Winter predation on two species of hibernating butterflies: monitoring rodent attacks with infrared cameras

Documentation of predator attacks in nature is important for understanding how specific antipredator defences have evolved, but previous accounts have been mostly anecdotal. Therefore, we monitored predation on two closely related butterfly species, Aglais urticae and Inachis io, during winter hibernation. Butterflies were placed singly close to the floor on walls in dark, seminatural hibernation sites (e.g. unheated outhouses). We used motion-initiated infrared-sensitive cameras to record predator attacks on the butterflies. The antipredator attributes of the two species have two characteristics: during rest the butterflies reduce predators’ attention by mimicking leaves but they can suddenly change their guise by repeatedly flicking their wings. The wing flicking also produces hissing sounds and ultrasonic clicks and, furthermore, I. io, but not A. urticae, have large eyespots on the dorsal wing surface. The two butterfly species suffer from mouse predation during the winter and mice have been suggested as potential targets for the butterflies’ sound production. Results showed that (1) mice (Apodemus spp.) were important predators on butterflies, (2) I. io often survived attacks by wing-flicking behaviour, and (3) both species moved to less accessible positions after interactions with mice and other small mammalian predators, but I. io more often so. The successful predator evasion in darkness by I. io suggests a multimodal defence; in addition to the large eyespots, which intimidate birds, we suggest that the hissing and/or click sounds produced during wing flicking may have evolved as defence against rodent attacks.