Mathies T, Andrews R M
Department of Biology, Virginia Polytechnic Institute and State University, 24061, Blacksburg, VA, USA.
Oecologia. 1995 Sep;104(1):101-111. doi: 10.1007/BF00365568.
Viviparity in squamate reptiles is presumed to evolve in cold climates by selection for increasingly longer periods of egg retention. Longer periods of egg retention may require modifications to other reproductive features associated with the evolution of viviparity, including a reduction in eggshell thickness and clutch size. Field studies on the thermal and reproductive biology of high (HE) and low (LE) elevation populations of the oviparous lizard, Sceloporus scalaris, support these expectations. Both day and night-time temperatures at the HE site were considerably cooler than at the LE site, and the activity period was 2 h shorter at the HE than at the LE site. The median body temperature of active HE females was 2°C lower than that of LE females. HE females initiated reproduction earlier in the spring than LE females, apparently in order to compensate for relatively low temperatures during gestation. HE females retained eggs for about 20 days longer than LE females, which was reflected by differences in the degree of embryonic development at the time of oviposition (stages 35.5-37.0 versus stages 31.0-33.5, respectively). These results support the hypotheses that evolution of viviparity is a gradual process, and is favored in cold climates. Females in the HE population exhibited other traits consistent with presumed intermediate stages in the evolution of viviparity; mean eggshell thickness of HE eggs (19.3 μm) was significantly thinner than that of LE eggs (26.6 μm) and the size-adjusted clutch sizes of HE females (9.4) were smaller than those of LE females (11.2).
有鳞目爬行动物的胎生现象被认为是在寒冷气候中通过选择越来越长的卵滞留期而进化而来的。更长的卵滞留期可能需要对与胎生进化相关的其他生殖特征进行调整，包括蛋壳厚度和窝卵数的减少。对卵生蜥蜴斯氏强棱蜥高海拔（HE）和低海拔（LE）种群的热生物学和生殖生物学进行的野外研究支持了这些预期。HE地点的白天和夜间温度都比LE地点低得多，并且HE地点的活动期比LE地点短2小时。活跃的HE雌性的平均体温比LE雌性低2°C。HE雌性比LE雌性在春季更早开始繁殖，显然是为了补偿妊娠期相对较低的温度。HE雌性比LE雌性多保留卵约20天，这在产卵时胚胎发育程度的差异中得到体现（分别为35.5 - 37.0阶段和31.0 - 33.5阶段）。这些结果支持了胎生进化是一个渐进过程且在寒冷气候中更受青睐的假设。HE种群中的雌性表现出与胎生进化假定中间阶段一致的其他特征；HE卵的平均蛋壳厚度（19.3μm）明显比LE卵（26.6μm）薄，并且经大小调整后的HE雌性窝卵数（9.4）比LE雌性（11.2）少。
