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测到卵内胚胎的热趋性（thermal taxis），该现象或可使胚胎通过行为实现体温调节。鉴于温度会对胚胎发育产生影响，此类行为性体温调节或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。本研究针对表现出胚胎热趋性（embryonic thermal taxis）的拟鳄龟（Chelydra serpentina），通过观测其天然巢穴与卵内的温度梯度，并更具普适性地模拟不同巢穴深度、卵尺寸与土壤类型下的巢穴内温度梯度，评估了胚胎在自然环境中通过行为实现体温调节的可能性。研究结果显示，拟鳄龟巢穴内部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，本研究的模拟结果未出现能够满足行为体温调节所需的足够大且稳定的温度梯度。具备生物学相关强度的温度梯度在全球范围内分布极为有限，即便存在，其方向每日也会反转两次，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实现行为性体温调节。据此我们提出，胚胎热趋性实际上属于一种游戏行为，该行为可能具有适应性或选择中性，其成因是孵化前自由生活阶段的行为体温调节机制提前被激活。