SR: The Adaptiveness of Defence Behaviour Flashcards
Peacock butterfly defence
Vallin et al
2005
Many butterflies and moths, including the peacock butterfly Inachis io, have large eyespots on their wings, which they expose suddenly to predators by opening the wings, or in moths by moving the front pair of wings forward. The insect may also move its whole body to better present the eyespots to the predator. The eyespots on the peacock butterfly are on the upper surface of both pairs of wings. The lower surfaces are cryptic.
When resting with its wings closed the butterfly resembles a dead leaf. In addition, the peacock butterfly can make a hissing sound by rubbing the lower edges of its forewings against the hind wings. This study investigates whether the eyespots and hissing sound protect the butterfly from predation by a common insectivorous bird, the blue tit Parus caeruleus, which lives in the same countries as the peacock.
This paper has a relatively long introduction that provides useful background to several aspects of the study. After a very general opening paragraph it provides some information on the peacock butterfly, and then describes some previous research by Blest investigating the role of the peacock’s eyespots in reducing predation by birds. There is some fair criticism of Blest’s study, which also provides a rationale for the current study. That is, they present the case that there is a need for a better study.
The introduction also points out that two forms of defence (hissing and eyespots) may be better than one and that there needs to be a study that determines if these two forms of defence work together in deterring predators. The final paragraph states what this study sets out to do. That is, to determine the defensive effect of eyespots and hissing separately and in unison.
The methods are quite straightforward and clearly described. One thing of note is the large number of birds needed, as each bird was only given one butterfly. The birds were not harmed. It was probably a great deal of work to catch and keep all these birds, and also required a permit. By contrast, no permit was needed to work with the butterflies and it is relatively easy to obtain many peacock butterflies by collecting larvae on stinging nettles and rearing them to adulthood. Then they waited until late winter to do the experiment. This was presumably because they wanted to study bird predation during the over-wintering period of the butterfly when it would be in a cavity such as a hollow tree where a bird might find it.
The main thing to note is how the six different treatments of butterflies were set up. Eyespots could be removed by using a black marker pen. The control for this was marking parts of the wing without eyespots. The ability to hiss could be removed by cutting away the lower edge of the forewing. The control for this was to cut away the lower edge of the hind wing, which is not involved in making the hissing sound. Peacock butterflies are edible and the butterflies were presented to the blue tit predators in a small room with natural light. The bird invariably located the butterfly. Each bird was only ever given one butterfly. Each butterfly was only presented to one bird. In all, they studied 54 presentations.
The main results are shown in table 1, which shows the three main comparisons made. The first compared butterflies without eyespots (inked out with a black marker pen) with controls in which marker pen had inked out another part of the wings. This comparison showed that eyespots significantly reduce predation risk, with 9/9 surviving versus 5/10 without eyespots. This difference in survival probability is significant (p = 0.03) using a Fisher’s exact test. (This is similar to a chi-square test.) The second compared butterflies with normal eyespots that could not make the hissing sound because the lower edge of the forewing had been cut away (8/8 survived) with those that could still hiss because it was the lower edge of the hind wing that had been cut away (7/8 survived). The difference is non-significant (p = 1.0). What little difference there was, was for slightly higher survival in the butterflies that could not hiss. The third comparison was between butterflies with eyespots and the ability to hiss (survival 9/9), with butterflies without eyespots that could not hiss (survival 2/10).
Overall, the data show that the eyespots greatly reduce predation risk but that hissing has no effect, at least against blue tits. Only one out of 34 (3%) of the butterflies with eyespots was killed whereas 13/20 (65%) of those without eyespots were killed. Another statistical analysis was carried out using GLIM (generalized linear model). This allows all the data to be put into a single analysis, rather than make three separate tests each on part of the data as above. GLIM gives the same conclusions: a significant effect of eyespots, no effect of hissing, no interaction between eyespots and hissing.
The results in Figure 2 are of minor importance and need not concern us here. Figure 3 shows that the butterfly will open its wings to an approaching bird at a greater distance on the second encounter than on the first.
This is in accord with the hypothesis in the introduction that on first encounter a butterfly may wait until the bird is close, relying initially on camouflage for defence. On the second encounter the camouflage has been overcome and the butterfly opens its wings sooner.
The discussion is straightforward. The authors make the interesting comment that a bird seemed visibly disturbed by the peacock’s eyespots. Presumably, blue tits have an inbuilt fear of large vertebrate predators and the peacock is in some sense tapping into this. It may be hard for a blue tit to switch off its fearful response to stimuli that normally would be associated with danger.
Hissing appears to have no effect on blue tits, and even if it did reduce predation the eyespots are already very effective, alone reducing predation to just 3%. Why bother with hissing at all? Maybe it acts as a defence to other predators, or predators foraging in the dark. In addition, it probably costs nothing to hiss so only a small benefit would be required for hissing to be favoured by natural selection.
