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Title: Evolution of the wave: aerodynamic and aposematic functions of butterfly wing motion

item Srygley, Robert

Submitted to: Proceedings of the Royal Society of London B
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2007
Publication Date: 4/7/2007
Citation: Srygley, R.B. 2007. Evolution of the wave: aerodynamic and aposematic functions of butterfly wing motion. Proceedings of the Royal Society of London B. 274(1612):913-917.

Interpretive Summary: Some distasteful passion-vine butterfly species mimic others in both color and the movement of their wings. How do butterflies offset the need to change their flapping styles with different flight behaviors as opposed to maintaining a constant recognizable signal? In natural settings, passion-vine butterflies cruise between habitats, search more slowly among flowers or hostplants within a habitat, flutter at evening roosting sites, or hover in courting. I found that they only flap their wings more slowly when they cruise, but a more subtle aspect of wing acceleration within the stroke remains constant through all of the behaviors. Hence mimetic butterflies wave their wings in a distinct fashion when they fly such that birds might recognize them.

Technical Abstract: Many unpalatable butterfly species use color to signal their distastefulness to birds, but motion cues may also be crucial to ward off predatory attacks. In previous research, captive passion-vine butterflies Heliconius mimetic in color pattern were also mimetic in motion. Here I investigate whether wing motion changes with the flight demands of different behaviors. If birds select for wing motion as a warning signal, aposematic butterflies should maintain wing motion independently of behavioral context. Members of one mimicry group (H. cydno and H. sapho) beat their wings more slowly and their wing strokes were more asymmetric than their sister-species (H. melpomene and H. erato, respectively), which were members of another mimicry group having a quick and steady wing motion. Within mimicry groups, wing beat frequency declined as its role in generating lift also declined in different behavioral contexts. In contrast, asymmetry of the stroke was not associated with wing beat frequency or behavioral context - strong indication that birds process and store the Fourier motion energy of butterfly wings. Although direct evidence that birds respond to subtle differences in butterfly wing motion is lacking, birds appear to generalize a motion pattern as much as they encounter members of a mimicry group in different behavioral contexts.