Sea Turtle Nest Sex Ratio: Temperature-Dependent Sex Determination

Beach & Nest Conditions

Surface sand temperature 31.0°C

Nest depth 35 cm

Vegetation shade cover 20%

Pivotal temperature (species) 29.2°C

Clutch Outcome

♀ 50%♂ 50%

Mean nest temp

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Hatch viability

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Top-of-nest temp

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Bottom-of-nest temp

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How it works

Sea turtles (and many other reptiles) have no sex chromosomes — hatchling sex is set entirely by the temperature the egg experiences during the middle third of incubation. This is called temperature-dependent sex determination (TSD). For sea turtles the rule is "hot chicks, cool dudes": warmer eggs become female, cooler eggs become male.

P(female) = 1 / (1 + exp(-(T - P) / k))
T = egg temperature (°C)
P = pivotal temperature — the temperature that gives a 50/50 split
k = transitional range constant (≈0.4-0.5°C for Cheloniidae)

Temperature inside a real nest is not uniform. Sun heats the sand surface, so eggs near the top of the clutch run warmer than eggs near the bottom — a single nest routinely produces a mix of both sexes from one thermal gradient:

T(depth) = T_surface − g · depth
T_surface = T_air − shade · ΔT_shade,max
g = thermal attenuation with depth (°C/cm)

Deeper nests and heavier shade cover both push the whole clutch cooler and toward more males; hot open beaches under a warming climate push clutches toward extreme, sometimes 90%+, female ratios. Hatch viability instead peaks near the pivotal range and drops off at both very cold and very hot extremes — real eggs simply fail above roughly 33-35°C or below roughly 25°C, so a beach can shift all-female long before it becomes literally lethal, and lethal well before it becomes all-male.

Surface sand temperature — the daily-mean heat load the beach absorbs from the sun.
Nest depth — how far below the surface the female buried the clutch; deeper eggs are insulated from the daily heat pulse.
Vegetation shade cover — canopy over the nest site, which cools the surface directly.
Pivotal temperature — varies by species and even by population; the slider lets you compare cooler-pivot and warmer-pivot populations.

Female hatchling Male hatchling Undetermined

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