Functional significance of elaborate secondary sexual traits and their evolution in the water strider genusRheumatobates

Abstract

AbstractSexual conflict may drive the evolutionary elaboration of sexually antagonistic traits that function in mating interactions. One of the most striking cases of elaboration of male morphology occurs in the water strider genusRheumatobatesBergroth (Hemiptera: Gerridae). The functional significance of the bizarre modifications of appendages in this group is not known. Here we focus on one of the more elaborate of the species,R. rileyiBergroth. We conduct observational and experimental studies aimed at determining the general sequence of mating behaviour, the role of females in the outcome of sexual interactions, and the functional significance of the highly modified appendages of males. We also map these traits on a known phylogeny of the genus to determine their pattern of evolution. Males repeatedly harass females and females respond with evasive skating or, if the male successfully grasps her, with a premating struggle. The dynamics of the struggle determine the success of mating attempts. Short struggles typically lead to mating, and long struggles typically result in disengagement of the pair. Following a short period of copulation, males withdraw their genitalia and dismount. Females that have been isolated from males for a period of time become less reluctant to mate. During the premating struggle, the antennae of males are used to grasp the females around the head, and the rear legs are used to lift the females' rear legs off the water surface. Neither antennae nor rear legs are used during copulation, thus they are not used for copulatory courtship. Mapping of these traits on the phylogeny indicates multiple independent origins and a pattern of escalation (16 origins, 7 losses). We conclude that these bizarre traits of males are sexually antagonistic and have evolved repeatedly in the genus.