Vleck C M, Vleck D
J Exp Zool Suppl. 1987;1:111-25.
Energy within an avian egg is used primarily to support the metabolic costs of growth and maintenance, deposited in embryonic tissues, or retained in residual yolk. In altricial species, embryonic growth rate and metabolic rate increase continuously during incubation, whereas in precocial species, embryonic growth rate declines shortly before hatching so that metabolic rate usually reaches a plateau before hatching. These differences mean that when altricial and precocial birds are compared using egg mass as a scaling variable, altricial species have lower costs of development, smaller hatchlings, and shorter incubation periods. Their eggs also have lower energy densities, however, and when egg energy content is used as the scaling variable altricial and precocial species cannot be distinguished except by egg size and energy density. This suggests that before hatching all avian embryos expand about the same percentage of the energy stored in their eggs. Altricial and precocial species do not appear to differ in energy cost per gram of yolk-free, dry hatchling produced (15.4 kJ/gm). The major difference between altriciality and precocity lies in the timing of hatching during the developmental process (earlier for altricial species than for precocial species) and in the amount of energy the parent originally deposits in the egg. We suggest that the first step in the evolution of altricial bird species from precocial ancestors was hatching early. This shortened the incubation period and reduced the amount of energy needed prior to hatching. Consequently, the female could reduce her energy investment per egg by decreasing size and increasing water content of the egg.
禽蛋内的能量主要用于支持生长和维持的代谢成本，沉积于胚胎组织中，或留存于剩余的卵黄中。在晚成雏物种中，胚胎生长速率和代谢速率在孵化期间持续增加，而在早成雏物种中，胚胎生长速率在孵化前不久下降，因此代谢速率通常在孵化前达到平稳状态。这些差异意味着，当以蛋重作为标度变量来比较晚成雏和早成雏鸟类时，晚成雏物种的发育成本较低、雏鸟较小且孵化期较短。然而，它们的蛋能量密度也较低，并且当以蛋的能量含量作为标度变量时，除了蛋的大小和能量密度外，无法区分晚成雏和早成雏物种。这表明在孵化前，所有鸟类胚胎消耗的能量约占其蛋中储存能量的相同比例。每克无卵黄干重雏鸟的能量成本（15.4千焦/克），晚成雏和早成雏物种之间似乎没有差异。晚成性和早成性之间的主要区别在于发育过程中孵化的时间（晚成雏物种比早成雏物种更早）以及亲代最初在蛋中沉积的能量数量。我们认为，晚成雏鸟类物种从早成雏祖先进化而来的第一步是提前孵化。这缩短了孵化期，并减少了孵化前所需的能量数量。因此，雌性可以通过减小蛋的大小和增加蛋的含水量来减少每个蛋的能量投入。
