Early survival is highly variable and strongly influences observed population growth rates in most vertebrate populations. One of the major potential drivers of survival variation among juveniles is body mass. Heavy juveniles are better fed and have greater body reserves, and are thus assumed to survive better than light individuals. In spite of this, some studies have failed to detect an influence of body mass on offspring survival, questioning whether offspring body mass does indeed consistently influence juvenile survival, or whether this occurs in particular species/environments. Furthermore, the causes for variation in offspring mass are poorly understood, although maternal mass has often been reported to play a crucial role. To understand why offspring differ in body mass, and how this influences juvenile survival, we performed phylogenetically corrected meta‐analyses of both the relationship between offspring body mass and offspring survival in birds and mammals and the relationship between maternal mass and offspring mass in mammals. We found strong support for an overall positive effect of offspring body mass on survival, with a more pronounced influence in mammals than in birds. An increase of one standard deviation of body mass increased the odds of offspring survival by 71% in mammals and by 44% in birds. A cost of being too fat in birds in terms of flight performance might explain why body mass is a less reliable predictor of offspring survival in birds. We then looked for moderators explaining the among‐study differences reported in the intensity of this relationship. Surprisingly, sex did not influence the intensity of the offspring mass–survival relationship and phylogeny only accounted for a small proportion of observed variation in the intensity of that relationship. Among the potential factors that might affect the relationship between mass and survival in juveniles, only environmental conditions was influential in mammals. Offspring survival was most strongly influenced by body mass in captive populations and wild populations in the absence of predation. We also found support for the expected positive effect of maternal mass on offspring mass in mammals (rpearson = 0.387). As body mass is a strong predictor of early survival, we expected heavier mothers to allocate more to their offspring, leading them to be heavier and so to have a higher survival. However, none of the potential factors we tested for variation in the maternal mass–offspring mass relationship had a detectable influence. Further studies should focus on linking these two relationships to determine whether a strong effect of offspring size on early survival is associated with a high correlation coefficient between maternal mass and offspring mass.

中文翻译：

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后代体重变化的原因和后果：鸟类和哺乳动物的荟萃分析

在大多数脊椎动物种群中，早期存活率变化很大，并且强烈影响观察到的种群增长率。幼鱼生存差异的主要潜在驱动因素之一是体重。体重较轻的幼鱼吃得更好，身体储备也更多，因此被认为比轻的个体生存得更好。尽管如此，一些研究未能发现体重对后代存活的影响，质疑后代体重是否确实持续影响幼鱼的存活，或者这是否发生在特定物种/环境中。此外，人们对后代体重变化的原因知之甚少，尽管经常有报道称母体质量起着至关重要的作用。要了解为什么后代的体重不同，以及这如何影响青少年的生存，我们对鸟类和哺乳动物的后代体重与后代存活率之间的关系以及哺乳动物中母体质量与后代质量之间的关系进行了系统发育校正的荟萃分析。我们发现后代体重对生存的总体积极影响得到了强有力的支持，对哺乳动物的影响比对鸟类的影响更明显。体重每增加一个标准差，哺乳动物后代存活的几率增加 71%，鸟类增加 44%。就飞行性能而言，鸟类太胖的代价可能解释了为什么体重对于鸟类后代存活的预测不太可靠。然后我们寻找主持人来解释在这种关系的强度中报告的研究之间的差异。出奇，性别不影响后代质量-生存关系的强度，系统发育仅占观察到的这种关系强度变化的一小部分。在可能影响幼鱼体重和存活率之间关系的潜在因素中，只有环境条件对哺乳动物有影响。在没有捕食的情况下，圈养种群和野生种群的体重对后代存活的影响最大。我们还发现支持母体质量对哺乳动物后代质量的预期积极影响（rpearson = 0.387）。由于体重是早期存活率的一个强有力的预测指标，我们预计体重较重的母亲会分配更多给后代，导致他们更重，因此存活率更高。然而，我们测试的母体质量-后代质量关系变化的潜在因素都没有可检测的影响。进一步的研究应侧重于将这两种关系联系起来，以确定后代大小对早期存活率的强烈影响是否与母体质量和后代质量之间的高相关系数有关。