Sexual Selection and Viability in the Wild: the Evolution of Sex Differences

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Sexual selection explains the evolution of sex differences such as ornamentation and sexually-competitive behaviour. Although important research investigating this process uses lab model organisms, there are some aspects that can only be fully addressed in the wild. These include tests of a central issue about sexual selection: is it opposed (Darwin 1871) or in the same direction as viability selection? Our research addresses this, and the factors thought to control sex differences, by using novel insect mating systems with sexual selection on females in competition for valuable donations from males. In two systems females possess "male-like" ornaments.******We propose to use formal selection analyses and field experiments to measure viability and sexual selection on female traits in the wild using: (a) female Rhamphomyia dance flies that risk predation by spiders when displaying ornaments that attract males bearing prey gifts (the only source of protein for females); (b) Female Oecanthus tree crickets at greater risk than males from wasp parasitoids when moving, as when approaching their mates to obtain valuable gift secretions. Additionally, behavioural observations and experiments will test the prediction that male choice in Oecanthus causes sexual selection on females. System (c) involves many species of Hemiandrus weta (related to crickets) where males deliver to ornamented females `gift' meals along with a sperm package. ******Sexual and viability selection will be estimated in the same time frame by comparing both mating success and mortality from predation. For the flies and crickets, we will analyze sperm stores of fresh-prey females compared to survivors using newly developed microsatellite DNA variation to determine lifetime female mating frequency and thus sexual selection on traits. A Darwinian prediction is that highly-ornamented, frequently mating females run a greater risk of predation. For viability selection we compare traits of our prey females to those of survivors. We will also estimate lifespans of females by marking and recapturing. As selection analyses on unmanipulated natural populations are correlative, sampling of prey and survivors will be followed by field experiments using established methods of releasing dance fly females with differing levels of trait expression near webs mounted in frames. Study system (c) involves a speciose weta genus, Hemiandrus. In addition to selection analyses, a molecular phylogeny developed by collaborators will allow a comparative approach to sexual selection and the evolution of female ornaments. Some species exhibit maternal care and some, abdominal ornamentation. Frequent female mating (gift acquisition) in species with maternal care is hypothesized to promote survival through a long period of starvation while caring. This predicts a phylogenetic association of ornaments with maternal care.**