Summary 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 covaries 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 covaried 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 covariation 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.

Spatial and temporal variation in ecological parameters selects for plastic responses to prevailing conditions. Males of web spiders have been shown to adjust developmental decisions to cues that relate to the degree of competition from other males. Here, we explore experimentally whether males of the spider Nephila senegalensis base behavioural decisions in the context of mate choice and sperm allocation to the presence of cues of a rival and to their own competitive dominance. Large and small males were offered a simultaneous choice between two penultimate instar females that varied in quality and in whether they were previously visited by a rival male. Large and small males avoided webs visited by a previous male, and this preference overrode differences in female quality. In a second experiment, males were assigned to copulate with a virgin female whose web was either previously visited by a rival male or not. We found substantial variation in male sperm allocation, but this variation was not related to our treatment. Our experiments demonstrate that males notice silk cues left behind by rival males on a female’s web and that they can utilize these cues in relation to decisions over whether to visit a web, but not in relation to sperm allocation.

No abstract

Offspring development is thought to be optimized if they are adapted towards the capacities of their parents. Here, we disrupted natural parent-offspring combinations by reciprocally exchanging whole broods of wild blue tits, and investigated whether and how any parentoffspring trait mismatch impinges on nestling development. Foster parents had a particularly strong impact on nestling growth via their provisioning rate. This relationship, however, also depended on the begging behavior of the nestlings.