Stabilizing sexual selection for female ornaments in a dance fly

Abstract

Ornamental traits function by improving attractiveness and are generally presumed to experience directional selection for mating success. However, given the greater investment of females in offspring than males, female-specific ornaments can in theory signal fecundity yet be constrained by fecundity costs. Theoretical work predicts that such constraints can lead to stabilizing selection via male choice for intermediately ornamented females. Female dance flies Rhamphomyia longicauda (Diptera: Empididae) display two female-specific ornaments in mating swarms - inflatable abdominal sacs and pinnate tibial scales. We investigated the intensity and form of sexual selection on female traits including ornaments and found no evidence for directional sexual selection. Instead, we found marginally nonsignificant quadratic selection for all three measures of ornament expression. Canonical analysis confirmed that the strongest vectors of nonlinear selection were associated with ornamental traits, although the significance of the quadratic coefficients associated with these vectors depended on the statistical approach. Direct Mitchell-Olds and Shaw tests for the location of the maximum fitted fitness value for both raw morphological traits and canonical axes revealed only one marginally nonsignificant result for the multivariate axis loading most heavily on pinnate leg scales. Together, these results provide the first tentative support for stabilizing selection on female-specific ornaments

The role of functional constraints in nonrandom mating patterns for a dance fly with female ornaments

“…Funk & Tallamy, ; LeBas et al., ), Wheeler et al. () showed stabilizing selection on ornaments with intermediately adorned females having the highest mating success. This apparent inconsistency could have several causes, including constraints on the operation of male choice for elaborate (over‐) ornamentation by females (Chenoweth, Doughty, & Kokko, ; Fitzpatrick, Berglund, & Rosenqvist, ; Herridge, Murray, Gwynne, & Bussiere, ; Murray et al., ).…”

“…In this subfamily (the Empidinae) not only is mating almost always accompanied by the presentation of nuptial gifts (Cumming, ), but adult females are not known to hunt, and must obtain their dietary protein from nuptial gifts (Hunter & Bussiere, ). Presumably as a consequence, females in many species compete intensely for the food gifts provided by males, and females have evolved a series of remarkable secondary sex characters that appear to improve their attractiveness to males (Collin, ; Cumming, ; Funk & Tallamy, ; LeBas, Hockham, & Ritchie, ; Murray, Wheeler, Gwynne, & Bussiere, ; Wheeler, Gwynne, & Bussière, ). In spite of the strong predicted covariance between sexual trait expression and mating success, studies of sexual selection on female dance flies report remarkable variation in the form and strength of selection (Bussière, Gwynne, & Brooks, ; Funk & Tallamy, ; LeBas et al., ; Sadowski, Moore, & Brodie, ; Wheeler et al., ).…”

“…Rhamphomyia longicauda display several elaborate female‐specific ornaments, which remain perplexing both in terms of their evolutionary origin (e.g. see Cumming, ; Funk & Tallamy, ) and for their relationship with mating success (Wheeler et al., ). Rhamphomyia longicauda forms mating swarms at dusk and dawn in which females display two sex‐specific ornaments: pinnate leg scales and inflatable abdominal sacs (Funk & Tallamy, ; Newkirk, ).…”

A novel method of comparing mating success and survival reveals similar sexual and viability selection for mobility traits in female tree crickets

“…Male preference may still help to shape sexual selection on females: males are expected to avoid mating with females that over‐invest in sexually selected traits (Fitzpatrick et al ., ; Chenoweth et al ., ), and this phenomenon may be responsible for the convex shape we found in both of the composite traits under a significant sexual selection. Similar convex or stabilizing sexual selection on females has been observed in field crickets ( Teleogryllus oceanicus ) (Thomas & Simmons, ), dance flies ( Rhamphomyia longicauda ) (Wheeler et al ., ), Mormon crickets (Robson & Gwynne, ) and fruit flies ( Drosophila serrata ) (Chenoweth et al ., ; Rundle & Chenoweth, ).…”

“…This similarity in selection is generally expected because females are not likely to develop sexually selected traits that incur viability costs (Fitzpatrick et al ., ), because females generally increase fitness more by increasing longevity than by attracting more mates (Bateman, ). As such, the relationship between viability and sexual selection is not expected to be opposing, as noted in two other species: the tibial scale ornaments of displaying female Rhamphomyia longicauda dance flies are under stabilizing sexual selection (Wheeler et al ., ), and positive directional viability selection from spider predators (Gwynne et al ., ); in pied flycatchers, where females expressing the ornament of a forehead patch had higher lifetime fecundity, higher survival (Potti et al ., ) and better health (Morales et al ., ). There was a possibility in female O. nigricornis that sexually selected traits may have carried viability costs because the risk of predation associated with those traits may have been offset by the fitness benefits of receiving more nuptial gifts, but this does not appear to be the case.…”