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.多项研究已在多种爬行动物中观察到卵内胚胎的热趋性（thermal taxis），该行为或可使胚胎通过行为调节自身体温。由于温度会影响胚胎发育进程，此类行为热调节或许能让胚胎对自身发育命运的调控能力远超既往认知。我们通过监测普通鳄龟（Chelydra serpentina，一种表现出胚胎热趋性的物种）天然巢穴及卵内的热梯度，并更广泛地模拟不同巢穴深度、卵径尺寸及土壤类型下的巢穴热梯度，评估了自然界中胚胎开展行为性体温调节的可行性。研究观察到，普通鳄龟巢穴内的空间热差异极小，且热梯度难以传递至卵内。此外，模拟结果显示，自然环境中难以形成强度足够且持续稳定、可供胚胎进行行为热调节的热梯度。具备生物学相关强度的热梯度在全球范围内分布十分有限，即便存在，其方向每日会发生两次反转，这一变化速率远快于胚胎在卵内移动换位的速度。本研究结果表明，爬行动物胚胎几乎没有机会通过在卵内移动来实现行为性体温调节。我们认为，胚胎热趋性实际上更可能是一种玩耍行为，该行为可能具有适应性或选择中性，其产生源于自由生活阶段的行为体温调节机制在孵化前便已提前激活。