Data from: Seasonal shifts along the oviparity-viviparity continuum in a cold-climate lizard population

Because squamate embryos require weeks of high temperature to complete development, cool climatic areas are dominated by viviparous taxa (in which gravid females can sun-bask to keep embryos warm) rather than oviparous taxa (which rely on warm soil to incubate their eggs). How, then, can some oviparous taxa reproduce successfully in cool climates – especially late in summer, when soil temperatures are falling? Near the northern limit of their distribution (in Sweden), sand lizards (Lacerta agilis) shift tactics seasonally, such that the eggs in late clutches complete development more quickly (when incubated at a standard temperature) than do those of early clutches. That acceleration is achieved by a reduction in egg size, and by an increase in the duration of uterine retention of eggs (especially, after cool weather). Our results clarify the ability of oviparous reptiles to reproduce successfully in cool climates, and suggest a novel advantage to reptilian viviparity in such conditions: by maintaining high body temperatures, viviparous females may escape the need to reduce offspring size in late-season litters.

由于有鳞目（Squamata）胚胎需要数周的高温才能完成发育，寒冷气候区域的爬行动物类群以卵胎生（viviparous）类群为主——妊娠雌性可通过晒太阳维持胚胎所需的适宜温度——而非卵生（oviparous）类群（后者依赖温暖土壤孵化卵）。那么，部分卵生类群如何能在寒冷气候中成功繁殖？尤其是在夏末土壤温度逐渐下降的时段。在其分布北限附近（瑞典境内），捷蜥蜴（Lacerta agilis）会随季节调整繁殖策略：在标准孵化温度下，晚期产卵窝的胚胎发育速度显著快于早期产卵窝。这种发育加速可通过两种方式实现：缩小卵的尺寸，以及延长卵在子宫内的滞留时长（尤其在凉爽天气过后）。本研究结果阐明了卵生爬行动物在寒冷气候中成功繁殖的能力，并揭示了此类环境下爬行动物卵胎生的一项全新演化优势：通过维持较高的体温，卵胎生雌性无需在季末的产仔中缩小幼崽体型。