting areas. I then used a variety of predictive models to link data on nesting behaviour collected in the field with the microclimate conditions experienced by nesting female tuatara and developing embryos. Using mechanistically modelled soil temperature data, I generated predictions of incubation temperatures, offspring sex ratios, and hatching dates for two populations of tuatara on environmentally distinct islands, Takapourewa and Hauturu, under current and projected future climate scenarios. Finally, I classified the thermal suitability of sites on Hauturu for facilitating successful embryonic development and created geospatial surfaces defining suitable nesting locations adjacent to tuatara habitats.  Offspring sex ratios on both islands are unlikely to become male-biased if the magnitude of climate warming observed over the next century more closely matches the minimum, rather than the maximum, projected warming scenario. On Takapourewa, the timing of nesting will be critical in determining whether sex ratios become male-biased under a scenario of maximum climate warming. Earlier nesting may also lead to shifts in hatching phenology under either scenario of climate warming. Warmer annual temperatures on Hauturu are more likely to lead to heavily male-biased offspring sex ratios under the maximum warming scenario. Female tuatara on Hauturu do not need to travel away from their current habitats to locate suitable nesting sites. Monitoring the population to quantify nesting behaviour on the island will be important for determining whether females' choices of incubation microclimates can compensate for the sex ratio-biasing effects of climate change.</p>