CANCELLED - Does butterfly wing morphology change after 40 years without bird predation?

Background/Question/Methods

Decline of vertebrate predators should, in theory, lead to relaxed selection and potentially rapid adaptation of prey. We focus on a predator-prey interaction between birds and butterflies. Birds are a major predator of butterflies, and are thought to influence traits such as wing shape and size since butterflies use flight to escape bird predators. Toxic butterflies use more gliding flight and have elongated forewings and larger body size than nontoxic butterflies which have shorter stubbier wings that facilitate evasive flight behavior to escape birds. Birds have been functionally extirpated from the island of Guam for over 40 years due to the invasive brown treesnake, releasing butterfly populations on Guam from nearly all avian predation. In contrast, butterfly populations in the adjacent Commonwealth of Northern Mariana Islands (CNMI) regularly experience bird predation. In our study we compare forewing and hindwing shape and size in two toxic and one nontoxic butterfly using landmark based geometric morphometrics. We conducted a cross island comparison (Guam v. CNMI) in two non-toxic species (Hypolimnas anomala and Melanitis leda), and one toxic species (Euploea eunice) from 1990-2021. In addition, we compared H. anomala morphology before and after birds were lost on Guam.

Results/Conclusions

We found that forewings of non-toxic butterflies differ in shape between Guam, without birds, and islands with birds whereas the shape and size of toxic butterflies have not changed between islands or over time. The shape of non-toxic butterfly hindwings are changing on both islands but in slightly different directions over time. In the CNMI, H.anomala forewing shape has barely changed whereas forewings of butterflies on Guam have changed much more over time. Interestingly, both hindwings and forewings of H. anomala are getting larger over time on all islands. These results shed light on the impact of birds on butterfly wing morphology, but the next steps must link changes in morphology to changes in flight.Furthermore, this study demonstrates the value of accidental experiments caused by global change for testing questions about the influence of relaxed selection on evolution.