Evolution of female ornamentation in dance flies: valuable gifts are worth dressing up for.

Elaborate female ornaments are rare in nature. One explanation for this is that female investment in ornamentation may take away crucial resources from other costly life history traits, such as fecundity, for which there is likely to be a higher fitness return. However, this trade-off between ornaments and fecundity may be less severe in species where the males offer the female an edible nuptial gift during mating. The nutrition gained from mating may make attracting mates with elaborate ornaments more cost-effective for the female. We investigated this link in dance flies in which there is large variation in nuptial gifts, as well as female ornaments. Our phylogenetic analysis showed that nuptial gift value is positively associated with the evolution of female ornaments. We found that species which lack nuptial gifts have no ornaments, and high levels of female ornamentation has evolved most frequently in species with a reliable access to an edible nuptial gift with each mating. Our results also suggest that female ornaments have most likely evolved following the evolution of nuptial gifts. We argue that the added benefits from each mating have helped the females to overcome the costs associated with the development and maintenance of ornaments.