Scaling and developmental integration of a secondary sexual sensory trait

Sexually dimorphic traits evolve due to differences in selective pressures on males and females. However, the scaling and covariation of traits that underlie sexual dimorphism varies considerably among species, and many questions remain about how these patterns are shaped by trait function. In particular, in contrast to weapon and display traits, sexual dimorphism, allometry and developmental integration of secondary sexual traits with sensory roles is rarely studied. We investigated the antennae of Leptotarsus costalis, which are ~ 5-fold longer than the body in males and ~ 36-fold longer in males relative to females. We found that antenna exaggeration in males results from a shift in the allometric intercept, rather than a difference in the allometric slope, between males and females. We also found evidence of greatly reduced developmental integration of the antenna with other morphological traits in males but not in females. In addition, we observed both micro- and macrostructural differences between male and female antennae that may relate to the distinct functions of the antennae in the sexes. Our findings show that a secondary sexual trait can be massively exaggerated without positive allometry, and perhaps without strong developmental trade-offs with other morphological traits.