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.

已在多种爬行动物中观测到卵内胚胎的热趋性（thermal taxis）行为，该现象可使胚胎通过行为性体温调节（behavioral thermoregulation）调控自身体温。由于温度会影响胚胎发育，此类体温调节机制或许能让胚胎对自身发育命运的调控能力远超以往学界的假设。本研究以表现出胚胎热趋性的拟鳄龟（*Chelydra serpentina*）为对象，通过观测其自然巢穴及卵内的温度梯度，并更宽泛地模拟不同巢穴深度、卵尺寸与土壤类型下的巢穴温度梯度，评估了胚胎在自然环境中进行行为性体温调节的可能性。我们发现拟鳄龟巢穴内几乎不存在空间温度差异，且温度梯度难以传递至卵内。此外，本研究的模拟结果显示，能够满足行为性体温调节所需的足够大且稳定的温度梯度并未出现。具有生物学相关强度的温度梯度在全球范围内的分布十分有限，即便存在，其方向每日会逆转两次，这一变化速率远快于胚胎在卵内移动的速度。本研究结果表明，爬行动物胚胎几乎（即便存在也极为罕见）没有机会通过在卵内移动来进行行为性体温调节。我们认为，胚胎热趋性行为实则属于一种玩耍行为，该行为可能具有适应性或选择中性，其产生源于孵化前便已激活的自由生活阶段行为性体温调节机制。