Repeatability, heritability, and age-dependence in the aggressiveness reaction norms of a wild passerine bird

Labile characters allow individuals to flexibly adjust their phenotype to changes in environmental conditions. There is growing evidence that individuals can differ both in average expression and level of plasticity in this type of character. Both of these aspects are studied in conjunction within a reaction norm framework. Theoreticians have investigated the factors promoting variation in reaction norm intercepts (average phenotype) and slopes (level of plasticity) of a key labile character: behaviour. A general prediction from their work is that selection will favour the evolution of repeatable individual variation in level of plasticity only under certain ecological conditions. While factors promoting individual repeatability of plasticity have thus been identified, empirical estimates of this phenomenon are largely lacking for wild populations. We assayed aggressiveness of individual male great tits (Parus major) twice during their egg-laying stage and twice during their egg-incubation stage to quantify each male's level of seasonal plasticity. This procedure was applied during six consecutive years; all males breeding in our plots during those years were assayed, resulting in repeated measures of individual reaction norms for any individual breeding in multiple years. We quantified among- and within-individual variation in reaction norm components, allowing us to estimate repeatability of seasonal plasticity. Using social pedigree information, we further partitioned reaction norm components into their additive genetic and permanent environmental counterparts. Cross-year individual repeatability for the intercepts (average aggressiveness) and slopes (level of seasonal plasticity) of the aggressiveness reaction norms were 0·574 and 0·516 respectively. The mean of the posterior distributions suggested modest heritabilities (h2 = 0·260 for intercepts; h2 = 0·266 for slopes), but these estimates were relatively uncertain. Males behaved more aggressively in areas with higher breeding densities, and became less aggressive and less plastic with increasing age; plasticity thus varied within individuals and was multidimensional in nature. This empirical study quantified cross-year individual repeatability, heritability and age-related reversible plasticity in behaviour. Acknowledging such patterns of multi-level variation is important not only for testing behavioural ecology theory concerning the evolution of repeatable differences in behavioural plasticity but also for predicting how reversible plasticity may evolve in natural populations.易变性状（Labile characters）可使个体灵活调整自身表型以适配环境条件的变化。越来越多的证据表明，同一类易变性状的个体间在平均表达水平与可塑性程度上均存在差异，上述两个维度均可通过反应规范（reaction norm）框架开展联合研究。 理论研究者已针对行为这一关键易变性状，探究了驱动其反应规范截距（即平均表型）与斜率（即可塑性程度）产生变异的各类因素。其研究得出的一般性预测为：仅在特定生态条件下，自然选择才会青睐可塑性程度上可重复的个体差异演化。尽管目前已明确了推动可塑性个体可重复性的相关因素，但针对野生种群该现象的实证估算仍极为匮乏。 本研究对雄性大山雀（Parus major）个体分别在产卵期与孵卵期各开展两次攻击性测定，以量化每只雄鸟的季节可塑性程度。该实验流程连续开展了六年，在此期间对研究样地内所有繁殖的雄鸟均进行了测定，由此可对多年繁殖的个体获取其反应规范的重复测量数据。我们对反应规范组成部分的个体间与个体内变异进行了量化，从而得以估算季节可塑性的可重复性。借助社会谱系信息，我们进一步将反应规范组成部分划分为加性遗传效应与永久环境效应两类。 攻击性反应规范的截距（即平均攻击性）与斜率（即季节可塑性程度）的跨年度个体可重复性分别为0.574与0.516。后验分布均值显示其遗传力（h²）处于中等水平（截距的h²=0.260，斜率的h²=0.266），但上述估算的不确定性相对较高。雄鸟在繁殖密度更高的区域攻击性更强，且随着年龄增长攻击性与可塑性均会降低；由此可见可塑性存在个体内变异，且本质上具有多维度特征。 本项实证研究量化了行为的跨年度个体可重复性、遗传力以及与年龄相关的可逆可塑性。认识这类多水平变异模式，不仅有助于检验行为生态学中有关行为可塑性可重复差异演化的理论，也可为预测自然种群中可逆可塑性的演化方向提供依据。