Prolonged Mating in the Monarch Butterfly Danaus plexippus and Nightfall as a Cue for Sperm Transfer

Abstract

Theory predicts that paternity assurance adaptatations should be associated especially with species in which there is strong male competition for females, e.g., as in insects like the monarch butterfly, Danaus plexippus, where males can enforce copulations on females which, in turn, exhibit strong polyandry. Observations on mating monarchs showed that copula termination almost exclusively occurred after darkness had fallen, and by interrupting mating pairs and dissecting females, we showed that sperm transfer did not start until after dark, irrespective of the time spent in copula in light. This, together with the observation that copula duration increased from 9 to 27 h when pairs mated in constant light, suggested that nightfall per se is the stimulus that initiates sperm transfer and determines the timing of copula termination. These prolonged matings in liaison with the duration of the night, allow time for sperm delivered by a male to be used for fertilizing eggs by the female on the day after copulation.