Data from: Selection and constraints on offspring size-number trade-offs in sand lizards (Lacerta agilis)

The trade-off between offspring size and number is a central component of life-history theory, postulating that larger investment into offspring size inevitably decreases offspring number. This trade-off is generally discussed in terms of genetic, physiological or morphological constraints; however, as among-individual differences can mask individual trade-offs, the underlying mechanisms may be difficult to reveal. In this study, we use multivariate analyses to investigate whether there is a trade-off between offspring size and number in a population of sand lizards by separating among- and within-individual patterns using a 15-year data set collected in the wild. We also explore the ecological and evolutionary causes and consequences of this trade-off by investigating how a female's resource (condition)- vs. age-related size (snout-vent length) influences her investment into offspring size vs. number (OSN), whether these traits are heritable and under selection and whether the OSN trade-off has a genetic component. We found a negative correlation between offspring size and number within individual females and physical constraints (size of body cavity) appear to limit the number of eggs that a female can produce. This suggests that the OSN trade-off occurs due to resource constraints as a female continues to grow throughout life and, thus, produces larger clutches. In contrast to the assumptions of classic OSN theory, we did not detect selection on offspring size; however, there was directional selection for larger clutch sizes. The repeatabilities of both offspring size and number were low and we did not detect any additive genetic variance in either trait. This could be due to strong selection (past or current) on these life-history traits, or to insufficient statistical power to detect significant additive genetic effects. Overall, the findings of this study are an important illustration of how analyses of within-individual patterns can reveal trade-offs and their underlying causes, with potential evolutionary and ecological consequences that are otherwise hidden by among-individual variation.

后代大小与数量之间的权衡（trade-off）是生活史理论的核心组成部分，该理论假定对后代大小的投入增加，必然会导致后代数量减少。这类权衡通常从遗传、生理或形态约束的角度进行讨论；然而，由于个体间差异可能掩盖个体层面的权衡现象，其背后的机制往往难以揭示。本研究依托野外采集的15年数据集，通过区分个体间与个体内的模式，利用多元分析方法探究沙蜥种群中是否存在后代大小与数量的权衡。本研究还通过以下方式探究该权衡的生态与进化成因及后果：分析雌性的资源（身体状况）相关体型与年龄相关体型（吻肛长）如何影响其对后代大小与数量的投入，探究这些性状是否具有可遗传性并处于选择压力之下，以及后代大小-数量权衡（offspring size vs. number, OSN）是否存在遗传基础。研究结果显示，雌性个体内部的后代大小与数量呈负相关，且物理约束（体腔大小）似乎限制了雌性可产卵的数量。这表明，由于雌性终生持续生长，因此会产出更多窝卵，OSN权衡由资源约束所驱动。与经典OSN理论的假设相悖，本研究未检测到针对后代大小的选择，但存在针对更大窝卵数的定向选择。后代大小与数量的重复力均较低，且未在这两个性状中检测到任何加性遗传方差。这一结果可能源于这些生活史性状受到了强烈的选择（过去或当前），或是统计效力不足，无法检测到显著的加性遗传效应。总体而言，本研究结果清晰地展示了：通过分析个体内模式，能够揭示权衡现象及其背后的成因，以及那些原本会被个体间差异掩盖的潜在进化与生态后果。