 The ‘ultimate explanation’ for the male trait is the prior existence of female preferences, proximately manifest in differential peahen mate choice decisions, whilst the ‘ultimate explanation’ for the peahens’ mating preferences is the prior existence of variation in the peacock's tail associated with fitness.This example illustrates how reciprocal causation is not a rejection of the proximate-ultimate distinction itself, but instead a rejection of the implication that developmental processes should not feature in evolutionary explanations.Reciprocal causation also applies in other domains of evolutionary biology.