Age-related reproductive performance in the parental burying beetle, Nicrophorus orbicollis

Age-related improvements in reproductive performance have been explained by the selection, constraint, and restraint hypotheses (increased performance related to greater effort and a declining residual reproductive value). Using a laboratory population of the burying beetle, Nicrophorus orbicollis, 6 measures of performance were assessed. Consideration of the selection and constraint hypotheses was minimized by reducing mortality to near zero and comparing reproductive outcomes of equal-sized females with equivalent mating and breeding experience. Young and old females were provided a resource for breeding (mouse carcass) and were confronted by a potentially infanticidal intruder (young or old) after offspring were present in the nest. Younger females were more likely to experience a brood failure and were slower to oviposit eggs. Younger intruding females also were less likely to usurp a carcass from a resident female. Age, however, did not affect any measure of reproductive performance once a female had a brood. There were no differences between young and old females in the number and mass of offspring in original or replacement broods, or in the probability of successfully defending offspring against an intruder. In a follow-up experiment with smaller carcasses, there was, again, no significant difference in the ability of young and old females to defend their brood. In N. orbicollis, the reproductive restraint observed in younger females appears to inhibit the initiation but not the continuance of reproduction. The usefulness of the residual reproductive, targeted reproductive effort and the maturational limits hypotheses to explain age-related variation in reproductive performance are discussed.