Families on the spot: sexual signals influence parent-offspring interactions.

In 1950, Tinbergen described the elicitation of offspring begging by the red spot on the bill of parent gulls, and this became a model system for behavioural studies. Current knowledge on colour traits suggests they can act as sexual signals revealing individual quality. However, sexual signals have never been studied simultaneously in relationship to parent-offspring and sexual conflicts. We manipulated the red-spot size in one member of yellow-legged gull pairs and observed their partners' feeding efforts in relationship to offspring begging. In the enlarged-spot group, partners doubled their effort compared with the other groups. Furthermore, in the reduced-spot group, partners provided food in relationship to offspring begging, contrasting with the fixed effort of the partners of enlarged-spot gulls. Manipulated gulls, independently of treatment, provided food in relationship to chicks begging only when the partner's investment was low, and performed a fixed effort when the partner's contribution was high. Results demonstrate that the red spot in yellow-legged gulls functions as a sexual signal and indicate that parental rules are plastic, depending on the information on offer. Previous evidence and this study indicate that this signal is used by all family members to adjust decision rules. The incorporation of sexual signals in parent-offspring interactions can be crucial in understanding intra-familial conflicts.