Data from: From gestation to weaning: combining robust design and multi-event models unveils cost of lactation in a large herbivore

1. The cost of current reproduction on survival or future reproduction is one of the most studied trade-offs governing resource distribution between fitness components. Results have often been clouded, however, by the existence of individual heterogeneity, with high-quality individuals able to allocate energy to several functions simultaneously, at no apparent cost. 2. Surprisingly, it has also rarely been assessed within a breeding season by breaking down the various reproductive efforts of females from gestation to weaning, even though resource availability and energy requirements vary greatly. 3. We filled this gap by using an intensively monitored population of Pyrenean chamois and by expanding a new methodological approach integrating robust design in a multi-event framework. We distinguished females that gave birth or not, and among reproducing females whether they lost their kid or successfully raised it until weaning. We estimated spring and summer juvenile survival, investigated whether gestation, lactation or weaning incurred costs on the next reproductive occasion, and assessed how individual heterogeneity influenced the detection of such costs. 4. Contrary to expectations if trade-offs occur, we found a positive relationship between gestation and adult survival suggesting that non-breeding females are in poor condition. Costs of reproduction were expressed through negative relationships between lactation and both subsequent breeding probability and spring juvenile survival. Such costs could be detected only once individual heterogeneity (assessed as two groups contrasting good vs poor breeders) and time variations in juvenile survival were accounted for. Early lactation decreased the probability of future reproduction, providing quantitative evidence of the fitness cost of this period recognized as the most energetically demanding in female mammals and critical for neonatal survival. 5. The new approach employed made it possible to estimate two components of kid survival that are often considered practically unavailable in free ranging populations, and also revealed that reproductive costs appeared only when contrasting the different stages of reproductive effort. From an evolutionary perspective, our findings stressed the importance of the temporal resolution at which reproductive cost is studied, and also provided insights on the reproductive period during which internal and external factors would be expected to have the greatest fitness impact.

1. 当前繁殖对自身存活或未来繁殖的成本，是调控适合度组分间资源分配的最受关注权衡之一。然而，个体异质性的存在往往使研究结果变得模糊：优质个体可同时将能量分配至多项生命活动，且无明显成本代价。
2. 令人意外的是，尽管资源可获得性与能量需求存在显著差异，学界仍极少在繁殖季内通过拆解雌性从妊娠到断乳的各阶段繁殖投入来开展相关评估。
3. 本研究以密集监测的比利牛斯臆羚（Pyrenean chamois）种群为研究对象，拓展了一种整合了稳健设计（robust design）与多事件框架的全新方法学手段，填补了这一研究空白。我们区分了产仔与未产仔的雌性个体，并在繁殖雌性中进一步划分出幼崽夭折与成功将幼崽抚育至断乳的个体。本研究估算了春季与夏季的幼体存活率，探究了妊娠、泌乳或断乳是否会对后续繁殖事件产生成本代价，并评估了个体异质性如何影响这类成本代价的检测效果。
4. 若存在繁殖权衡，理论预期的结果与本研究发现相悖：我们发现妊娠与成体存活率呈正相关，这表明未繁殖的雌性个体身体状况较差。繁殖成本通过泌乳与后续繁殖概率、春季幼体存活率之间的负相关关系得以体现。仅当我们控制个体异质性（划分为优质繁殖者与劣质繁殖者两个组别）以及幼体存活率的时间变异后，这类成本代价才能被检测到。泌乳早期会降低未来繁殖概率，这为该阶段的适合度成本提供了定量证据——该阶段被认为是雌性哺乳动物能量消耗最高、且对新生幼体存活至关重要的时期。
5. 本研究采用的全新方法，得以估算出野外自由种群中通常被认为几乎无法获取的两项幼崽存活率组分，同时还揭示出：仅当对比繁殖投入的不同阶段时，繁殖成本才会显现。从进化生态学视角来看，本研究结果强调了研究繁殖成本时时间分辨率的重要性，并为明确内外因素对适合度产生最大影响的繁殖阶段提供了新见解。