Catch me if you can: wild Morpho butterflies trade speed for erraticity in escape flight

Interactions with predators shape the evolution of escape behaviors in prey. Prey may exhibit evasive movements enhancing the unpredictability in their fleeing directions and trajectories, referred to as protean motion. Such behaviors may enhance survival of flying prey in the wild but quantifying them under natural conditions remains challenging. Here we used stereoscopic high-speed videography to record the escape flight behavior of wild males of the butterfly species Morpho menelaus in the Amazonian rainforest, and reconstructed 3D flight trajectories using artificial-neural-network-based tracking. During the experiments, we used a blue iridescent lure to attract freely patrolling male butterflies and elicited escape flights by performing an upward-directed entomological net swing. We then compared escape flight kinematics with pre-attack approach flight, using a measure of erraticity to quantify escape unpredictability for the first time in wild flying insects. Attacks first induced a rapid upward flight maneuver, directly followed by a horizontal turn, leading to a large range of escape directions. The following escape flight trajectories showed increased horizontal erraticity as compared to the pre-attack flight. The mean speed decreased during the escape phase, notably in the horizontal plane: a significant negative association between horizontal trajectory complexity and flight speed was indeed detected, consistent with a speed-erraticity trade-off. Because these large and relatively slow-flying butterflies display bright iridescent blue coloration on their dorsal wing side, erraticity during escape flight might enhance the dynamic flash coloration, likely limiting accurate targeting by predators.