Flexible communication within bird families-the consequences of behavioural plasticity for parent-offspring coadaptation

Offspring are selected to demand more resources than what is optimal for their parents to provide, which results in a complex and dynamic interplay during parental care. Parent-offspring communication often involves conspicuous begging by the offspring which triggers a parental response, typically the transfer of food. So begging and parental provisioning reciprocally influence each other and are therefore expected to coevolve. There is indeed empirical evidence for covariation of offspring begging and parental provisioning at the phenotypic level. However, whether this reflects genetic correlations of mean levels of behaviours or a covariation of the slopes of offspring demand and parental supply functions (= behavioural plasticity) is not known. The latter has gone rather unnoticed - despite the obvious dynamics of parent-offspring communication. In this study we measured parental provisioning and begging behaviour at two different hunger levels using canaries (Serinus canaria) as a model species. This enabled us to simultaneously study the plastic responses of the parents and the offspring to changes in offspring need. We first tested whether parent and offspring behaviours covary phenotypically. Then, using a covariance-partitioning approach, we estimated whether the covariance predominantly occurred at a between-nest level (i.e. indicating a fixed strategy) or at a within-nest level (i.e. reflecting a flexible strategy). We found positive phenotypic covariation of offspring begging and parental provisioning, confirming previous evidence. Yet, this phenotypic covariation was mainly driven by a covariance at the within-nest level. That is parental and offspring behaviours covary because of a plastic behavioural co-adjustment, indicating that behavioural plasticity could be a main driver of parent-offspring coadaptation.

子代经演化选择后，会索要超出双亲抚育最优供给量的资源，这使得双亲抚育过程中形成复杂且动态的互作关系。亲代与子代的交流通常包含子代显眼的乞食行为，该行为会触发双亲的抚育回应，通常表现为食物投喂。因此，乞食行为与双亲育幼供给行为互为因果、彼此影响，二者被推测会发生协同演化。目前已有实证证据表明，子代乞食与双亲育幼供给在表型层面存在协变关系。然而，这种表型协变究竟反映的是行为平均水平的遗传相关，还是子代需求与双亲供给函数的斜率协变（即行为可塑性（behavioural plasticity)），目前仍不明确。尽管亲代-子代交流具有显著的动态特征，但后者长期以来未受到足够关注。本研究以金丝雀（Serinus canaria）为模式物种，设置两种不同的饥饿水平，对双亲育幼供给行为与子代乞食行为进行了量化测定。该实验设计使我们得以同步研究双亲与子代对子代需求变化的可塑性响应。我们首先验证了亲代与子代的行为是否存在表型协变关系。随后，我们采用协方差分解方法，评估该协变主要发生在巢间水平（即代表固定行为策略）还是巢内水平（即反映灵活行为策略）。实验结果显示，子代乞食与双亲育幼供给存在正向表型协变，印证了此前的实证证据。但该表型协变主要由巢内水平的协变所主导。换言之，亲代与子代的行为协变源于可塑性的行为协同调整，这表明行为可塑性可能是亲代-子代协同适应的主要驱动因素。