Are older parents less flexible? Testing age-dependent plasticity in Nicrophorus vespilloides burying beetles

Abstract

Phenotypic plasticity is an important mechanism facilitating adaptation to environmental change that often varies among individuals. One reason for this individual variation is that plasticity may depend on state variables, such as size, condition or age, which affect the costs and benefits of plasticity. Recent theoretical work predicts that plasticity will decrease as an organism ages because costs of plasticity mean that flexible phenotypic adjustments by individuals to environmental change will be less beneficial as age-related survival prospects decrease. Here we used Nicrophorus vespilloides burying beetles to test this prediction in the context of parental care. Burying beetles use the carcasses of small vertebrates as resources for breeding and have complex, extended, flexible parental care. Our experiment manipulated female age and (the order of presentation of) carcass size in a repeated-measures design to test the prediction that older beetles are less plastic than younger beetles in parental care. We found evidence in support of our central prediction: young females showed greater mean levels of plasticity than older beetles for all traits (parental care, number of offspring, brood mass) except mean larval mass (i.e. size of offspring), with the response to changes in carcass size dependent on the order of carcass presentation for young females but not for older females. Between-trait correlation analysis revealed age-related trade-offs between the size and number of offspring for older, but not young, mothers. The three age-dependent traits, which were intercorrelated, were also repeatable, indicating potential for coevolutionary responses to selection.