Figure 1.

Illustration of parental supply (solid blue lines) and offspring demand (dashed red lines) functions, and the evolution of parental provisioning and offspring begging as interacting behavioural reaction norms. (a) A simple example in which the observed supply (S) and demand (D) levels are defined at the intersect. Genetic variation between individuals can occur as variation in (b) intercept, (c) slope and (d) shape of the functions. Each intersect represents a possible outcome of the supply–demand interaction. Assuming an ancestral population with mean values represented by the circle, an increase in the level of parental provisioning by ΔS can be produced via changes in the supply function's intercept (b; square—increased parental supply to all levels of offspring begging), slope (c; square—higher parental responsiveness to begging) or shape (d; square—higher responsiveness to begging especially at low levels). In these cases, the behavioural level of offspring begging is assumed to passively follow according to an evolutionarily unchanged demand function. Alternatively, an increase in the level of parental provisioning by ΔS could also be achieved through evolutionary changes in the demand function intercept (b; triangle—increased offspring begging to all levels of parental supply), slope (c; triangle—reduced offspring sensitivity to supply) or shape (d; triangle—reduced sensitivity to supply especially at low levels). In the latter three scenarios, parental provisioning is assumed to passively follow according to an evolutionarily unchanged supply function. Finally, an increase in the level of parental provisioning by ΔS could occur due to changes in either intercept (b; plus), slope (c; plus) or shape (d; plus) of both parental and offspring response functions.