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 (≈0.45°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.
- Top-view panel — drag it left/right to rotate the clutch, the 2D equivalent of orbiting the buried nest.