Sockman Keith W, Sharp Peter J, Schwabl Hubert
Department of Biology, University of North Carolina, Chapel Hill, NC 27599-3280, USA.
Biol Rev Camb Philos Soc. 2006 Nov;81(4):629-66. doi: 10.1017/S1464793106007147. Epub 2006 Oct 12.
How much effort to expend in any one bout of reproduction is among the most important decisions made by an individual that breeds more than once. According to life-history theory, reproduction is costly, and individuals that invest too much in a given reproductive bout pay with reduced reproductive output in the future. Likewise, investing too little does not maximize reproductive potential. Because reproductive effort relative to output can vary with predictable and unpredictable challenges and opportunities, no single level of reproductive effort maximizes fitness. This leads to the prediction that individuals possessing behavioural mechanisms to buffer challenges and take advantage of opportunities would incur fitness benefits. Here, we review evidence in birds, primarily of altricial species, for the presence of at least two such mechanisms and evidence for and against the seasonal coordination of these mechanisms through seasonal changes in plasma concentrations of the pituitary hormone prolactin. First, the seasonal decline in clutch size of most bird species may partially offset a predictable seasonal decline in the reproductive value of offspring. Second, establishing a developmental sibling-hierarchy among offspring may hedge against unpredictable changes in resource availability and offspring viability or quality, and minimize energy expenditure in raising a brood. The hierarchy may be a product, in part, of the timing of incubation onset relative to clutch completion and the rate of yolk androgen deposition during the laying cycle. Because clutch size should influence the effects of both these traits on the developmental hierarchy, we predicted and describe evidence in some species that females adjust the timing of incubation onset and rate of yolk androgen deposition to match clutch size. Studies on domesticated precocial species reveal an inhibitory effect of the pituitary hormone prolactin on egg laying, suggesting a possible hormonal basis for the regulation of clutch size. Studies on the American kestrel (Falco sparverius) and other species suggest that the seasonal increase in plasma concentrations of prolactin may regulate both a seasonal advance in the timing of incubation onset and a seasonal increase in the rate of yolk androgen deposition. These observations, together with strong conceptual arguments published previously, raise the possibility that a single hormone, prolactin, functions as the basis of a common mechanism for the seasonal adjustment of reproductive effort. However, a role for prolactin in regulating clutch size in any species is not firmly established, and evidence from some species indicates that clutch size may not be coupled to the timing of incubation onset and rate of yolk androgen deposition. A dissociation between the regulation of clutch size and the regulation of incubation onset and yolk androgen deposition may enable an independent response to the predictable and unpredictable challenges and opportunities faced during reproduction.
在任何一次繁殖中投入多少精力，是多次繁殖的个体所做出的最重要决策之一。根据生活史理论，繁殖成本高昂，在某一特定繁殖阶段投入过多的个体会在未来付出繁殖产出减少的代价。同样，投入过少也无法使繁殖潜力最大化。由于相对于产出的繁殖投入会随着可预测和不可预测的挑战与机遇而变化，不存在单一的繁殖投入水平能使适应性最大化。这就导致了一个预测，即拥有行为机制来缓冲挑战并利用机遇的个体将获得适应性益处。在此，我们回顾鸟类（主要是晚成雏物种）中存在至少两种此类机制的证据，以及支持和反对通过垂体激素催乳素血浆浓度的季节性变化对这些机制进行季节性协调的证据。首先，大多数鸟类物种窝卵数的季节性下降可能部分抵消后代繁殖价值可预测的季节性下降。其次，在后代中建立发育性的同胞等级制度，可能有助于应对资源可用性以及后代生存能力或质量的不可预测变化，并将养育一窝幼雏的能量消耗降至最低。这种等级制度可能部分是孵化开始时间相对于窝卵完成时间的早晚以及产卵周期中卵黄雄激素沉积速率的产物。由于窝卵数应会影响这两个特征对发育等级制度的作用，我们预测并描述了一些物种中的证据，即雌性会调整孵化开始时间和卵黄雄激素沉积速率以匹配窝卵数。对家养早成雏物种的研究揭示了垂体激素催乳素对产卵的抑制作用，这表明调节窝卵数可能存在激素基础。对美洲隼（Falco sparverius）和其他物种的研究表明，催乳素血浆浓度的季节性升高可能会调节孵化开始时间的季节性提前以及卵黄雄激素沉积速率的季节性增加。这些观察结果，连同先前发表的有力概念性论证，增加了一种可能性，即单一激素催乳素作为繁殖投入季节性调整的共同机制基础发挥作用。然而，催乳素在任何物种中调节窝卵数的作用尚未得到确凿证实，并且一些物种的证据表明窝卵数可能与孵化开始时间和卵黄雄激素沉积速率无关。窝卵数调节与孵化开始时间和卵黄雄激素沉积调节之间的分离，可能使个体能够对繁殖过程中面临的可预测和不可预测的挑战与机遇做出独立反应。
