Correlated response to selection on paternity assurance behaviour on parental care in females, but not males

Classical parental investment (PI) theory predicts that males should respond to a low probability of parentage by reducing paternal effort. However, empirical evidence for this predicted relationship is equivocal and it has recently been suggested that this may be because adjustments in behaviour by females to changes in male behaviour, and vice versa, during mating and parental care interactions have not been accounted for. Recent theoretical work predicts that social interactions between the sexes will modify the co-evolution between traits involved in mating and parental care as a result of costs of these interactions (i.e., sexual conflict). Here we use artificial selection on a paternity assurance trait, repeated mating rate, and crosses within and between selection lines, to test these ideas using a model species, Nicrophorus vespilloides burying beetles. The experimental design allowed us to assess whether mating traits and parental care traits co-evolved (specifically testing whether paternity assurance behaviours and paternal care are positively related), and if so, what role social interactions between the sexes play in this co-evolution. Our results show that selection acting on females, not males, can drive the co-evolution of paternity assurance traits and parental care. Males do not care more in response to selection on mating rate. Instead, patterns of parental care change as an indirect response to costs of mating for females. In species with biparental care costs to females may therefore be more important than costs to males in explaining variation in patterns of parental care within and between species.