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

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: Keywords: apyrene sperm Danaus plexippus eupyrene sperm mate assessment monarch butterfly sperm competition sperm transfer spermatophore strategic mating effort 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. Studies of sexual selection traditionally focused on precopula-tory processes, but in the past few decades researchers have recognized that both female choice and male competition continue after copulation (Parker 1970; Eberhard 1996). Sperm competition occurs when the ejaculates of two or more males are simultaneously available to a female for fertilization. This competition often approximates a raffle in which males delivering more sperm are likely to fertilize more eggs (Parker 1990; Gage & Morrow 2003). However, the energetic demands of producing large quantities of sperm can limit male reproductive success (Dewsbury 1982; Wedell et al. 2002) and males are consequently predicted to allocate ejaculates strategically based on male condition, risk and intensity of sperm competition and relative reproductive value of Recent models distinguish between sperm competition risk (i.e. the probability of sperm competition occurring) and intensity (i.e. the number of competing sperm …