What is a paternal effect?

Maternal effects are now universally recognised as a form of nongenetic parental influence on offspring but, until recently, paternal effects were regarded as an anomaly. Although it is now clear that paternal effects are both widespread and important, their proximate basis and evolutionary consequences have received little attention and remain poorly understood. In particular, because many paternal effects are mediated by maternal responses such as differential allocation, the boundary between paternal and maternal effects is sometimes blurred. We distinguish here three basic types of paternal effect and clarify the role of maternal responses in these effects. We also outline key questions that can serve as a road map for research on the proximate basis and evolutionary implications of paternal effects.