Q15 - Sea Turtle Temperature-Dependent Sex Determination

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Sex Determination & Sex-Linked Inheritance Ecology

Hawksbill turtles breed on Saadiyat Island in Abu Dhabi.

Sex in sea turtles is controlled by the temperature of the developing eggs (temperature-dependent sex determination). Higher temperatures lead to more females, and lower temperatures, to more males.

As the global climate is rapidly heating, there is a risk that many populations of long-lived sea turtles could develop biased sex ratios.

On average, female sea turtles, which have a prolonged reproductive period extending to 60 to 70 years, mate once every three years, storing sperm within their bodies during this period, whilst the males try to mate every year. The figure below models the influence of temperature on the probability of eggs developing into females in two species of sea turtle.

Sigmoid curves for two species. Species 1 (curve a) has a steeper transition from low to high probability of female hatchlings across a narrow temperature range. Species 2 (curve b) has a more gradual transition across a wider temperature range. Both curves cross 0.5 probability near 29 degrees C. X = Temperature (°C), Y = Probability of a female hatchling. Curve a = Species 1, Curve b = Species 2.

Part A: If an individual of Species 1 has 150 eggs in a clutch, then, based on the model above, what would be the expected number of female hatchlings at 30°C?

Part B: True or false?

Close-up of the sigmoid curves showing the steeper slope for Species 1 and the shallower slope for Species 2 near the pivotal temperature. Detail of the two sigmoid curves near the pivotal temperature.

Q15.1. If an individual of Species 1 has 150 eggs in a clutch, what would be the expected number of female hatchlings at 30 degrees C?

Q15.2.1. Species 2 will be more vulnerable to skewed sex ratios as temperatures rise, compared to Species 1.

Q15.2.2. Dark-coloured beaches are likely to produce more female hatchlings than pale-coloured beaches.

Q15.2.3. The production of more female hatchlings, due to climate change, is likely to enhance the natural rate of population growth of turtles in the next 10 years.

Q15.2.4. Rising temperatures during climate change are more likely to skew the operational sex ratio of sea turtles than the absolute sex ratio at hatching.

Q15.2.5. Increasing sand temperatures will lead to a significant increase in metabolic heating of the developing turtle eggs.

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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF