Data from: 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 direction twice daily when they do exist, which is substantially
faster than embryos can shift position within the egg. Our results imply
that reptile embryos will rarely, if ever, have the opportunity to
behaviorally thermoregulate by moving within the egg. We suggest that
embryonic thermal taxis instead represents a play behavior, which may be
adaptive or selectively neutral, and results from the mechanisms for
behavioral thermoregulation in free-living stages "coming
online" prior to hatching.