Department of Biology, University of Missouri, St. Louis, Missouri 63121, USA.
Am Nat. 2010 Mar;175(3):350-61. doi: 10.1086/650371.
Beginning with George Williams's concept of present and residual (future) reproductive value, life-history theory has considered that the optimized level of parental investment (i.e., assumed risk) should increase in proportion to the annual mortality rate of parent individuals. However, when the survival of young from independence to maturity is separated from parental reproductive success, optimized parental investment is proportional, instead, to prereproductive survival when reproductive and nonreproductive components of adult mortality are additive (simultaneous risk) or to the ratio of prereproductive survival to adult nonreproductive survival when the adult mortality components are multiplicative (independent risk). Applied to the lower, and largely nonoverlapping, brood sizes of tropical compared to temperate and boreal birds, estimates of both adult and prereproductive survival do not predict different levels of reproductive investment. Accordingly, the pervasive increase in clutch size with latitude would appear to reflect increasing availability of food resources to provision offspring.
从乔治·威廉姆斯（George Williams）提出的现生（未来）生殖值概念开始，生活史理论就认为，亲代投资（即假定风险）的最佳水平应该与亲代个体的年死亡率成比例增加。然而，当幼鸟从独立到成熟的生存与亲代生殖成功分离时，优化的亲代投资与生殖和非生殖成分的成比例死亡率是相加的（同时风险），或者与生殖死亡率成分是相乘的（独立风险）的生殖前生存与成年非生殖生存的比例成比例。应用于热带地区与温带和寒带鸟类相比，较低且在很大程度上不重叠的窝卵数，对成年和生殖前生存的估计并不能预测不同水平的生殖投资。因此，随着纬度的增加，普遍存在的窝卵数增加似乎反映了食物资源对后代供应的增加。
