Male monarch butterflies, Danaus plexippus, adjust ejaculates in response to intensity of sperm competition

During mating, male Lepidoptera transfer spermatophores that consist of accessory gland material, eupyrene (nucleated) sperm and apyrene sperm that is incapable of fertilizing eggs. Sexual selection theory predicts that males should allocate these materials strategically based on the risk and intensity of sperm competition. We studied the relationship between behavioural and physiological cues and material allocation by male monarch butterflies, Danaus plexippus. Males that had waited longer between matings transferred larger spermatophores and more apyrene and eupyrene sperm. Eupyrene sperm number was also correlated with female mating history, with males transferring more sperm to females that had larger amounts of spermatophore material stored from previous mates, regardless of whether this came from one or three mates. This result suggests that males use stored ejaculates to assess female mating history and increase eupyrene sperm investment under increased sperm competition intensity. Male monarchs appear to be capable of independently manipulating the different components of their ejaculates. Ejaculate allocation patterns suggest that males benefit by maximizing spermatophore size and apyrene sperm number, possibly to delay future female remating. However, males allocate more eupyrene sperm to females when sperm competition is more intense, which is consistent with predictions from recent sperm competition models.