Male monarch butterfly spermatophore mass and mating strategies

Male monarch butterflies, Danaus plexippus, produce a spermatophore which can represent approximately 10% of their body mass. Spermatophore mass increased with age in virgin males, and with the time since last mating in non-virgin males. Male monarchs did not delay re-mating until they were able to produce a large spermatophore. Recently mated males were as likely as virgins to copulate with both virgin and non-virgin females. Monarchs provide an example of Bateman's principle, mating whenever possible, despite the non-trivial cost involved. Bateman's (1948) study of reproductive success in Drosophila melanogaster showed that male reproductive success was correlated positively with the number of copulations they obtained. His study provided a quantitative example of Darwin's (1871) suggestion that males maximize their fitness by copulating as often as possible. However, there are exceptions to Bateman's principle. In species whose females mate synchronously, it may be impossible for males to show a high degree of polygyny (Knowlton 1979). In other species, it may be in the male's best interest to be monogamous if he can provide parental care that substantially increases offspring survival, or if guarding a mate prevents her from copulating with another male (Emlen & Oring 1977). A third category of exceptions may occur in polygynous species in which males delay re-mating to increase the expected number of offspring they gain from each mating. Bateman's principle is based on the assumption that males incur little or no cost in ejaculate production, but mating success in some species is affected by the transfer of substances that must be replenished after mating. Therefore, selection might favour males that wait between rantings until the amount of material available for transfer to the female ensures paternity of more of her offspring (Dewsbury 1982). In this paper I report a study of re-mating strategies in captive monarch butterflies, Danaus plexippus. Male monarchs, like all Lepidoptera, produce spermatophores consisting of a sperm sac embedded in a gelatinous body formed from accessory gland secretions. Several aspects of monarch biology suggest that males may increase their reproductive success by delaying re-mating: spermatophore size increases with the time elapsed since the last mating by a male (see below), monarchs are highly polyandrous (Pliske 1973; personal observation), and large spermatophores delay female re-mating (Oberhauser, unpublished data). Thus by waiting to re-mate, males might increase the number of ova from each mate that are fertilized by their sperm, thereby violating Bateman's principle.