Data for Adaptive seasonal shift towards investment in fewer, larger offspring: Evidence from field and laboratory studies

1. Seasonal changes in reproduction have been described for many taxa. As reproductive seasons progress, females often shift from greater energetic investment in many small offspring towards investing less total energy into fewer, better provisioned (i.e. larger) offspring. The underlying causes of this pattern have not been assessed in many systems. 2. Two primary hypotheses have been proposed to explain these patterns. The first is an adaptive hypothesis from life-history theory: early offspring have a survival advantage over those produced later. Accordingly, selection favours females that invest in offspring quantity early in the season and offspring quality later. The second hypothesis suggests these patterns are not intrinsic but result from passive responses to seasonal changes in the environment experienced by reproducing females (i.e. maternal environment). 3. To disentangle the causes underlying this pattern, which has been reported in brown anole lizards (Anolis sagrei), we performed complementary field and laboratory studies. The laboratory study carefully controlled maternal environments and quantified reproductive patterns throughout the reproductive season for each female. The field study measured similar metrics from free ranging lizards across an entire reproductive season. 4. In the laboratory, females increased relative effort per offspring as the reproductive season progressed; smaller eggs were laid earlier, larger eggs were laid later. Moreover, we observed significant among-individual variation in seasonal changes in reproduction, which is necessary for traits to evolve via natural selection. Because these patterns consistently emerge under controlled laboratory conditions, they likely represent an intrinsic and potentially adaptive adjustment of reproductive effort as predicted by life-history theory. 5. The field study revealed similar trends, further suggesting that intrinsic patterns observed in the laboratory are strong enough to persist despite the environmental variability that characterizes natural habitats. The observed patterns are indicative of an adaptive seasonal shift in parental investment in response to a deteriorating offspring environment: allocating greater resources to late-produced offspring likely enhances maternal fitness.

1. 诸多类群（taxa）的繁殖季节性变化已被广泛报道。随着繁殖季的推进，雌性个体通常会调整能量投入策略：从最初对大量小型后代的高能量投入，转向总能量投入减少但投向更少、抚育更充分（即体型更大）的后代。目前该繁殖模式的潜在成因在多数类群中尚未得到系统阐明。
2. 目前已有两种主流假说用于解释该繁殖模式。第一种源自生活史理论（life-history theory）的适应性假说：早期产出的后代相较于后期产出的后代具有更高的存活优势。因此，自然选择会青睐于繁殖季早期侧重后代数量投入、晚期则提升后代质量的雌性个体。第二种假说则认为该模式并非内在固有，而是繁殖雌性所经历的环境随季节变化产生的被动响应，即母体环境（maternal environment）。
3. 为厘清该模式的潜在成因（该模式已在棕安乐蜥（Anolis sagrei）中被报道），我们开展了互补的野外与实验室研究。实验室研究严格控制母体环境，并量化了每只雌性个体在整个繁殖季的繁殖模式。野外研究则在完整繁殖季内，对自由栖息的蜥蜴开展了相似指标的测定。
4. 实验室实验结果显示，随着繁殖季推进，雌性个体对单只后代的相对投入量逐渐提升：早期产出的卵体型更小，晚期产出的卵体型更大。此外，我们观测到繁殖季节性变化存在显著的个体间差异——这正是性状通过自然选择演化的必要前提。由于该模式在受控实验室条件下稳定重现，其大概率代表了生活史理论所预测的、繁殖投入的内在且潜在具有适应性的调控调整。
5. 野外研究同样观测到了相似的趋势，进一步表明实验室中观测到的内在模式足够稳定，即便在自然栖息地典型的环境波动下仍能得以维持。本次观测到的模式暗示，亲本投入会随季节发生适应性转变，以应对后代生存环境的恶化：将更多资源分配给晚期产出的后代，或可提升母体适合度（maternal fitness）。