Data from: Within-family parent-offspring co-adaptation in a wild bird: on static traits, behavioural reaction norms and sex differences

Parental care, a central component of reproduction in a wide range of animal species, often involves elaborate behavioural interactions between parents and their offspring. Due to the reciprocal nature of these interactions, it has been hypothesized that parental and offspring behaviours (e.g. parental food provisioning and offspring begging) are not only target but also agent of selection. These traits are therefore expected to co-evolve, ultimately leading to co-adaptation of parent and offspring behaviours within families. However, empirical data on such parent-offspring co-adaptation are limited, particularly for wild populations. Furthermore, mean levels of behaviour (as measured in previous studies) may not adequately describe the dynamic nature of the reciprocal interplay between parents and their offspring, and instead rather the behavioural reaction norms for provisioning and begging may be co-adapted. We applied a large-scale cross-fostering study over 3 consecutive breeding seasons to investigate whether provisioning behaviour of wild blue tit (Cyanistes caeruleus) parents co-varies with the begging behaviour of their genetic, cross-fostered offspring. We simultaneously analysed parent and offspring behaviours, both as static traits (mean levels) and behavioural reaction norms (offspring begging as a function of food deprivation and parental provisioning as a function of short-term experimental changes in brood size). Neither maternal nor paternal provisioning rates co-varied with the begging intensity of their genetic offspring when analysed as mean levels of behaviour. However, the slopes of the reaction norms for provisioning and begging were negatively correlated between male, but not female, parents and their genetic offspring. Thus, fathers that change their provisioning rate strongly with brood size sire offspring whose level of begging only weakly increases with hunger, and vice versa. The observed co-variation suggests the existence of sex-specific optima for parent-offspring trait combinations. Thus, our study not only highlights the importance of a behavioural reaction norm approach when investigating parent-offspring interactions, but also stresses the relevance of considering parents as separate units, at least for biparental species.双亲抚育是诸多动物类群繁殖过程中的核心组成部分，通常涉及亲代与子代间复杂的行为互动。鉴于这类互动具有相互性，学界曾提出假说：亲代与子代行为（例如亲代育食行为与子代乞食行为）不仅是自然选择的靶标，同时也是自然选择的动因。因此，这些性状预计将发生协同进化，最终实现家庭内部亲代与子代行为的协同适应。不过，针对这类亲子协同适应的实证数据较为匮乏，针对野生种群的相关研究尤为稀缺。此外，过往研究中所测定的行为平均水平或许无法充分刻画亲代与子代间相互作用的动态本质，相较而言，育食与乞食的行为反应规范（behavioural reaction norms）或许才是协同适应的核心对象。 本研究在连续3个繁殖季开展大规模交叉抚育实验，旨在探究野生蓝山雀（Cyanistes caeruleus）亲代的育食行为是否与其亲生子代、寄养子代的乞食行为存在共变关系。研究同时从两个维度同步分析亲代与子代行为：一是将行为平均水平这一静态性状，二是行为反应规范——子代乞食行为随食物剥夺的变化规律，以及亲代育食行为随育雏数短期实验扰动的变化规律。 当以行为平均水平为分析维度时，母本与父本的育雏率均未与其亲生子代的乞食强度呈现共变关系。然而，雄性亲代的育食行为反应规范的斜率与其亲生子代的乞食行为反应规范的斜率呈显著负相关，雌性亲代则未出现这一关联。具体而言，育雏率随育雏数变化幅度较大的父亲，其所育子代的乞食水平仅随饥饿程度上升的幅度较小，反之亦然。 本研究观测到的共变现象表明，亲子性状组合存在性别特异性的最优解。因此，本研究不仅凸显了采用行为反应规范方法开展亲子互动研究的重要性，同时也强调了将双亲视为独立个体的必要性——至少对于双亲育幼物种而言。