Tuatara Translocation: Temperature-Dependent Sex Determination
A translocation of tuatara from a predator-free island to a mainland fenced sanctuary is being planned. Source nests average 21.3 °C and produce 60% males. Constant-temperature incubation data show 100% female at ≥22.3 °C, 100% male at ≤21.0 °C, and a pivotal temperature of 21.7 °C. Soil loggers at the recipient site record a mean nest temperature of 21.9 °C, with 30% of nests deeper and 0.8 °C cooler than the rest. Climate projections add 1.5 °C by 2100. A pilot using 70% shade cloth lowered nest temperature by 1.2 °C but delayed hatching by 9 days and reduced hatchling mass by 12%. The sanctuary can shade about 40% of the nesting area. The management plan must fix a sex-ratio target of 50:50 ± 10% for the first 20 years.
1. Project the primary sex ratio at the recipient site now and under +1.5 °C, allowing for nest-depth variation. State your assumptions and where the uncertainty lies.
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The transition from all-male (21.0 °C) to all-female (22.3 °C) spans 1.3 °C. A nest at 21.9 °C is (21.9 − 21.0)/1.3 ≈ 69% of the way along that span, so a linear approximation gives roughly 75–85% female. Deeper nests are 0.8 °C cooler at 21.1 °C, near the male end (about 10–20% female).
Population average ≈ 0.7 × 80% + 0.3 × 15% ≈ 60% female now. Under +1.5 °C, even the deepest nests reach 22.6 °C, above 22.3 °C, so the projection is effectively 100% female - no male recruitment.
Assumptions: constant-temperature lab data approximate fluctuating field nests (they tend to overestimate how sharp the transition is), the nest-depth distribution stays constant, and vegetation and shade do not change. Uncertainty is dominated by microhabitat thermal variation and by the true shape of the TSD curve, so these should be presented as ranges, not point predictions. The +1.5 °C scenario is robust: no plausible microhabitat buffering rescues male production.
2. Given the pilot trade-offs, evaluate shading as a management tool and recommend a specific design.
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Shading works directionally: −1.2 °C takes the mean from 21.9 to 20.7 °C, below the pivotal, so shaded nests would be male-biased while unshaded nests stay female-biased. Shading 40% of the area would therefore create thermal heterogeneity and produce both sexes - a sensible buffer against warming.
The trade-offs are real, not trivial. Nine days' delayed hatching shifts emergence into a different thermal and predator window, and 12% lower hatchling mass is likely to reduce overwinter survival and later fecundity. A treatment that fixes the sex ratio but lowers recruitment is not a fix.
Recommendation: patchy, block shading of about 40% of the nesting area rather than uniform shade over everything, using removable structures, so females can choose among thermal environments and the sex ratio does not flip to all-male. Pair this with nest-temperature loggers and hatchling mass monitoring in shaded and unshaded blocks so the treatment can be tuned - the relevant test is whether recruitment, not just sex ratio, is maintained.
3. Is the 50:50 ± 10% target biologically defensible? What would you monitor, and what would trigger a change of plan?
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The target is operationally convenient but biologically ambiguous. Temperature determines the primary sex ratio; the population sex ratio also depends on juvenile and adult mortality, which may differ between the sexes. In a long-lived species with high adult survival, a moderate hatchling female bias can still yield an even adult ratio - so 50:50 at hatching is not automatically the right target, and forcing it could over-produce males.
A more defensible target is a primary sex ratio of 40–60% female, plus an explicit check on adult sex ratio each generation and a rule that no cohort is 100% one sex.
Monitor: temperature loggers in at least 30 nests per year; hatchling emergence date and mass; juvenile recruitment from marked nests; adult sex ratio from capture–recapture. Triggers: if the three-year running mean of predicted female production exceeds 70%, expand shading; if hatchling mass falls more than 10% below the pilot baseline, reduce shading and consider nest-site supplementation or assisted movement of nests instead.