In many insects, males court more actively than females, but invest less in their offspring (Thornhill and Alcock, 1983). Because the offspring yielded by a female are always her own, reproductive investment by the female is much more easily selected for than that by the male in an evolutionary sense. However, females of several species behave more actively than males in courtship. Such species are called courtship-role-reversed species, males of which invest in reproduction, e.g., donate nutrients to females and/or offspring (Gwynne, 1981, 1982), and take care of the offspring (Smith, 1979). Generally, maternity is self-evident, while paternity is not always so. Therefore, males with paternal investment inevitably have some risk of being cheated by other males. In other words, there must exist some mechanism to assure the paternity of the investing male, particularly in courtship-rolereversed species.
There are at least two basic strategies for investing males to assure reproductive advantage, i.e., acquire a higher opportunity to mate and ensure paternity. Studies on courtship-role-reversed species have provided good examples of these strategies: high investing katydids males through nutritional spermatophores are preferred by females (Gwynne, 1982); male scorpionflies donating larger nuptial gifts can copulate longer and thus can inject more sperm (Thornhill, 1976); and the male of the giant water bug, which cares for its offspring, repeatedly mates with the same female before each oviposition bout to ensure its paternity (Smith, 1979).
All of these mechanisms are rather apparent and observable during mating, so that special attention has been paid to them for nearly two decades. On the other hand, most insect species do not show such apparent behavior. This is also true even in some species with courtship-role-reversed mating systems. Why don’t investing males of these species show any apparent mechanism of paternity assurance or gaining more mating opportunity? Recently, studies of sexual selection have focused on the potential effects of cryptic female choice on sperm competition, even if no apparent male-male competition or female choice is observed (Thornhill, 1983, 1984; Birkhead and Mollar, 1993; Eberhard, 1996). If the female preferentially uses some sperm derived from a given male, or if the female is reluctant to mate after copulating with a male of given traits while not showing any mate preference, it is considerably harder to detect female choice or resultant sperm precedence based solely on traditional mate choice experiments. These findings imply the importance of cryptic female choice on sperm competition.
Bruchidius dorsalis (Fahraeus) is a member of the Bruchidae, some species of which are serious pests of stored crops, such as azuki, lentil and mung beans. This species feeds on seeds of a woody wild legume, Greditsia japonica Miq. The females exhibit active courtship behavior, while the males ejaculate a large amount of seminal fluid, approximately equal to 7% of the male body weight, greatly enhancing female fecundity (Takakura, 1999). Neither males nor females of B. dorsalis exhibit pre- or post-copulatory guarding of mates, and probably have an opportunity to choose their mates because the density is fairly low in the field (Takakura, unpublished). However, a reliable mechanism favoring investing males over non-investing males must exist since paternal investment can not be evolutionarily stable without such a mechanism.
This study aims to examine: 1) whether B. dorsalis females choose greater investing males as their mates on a behavioral basis, and 2) whether the amount of male investment affects the postcopulatory behavior of females, and consequently changes paternity. I examined the latter question by measuring the sperm precedence of the last male to mate (P2 value), because cryptic female choice made after copulation can not be detected otherwise. In addition, I discuss the adaptive significance of male reproductive investment both for the donating male and the receiving female, and try to figure out the evolutionary scenario of courtshiprole- reversal in B. dorsalis.