The handicap hypothesis of sexual selection predicts that sexual ornaments have evolved heightened condition-dependent expression. The prediction has only recently been subject to experimental investigation. Many of the experiments are of limited value as they: (i) fail to compare condition dependence in sexual ornaments with suitable non-sexual trait controls; (ii) do not adequately account for body size variation; and (iii) typically consider no stress and extreme stress manipulations rather than a range of stresses similar to those experienced in nature. There is also a dearth of experimental studies investigating the genetic basis of condition dependence. Despite the common claim that sexual ornaments are conditiondependent, the unexpected conclusion from our literature review is that there is little support from welldesigned experiments.

The last decade has witnessed considerable theoretical and empirical investigation of how male sexual ornaments evolve. This strong male-biased perspective has resulted in the relative neglect of variation in female mate preferences and its consequences for ornament evolution. As sexual selection is a co-evolutionary process between males and females, ignoring variation in females overlooks a key aspect of this process. Here, we review the empirical evidence that female mate preferences, like male ornaments, are condition dependent. We show accumulating support for the hypothesis that high quality females show the strongest mate preference. Nonetheless, this is still an infant field, and we highlight areas in need of more research, both theoretical and empirical. We also examine some of the wider implications of condition-dependent mating decisions and their effect on the strength of sexual selection.

We used the stalk-eyed fly Cyrtodiopsis dalmanni to examine predictions made by condition-dependent handicap models of sexual selection. Condition was experimentally varied by manipulation of larval food availability. Cyrtodiopsis dalmanni is a highly dimorphic species exhibiting strong sexual selection, and the male sexual ornament (exaggerated eyespan) showed strong condition-dependent expression relative to the homologous trait in females and nonsexual traits. Male eyespan also showed a great increase in standardized variance under stress, unlike nonsexual traits. The inflated variance of the male ornament was primarily attributable to condition-dependent (but body-size-independent) increase in variance. Thus, evaluation of male eyespan allows females to gain additional information about male condition over and above that given by body size. These findings accord well with condition-dependent handicap models of sexual selection.

When sex determination in a species is predominantly genetic but environmentally reversible, exposure to (anthropogenic) changes in the environment can lead to shifts in a population's sex ratio. Such scenarios may be common in many fishes and amphibians, yet their ramifications remain largely unexplored.We used a simple model to study the (short-term)

No abstract

Handicap models of sexual selection propose that females use male sexual ornaments as a cue in mate choice because they reflect commodities that increase female fitness, either directly or indirectly. In contrast to studies on vertebrates, most investigations of ornaments in insects and other invertebrate taxa have been conducted under laboratory conditions. There is a pressing need to address questions relating to sexual signalling of quality in natural populations, as the arbitrary and uniform environments found in the laboratory fail to reflect the world under which animals have evolved. We investigated associations between male ornaments (exaggerated eyespan), attractiveness, and reproductive quality in a wild population of the sexually ornamented stalk-eyed fly, Teleopsis dalmanni. We also explored the relationship between eyespan and reproductive quality in females to evaluate the potential for sexually antagonistic selection on eyespan. We show that eyespan is a generic correlate of reproductive quality, acting as a reliable mirror of variation in reproductive fitness in both sexes. Our findings suggest that male ornaments signal commodities that are of interest to females in the natural environment in which they, and mate preferences for them, have evolved. In addition, the covariance between female eyespan and reproductive output suggests that the former may be a reliable cue of quality in its own right. Our data provide important insights into the evolutionary forces that shape the evolution of exaggerated eyespan in wild populations of this species.

No abstract

This study provides the first direct evidence from wild populations of stalk-eyed flies to support the hypothesis that male eyespan is a signal of meiotic drive. Several stalk-eyed fly species are known to exhibit X-linked meiotic drive. A recent quantitative trait locus analysis in Teleopsis dalmanni found a potential link between variation in male eyespan, a sexually selected ornamental trait, and the presence of meiotic drive. This was based on laboratory populations subject to artificial selection for male eyespan. In this study, we examined the association between microsatellite markers and levels of sex ratio bias (meiotic drive) in 12 wild T. dalmanni populations. We collected two data sets: (a) brood sex ratios of wild-caught males mated to standard laboratory females and (b) variation in a range of phenotypic traits associated with reproductive success of wild-caught males and females. In each case, we typed individuals for eight X-linked microsatellite markers, including several that previously were shown to be associated with male eyespan and meiotic drive. We found that one microsatellite marker was very strongly associated with meiotic drive, whereas a second showed a weaker association. We also found that, using both independent data sets, meiotic drive was strongly associated with male eyespan, with smaller eyespan males being associated with more female-biased broods. These results suggest that mate preference for exaggerated male eyespan allows females to avoid mating with males carrying the meiotic drive gene and is thus a potential mechanism for the maintenance and evolution of female mate preference.