Deceived, but not betrayed: static allometry suggests female ornaments in the long-tailed dance fly (Rhamphomyia longicauda) exaggerate condition to males

Abstract

Despite their prevalence in nature, the evolution of sex-specific female ornaments is still not well understood. Although in some cases (often carotenoid-based ornaments) they appear to honestly signal quality, such as fecundity, it has been suggested that some female ornaments evolved to deceptively obtain matings. One such case is the long-tailed dance fly (Rhamphomyia longicauda) where females possess two sex-specific ornaments: pinnate scales on the hind femur and tibia and abdominal sacs that are inflated in female-biased “display” swarms. Because females rely on male nuptial food gifts to initiate and sustain egg development, female ornaments are thought to have evolved in the context of deceiving males to obtain gifts. For males, the costs of being deceived may be reduced if female ornaments on average provide valuable information about female quality such as fecundity to males. Here, we use static allometry (with body size as a proxy for condition) of both ornamental and non-ornamental traits in females (and homologous non-ornamental traits in males) in order to determine whether they indicate condition to males. Most male traits scaled isometrically with body size, however, as often expected for sexually selected traits, female ornaments (abdomen area and tibia scale length) showed significant positive allometry and had steep slopes relative to non-ornamental traits. In addition, male leg hairs (homologous with female scales) showed positive static allometry, probably because they are involved in nuptial-prey capture or in grasping mates. As larger females invest more in ornamentation relative to smaller females, their ornaments may exaggerate differences in female condition and thus inform male mating decisions.