Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

Kipp C. Kruse


Sexual selection theory predicts that in the few species where males make a larger parental investment than females and limit female reproduction, females should court and compete for males (i.e. we should observe "sex-role reversal"--Trivers, 1972). These predictions were tested in the laboratory with the giant waterbug Belostoma flumineum Say (Hemiptera: Belostomatidae) . Males of this species characteristically carry and brood eggs on their dorsa, thus making a substantial parental investment.

The courtship sequence in "one male:one female" pairings began with the male "pumping" (rapidly raising and lowering his abdomen at the water's surface). This presumably attracted the female, who then moved to and contacted the male. Placing his hind leg over the female's back, the male positioned the female and copulation occurred. Oviposition invariably followed. After the female deposited approximately four eggs, the male forced the female back to copulate again. This cycle continued virtually uninterrupted until the male was approximately 80% encumbered (x̄=102 eggs) .

"Two male: one female" trios were set up to simulate a situation in which access to females limited male reproduction. The same general courtship sequence was observed, with minor variations. The rate of male pumping (i.e. pumps per minute) was significantly higher than in any other sex ratio situation observed. Oviposition frequently did not follow copulation. Both components of sexual selection appeared to be observed: 1) intrasexual selection was seen in males "striking" at each other, and 2) intersexual selection (i.e. courtship competition) was observed in male pumping displays.

A situation in which access to males limited female reproduction was simulated in "one male:two female" trios. Again, the same general courtship sequence was followed with minor variations. The rate of male pumping did not differ significantly from "one male: one female" pairings. Oviposition frequently did not follow copulation. Intrasexual selection was observed in the form of a "pushing" behavior, where one female would attempt to push another female off of the male's back. However, no intersexual selection (i.e. no female courtship) was seen. Thus what is observed may be "partial" sex-role reversal. Sex roles may rely as much on proximate circumstances as on pre-programmed genetic behaviors.

In order to assess preference for or dominance of partially-encumbered males and heavily-gravid females, additional trios were set up. Partially-encumbered males paired with unencumbered males and females did not breed significantly more than the unencumbered males. Heavily-gravid females paired with less-gravid females and males did breed significantly more than the less-gravid females. The manner in which these results relate to sexual selection theory are discussed.