Altered embryonic development in northern bobwhite quail (Colinus virginianus) induced by pre-incubation oscillatory thermal stresses mimicking global warming predictions

Global warming is likely to alter reproductive success of ground-nesting birds that lay eggs normally left unattended for days or even weeks before actual parental incubation, especially in already warm climates. The native North American bobwhite quail (Colinus virginianus) is such a species, and pre-incubation quail eggs may experience temperatures >45°C. Yet, almost nothing is known about embryonic survival after such high pre-incubation temperatures. Freshly laid bobwhite quail eggs were exposed during a 12 day pre-incubation period to one of five thermal regimes: low oscillating temperatures (25-40°C, mean=28.9°C), high oscillating temperatures (30-45°C, mean=33.9°C), low constant temperatures (28.85°C), high constant temperatures (mean=33.9°C), or commercially employed pre-incubation temperatures (20°C). After treatment, eggs were then incubated at a standard 37.5°C to determine subsequent effects on embryonic development rate, survival, water loss, hatching, and embryonic oxygen consumption. Both quantity of heating degree hours during pre-incubation and specific thermal regime (oscillating vs. non-oscillating) profoundly affected important aspects of embryo survival and indices of development and growth Pre-incubation quail eggs showed a remarkable tolerance to constant high temperatures (up to 45°C), surviving for 4.5±0.3 days of subsequent incubation, but high oscillating pre-incubation temperature increased embryo survival (mean survival 12.2±1.8 days) and led to more rapid development than high constant temperature (maximum 38.5ºC), even though both groups experienced the same total heating degree-hours. Oxygen consumption was ~200-300 μl O2.egg.min-1 at hatching in all groups, and was not affected by pre-incubation conditions. Oscillating temperatures, which are the norm for pre-incubation quail eggs in their natural habitat, thus enhanced survival at higher temperatures. However, a 5°C increase in pre-incubation temperature, which equates to the predicted long-term increases of 5°C or more, nonetheless reduced hatching rate by approximately 50%. Thus, while pre-incubation bobwhite eggs may be resiliant to moderate oscillating temperature increases, global warming will likely severely impact wild bobwhite quail populations, especially in their strongholds in southern latitudes.

全球变暖或会改变地面筑巢鸟类的繁殖成功率——这类鸟类的鸟卵在亲鸟启动实际孵化前，通常会被遗弃数天乃至数周，在本就温暖的气候区域这一现象尤为突出。北美本土的山齿鹑（Colinus virginianus）正是这类物种，其孵化前的鸟卵可能会暴露于45℃以上的高温环境中。然而，目前学界对这类极端高温暴露后的胚胎存活情况几乎一无所知。
本研究将新鲜产下的山齿鹑鸟卵在为期12天的预孵化阶段，置于五种热环境之一中处理：低波动温度组（25~40℃，平均28.9℃）、高波动温度组（30~45℃，平均33.9℃）、低恒温组（28.85℃）、高恒温组（平均33.9℃），以及商业孵化常用的预孵化温度组（20℃）。处理完成后，所有鸟卵均在标准37.5℃条件下进行孵化，以探究预孵化处理对胚胎发育速率、存活率、水分流失、孵化情况以及胚胎耗氧量的后续影响。
预孵化阶段的积热小时数与具体热环境（波动式vs非波动式）均会显著影响胚胎存活及发育生长的关键指标。预孵化的山齿鹑鸟卵对恒定高温（最高可达45℃）展现出显著耐受性，在后续孵化阶段可存活4.5±0.3天；但与高恒温组（最高温度38.5℃）相比，高波动式预孵化温度虽总积热小时数与之相同，却提升了胚胎存活率（平均存活12.2±1.8天）并加快了发育速度。所有组别的孵化期胚胎耗氧量约为200~300 μl O₂·卵⁻¹·min⁻¹，且不受预孵化处理条件的影响。
波动式温度是自然生境中山齿鹑预孵化阶段的常态温度条件，因此在高温环境下可提升胚胎存活率。然而，预孵化温度升高5℃（这一数值对应预测的长期升温幅度5℃及以上）仍会使孵化率降低约50%。综上，尽管山齿鹑的预孵化鸟卵或许可耐受适度的波动温度升高，但全球变暖仍可能对野生山齿鹑种群造成严重冲击，尤其是在其分布核心的低纬度南部区域。