Between- and within-individual variation of maternal thyroid hormone deposition in wild great tits (Parus major)

Maternal hormones are often considered a mediator of anticipatory maternal effects, namely mothers adjust maternal hormone transfer to prepare the offspring for the anticipated environment. The flexibility for mothers to adjust hormone transfer is therefore a prerequisite for such anticipatory maternal effects. Nevertheless, previous studies have only focused on the average differences of maternal hormone transfer between groups and neglected the substantial individual variation, despite that individual plasticity in maternal hormone transfer is actually the central assumption. In this study, we studied the between- and within-individual variation of maternal thyroid hormones (THs) in egg yolk of wild great tits (Parus major) and estimated the individual plasticity of maternal yolk THs across environmental temperature, clutch initiation dates and egg laying order using linear mixed-effects models. Interestingly, our models provide statistical evidence that the two main THs –
the main biologically active hormone T3, and T4, which is mostly considered as a prohormone – exhibited different variation patterns. Yolk T3 showed significant between-individual variation on the average levels, in line with its previously reported moderate heritability. Yolk T4, however, showed significant between-clutch variation in the pattern over the laying sequence, suggesting a great within-individual plasticity. Our findings suggest that the role and function of the hormone within the endocrine axis likely influences its flexibility to respond to environmental change. Whether the flexibility of T4 deposition brings fitness advantage should be examined along with its potential effects on offspring, which remains to be further investigated.

母体激素常被视为预期性母性效应的介导因子，即母体通过调整激素传递量，使后代适配其所预期的生存环境。因此，母体调控激素传递的灵活性，正是此类预期性母性效应得以实现的必要前提。然而，过往研究仅关注组间母体激素传递的平均差异，却忽略了显著的个体间变异——尽管母体激素传递的个体可塑性，实则是该领域研究的核心假设。本研究以野生大山雀（Parus major）的卵黄母体甲状腺激素（THs）为研究对象，分析了其个体间与个体内的变异模式，并通过线性混合效应模型，探究了母体卵黄THs在环境温度、窝卵启动日期以及产卵顺序梯度下的个体可塑性。值得注意的是，本研究的模型提供了统计学证据，表明两种主要甲状腺激素——即主要生物活性激素T3，以及通常被视为前体激素的T4——呈现出截然不同的变异模式。卵黄T3的平均水平存在显著的个体间变异，这与其此前报道的中等遗传力特征相符。而卵黄T4则在产卵序列的变化模式中呈现出显著的窝间变异，这暗示其具有较强的个体内可塑性。本研究结果表明，激素在内分泌轴中的角色与功能，或会影响其响应环境变化的灵活性。T4沉积的灵活性是否能带来适合度优势，以及其对后代的潜在影响，仍有待进一步的研究验证。