Rhymer Judith M
Department of Zoology, University of Manitoba, R3T 2N2, Winnipeg, Manitoba, Canada.
Oecologia. 1988 Feb;75(1):20-24. doi: 10.1007/BF00378809.
There is a range of egg size phenotypes in Mallards (Anas platyrhynchos) that has a large genetic component. It was hypothesized that egg size variation could play an important role in survival of newly hatched ducklings during their first few days out of the nest when they are most susceptible to thermal stress and starvation. Precocial young must be physiologically capable of maintaining homeothermy in order to spend adequate time foraging. Duckling size at hatching was highly correlated with egg mass, and those hatching from heavier eggs were able to maintain homeothermy at colder environmental temperatures than those from lighter eggs. Heavy ducklings had significantly lower mass-specific cooling rates, but lower critical temperature did not vary significantly among ducklings of different size. Although insulation and energy reserves were not proportionally greater in larger ducklings, those hatching from heavier eggs can survive starvation longer than those from lighter eggs. The relative cold tolerance of young from light and heavy eggs will affect the ratio of time spent foraging to time spent being brooded by the female parent. Although there is no direct evidence that selection is acting on egg size, variation in this trait within a population could be maintained by fluctuating environmental conditions at hatch.
绿头鸭（Anas platyrhynchos）存在一系列蛋大小的表型，且该性状具有很大的遗传成分。据推测，蛋大小的变化可能在新孵化的雏鸭出巢后的头几天生存中发挥重要作用，此时它们最易受到热应激和饥饿的影响。早成雏必须在生理上有能力维持体温恒定，以便有足够的时间觅食。孵化时雏鸭的大小与蛋的质量高度相关，那些从较重的蛋孵化出的雏鸭在比从较轻的蛋孵化出的雏鸭更冷的环境温度下能够维持体温恒定。较重的雏鸭具有显著更低的单位质量冷却速率，但不同大小的雏鸭之间的临界温度下限没有显著差异。尽管较大的雏鸭的隔热和能量储备并非成比例地更大，但那些从较重的蛋孵化出的雏鸭比从较轻的蛋孵化出的雏鸭能够在饥饿中存活更长时间。轻蛋和重蛋孵化出的雏鸭的相对耐寒性会影响觅食时间与由雌性亲代孵育时间的比例。尽管没有直接证据表明选择作用于蛋的大小，但种群内该性状的变异可能通过孵化时波动的环境条件得以维持。
