Sending mixed signals: convergent iridescence and divergent chemical signals in sympatric sister-species of Amazonian butterflies

The evolution of traits in closely related species living in sympatry strongly depends on both shared selective pressures and reproductive interference. In closely-related Morpho butterfly species living in the understory of the neo-tropical rainforest, the blue iridescent coloration of the wings is likely involved in predation evasion, as well as in mate recognition and courtship. We used spectrophotometry, behavioral experiments, visual modeling and chemical analyses to characterize the evolution of visual and chemical traits in two closely-related species, Morpho helenor and Morpho achilles. We specifically compared trait variation between samples from allopatric and sympatric populations of M. helenor to test the effect of ecological interactions with M. achilles on trait evolution. We quantified the differences in wing iridescence and tested for variations in the sexual preference for this trait. We found a strong similarity in iridescence between M. helenor and M. achilles in sympatry while the iridescence of M. helenor diverged in allopatry, suggesting that predation favors local resemblance. Although intraspecific behavioral experiments showed that iridescent signals could be used as visual cues during intraspecific mate choice, the strong resemblance of the iridescent signals between species may impair these species visual recognition. In contrast, the divergent chemical bouquets detected between species suggest that the visual similarity of sympatric Morpho species may have favored the divergence of alternative traits involved in species recognition, such as chemical cues.