Reptile embryos lack the opportunity to thermoregulate by moving within the egg

Thermal taxis by egg-bound embryos has been observed in multiple reptiles and might allow embryos to behaviorally thermoregulate. Because temperature affects development, such thermoregulation could allow embryos to control their fate far more than historically assumed. We assessed the opportunity for embryos to behaviorally thermoregulate in nature by examining thermal gradients within natural nests and eggs of the common snapping turtle (Chelydra serpentina, a species that displays embryonic thermal taxis) and, more generally, by simulating thermal gradients within nests across a range of nest depths, egg sizes, and soil types. We observed little spatial thermal variation within C. serpentina nests, and thermal gradients were poorly transferred to eggs. Furthermore, thermal gradients sufficiently large and constant for behavioral thermoregulation were not predicted to occur in our simulations. Gradients of biologically-relevant magnitude have limited global occurrence, and reverse...