Being long-lived insects that overwinter in Britain and other cold places, peacock butterflies are popular food items for hungry overwintering birds, particularly blue tits, Cyanistes caeruleus. So these insects have evolved a number of ways to reduce their likelihood of being eaten by predators. First, the hibernating butterfly sits immobile amongst a pile of dead leaves with its wings closed. Since its wings strongly resemble dead leaves, this insect is very difficult to see. Second, when peacock butterflies are disturbed, they flip their wings open so their dramatic eyespots are suddenly revealed. For obvious reasons, this is usually referred to as the “startle display”, and studies have shown that eyespots on butterfly wings are very effective at intimidating would-be predators. But as you noted in the above video, peacock butterflies have evolved a third line of defense: hissing. Why?
A group of Swedish scientists designed a study to examine the peacock butterfly’s eyespots and hissing to determine whether they both truly are intimidating to wild blue tits [doi:10.1098/rspb.2004.3034]. To do this, they manipulated six groups of live captive-reared peacock butterflies, three experimental groups with their control groups, and presented these insects to wild-caught blue tits.
The groups of peacock butterflies were as follows:
- butterflies with eyespots painted over (figure 1a) and their controls (figure 1d) painted on another part of the wing
- butterflies with their sound production capacity removed (figure 1b) (small part of wings removed) and their controls (figure 1e; with a non-sound producing part of the wing removed)
- butterflies with eyespots painted over and sound production aborted (figure 1c) and their controls (figure 1f)
as you see in this diagram (figure 1; larger view):
Figure 1. The six different morphs produced by a black marker pen and scissors. Upper row: experimental morphs. Lower row: control morphs. Upper row: (a) no eyespots, (b) no sound, (c) no eyespots or sound, (d) eyespots, (e) sound, (f) eyespots and sound. [doi:10.1098/rspb.2004.3034]
The team found that eyespots alone, or in combination with sound, were an effective defence against blue tits; only 1 out of 34 butterflies with intact eyespots was killed, whereas 13 out of 20 butterflies without eyespots were killed. Since the blue tits ate the killed peacocks, the scientists inferred that the butterflies were rather tasty.
Since the non-hissing peacock butterflies were nearly as good at defending themselves against hungry birds, three of the original four scientists decided that hissing may work better as a defence against nocturnal predators — mice and other small rodents. They designed a series of studies and found that, even in total darkness, peacock butterflies’ wing-flicking has an anti-predator function against mice; 75% of the mice ran away, but mice ran further away from butterflies that could also hiss. For this reason, the Swedish team suggests that intense predation pressure from rodents during the butterflies’ hibernation has been the driver of the evolution of hissing in the peacock butterfly [doi:10.1007/s00265-011-1268-1]. Isn’t evolution interesting?
Because these are such gorgeous butterflies, and also because a beautiful HD video exists, I have to share this second video with you, which shows a peacock butterfly emerging from its chrysalis:
Vallin A., Jakobsson S., Lind J. & Wiklund C. (2005). Prey survival by predator intimidation: an experimental study of peacock butterfly defence against blue tits, Proceedings of the Royal Society B: Biological Sciences, 272 (1569) 1203-1207. doi:10.1098/rspb.2004.3034
Olofsson M., Jakobsson S. & Wiklund C. (2012). Auditory defence in the peacock butterfly (Inachis io) against mice (Apodemus flavicollis and A. sylvaticus), Behavioral Ecology and Sociobiology, 66 (2) 209-215. doi:10.1007/s00265-011-1268-1
Many thanks also to @EricRietzschel for taking the time to download and email a PDF that I requested on this gloomy Saturday morning.
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