Individual Variation in Parental Care Reaction Norms: Integration of Personality and Plasticity

Personality (consistent differences between individuals in behavior) and plasticity (changes within individuals in behavior) are often viewed as separate and opposing phenomena. We tested this idea by analyzing parental care reaction norms in a bird that exhibits biparental care. Personality in provisioning behavior existed (r(IC) = 0.11) and persisted despite being reduced after accounting for individual differences in environment. Plasticity was also evident and differed between the sexes. Male visit rate was associated with changes in brood size and time of day, but female visit rate was associated with changes in nestling age and date. In both sexes changes in visit rate were positively correlated with changes in their partner's visit rate. Both sexes also exhibited multidimensional reaction norms; interaction terms revealed that within-individual visit rates increased more steeply with brood size when nestlings were older, and the effect of the partner's visit rate was sensitive to variation in date, precipitation, and the focal bird's age. Individuals also varied in how they responded (reaction norm slope) to changes in nestling age and partner visits. Moreover, parental personality was interdependent with individual plasticity in several ways. Individuals of both sexes with a high visit rate also responded more positively to changes in nestling age, and males also showed this pattern with changes in partner visit rate. Explicit use of the behavioral reaction norm integrated personality and plasticity, revealed that these are not opposing concepts, and stimulated new hypotheses about sexual conflict over care and provisioning as a life-history trait.