The Signalling Function Of Eyespan In Stalk-Eyed Flies (Diptera: Diopsidae)

Genetic models of the handicap theory of sexual selection propose that ornaments signal heritable male quality, so females mating with the most ornamented males acquire fitness benefits for their offspring. Male ornaments are predicted to have co­ evolved with female preference to be larger, and so more costly. The key prediction made by the handicap hypothesis is that male sexual traits have evolved heightened condition dependence, a result of the higher differential cost of ornaments relative to other traits. I investigated evidence for condition-dependent sexual ornaments and found little support from well-designed experiments. Most studies had neglected to 1) compare condition dependence in sexual traits with suitable non-sexual controls, 2) adequately account for body size variation, and 3) assess individuals under a range of stresses representative of those experienced in nature. There was also a dearth of experimental studies exploring the genetic basis of condition dependence. I used experiments with stalk-eyed flies to examine predictions made by condition-dependent handicap models of sexual selection. Cyrtodiopsis dalmanni is highly sexually dimorphic for eyespan, and females exhibit strong mating preferences for males with large eyespans. Condition was varied experimentally by manipulating larval food availability. I found that male eyespan was more sensitive to changes in condition than female eyespan and other non-sexual traits. Male eyespan also showed a great increase in standardized phenotypic variance under stress, unlike non-sexual traits. These patterns persisted before and after controlling for body size. In contrast, there was no heightened condition dependence of male eyespan in Sphyracephala