The Comma (Polygonia c–album) is a familiar butterfly, seen throughout most of England and Wales in the summer. Most people will have looked at the white ‘comma’ mark on the underside of the hindwing, and many will have wondered what is it for? One, slightly controversial theory, is that it is a ‘distractive’ marking; something noticeable, which draws attention our attention away from the rest of the butterfly.
“Distractive markings direct the “attention” or gaze of the receiver from traits that would give away the animal (such as the outline), or interfere with visual mechanisms that an observer could use to detect or recognize an object by virtue of the distractive markings’ high-contrast or conspicuousness”. (Stevens et al., 2013).
I must confess that I rather like this theory of distractive markings, although it is not without its critics, and some clever experiments have failed to prove it. The idea was first put forward by Abbott Handerson Thayer (1849 – 1921), an American artist and naturalist, who noticed that many animals have small, conspicuous markings which “tend to draw and hold the eye’s attention” such that we discern the animal itself less clearly. The theory is that these distractive markings may have evolved to attract the attention of a would be predator away from other details – such as the outline of the animal – that would reveal it. In other words, they are a form of camouflage (Thayer, 1909). Another lepidopteran with a possible distractive mark, is the Silver Y (Autographa gamma), with a silvery Y-shaped mark on each of its forewings (below).
The background markings on both the Comma butterfly and Silver Y moth, are fairly similar: brown and rather cryptic. Although, to my mind, the Comma has the better camouflage. It’s worth noting in passing that there is another butterfly, the Southern Comma (Polygonia egea), which has a much less prominent white mark on its wings (more of a V-shape).
Experiments carried out Martin Olofsson and colleagues in Sweden, using blue tits (Cyanistes caeruleus) as stand-in predators, showed that the presence of the white comma marked, increased survival of the butterflies. The blue tits attacked butterflies with no white mark (i.e. an obliterated comma) significantly more often than they did butterflies with intact white commas. In other words, the white
comma on a brown background had a preventive effect against bird attacks.
Unfortunately, for the theory, other researchers have concluded that conspicuous markings are more likely to impair camouflage than enhance it, and do not work as a camouflage strategy (Stevens et al., 2008, 2013). They did concede that more work is needed, but came to the conclusion that there is little evidence that high contrast markings aid concealment. But I think it is fair to say that the jury is still out. Other researchers, again using blue tits, came down on the side of the theory, i.e. that distractive features may improve camouflage (Dimitrova et al., 2009; Merilaita et al., 2013).
So let’s leave the researchers to battle it out, and turn our attention to other species which might have distractive markings. One candidate which comes to my mind, is the Chocolate pansy (Junonia iphita)(below).
The Chocolate pansy also has a prominent white mark, more of a smudge really, on its hindwing, which is placed very much at the centre of its wings, seen in profile.
On reflection, I think that this white mark may not be sufficiently bright (high-contrast) and prominent enough to function as a distractive feature. Would it draw or distract the attention of a predator away from the body outline, leading to a failure to detect the object itself? Another piece of evidence, if I can call it that, is a photo of an Oriental Chocolate Pansy that I took in Sarawak, Malaysia (below). In this image, there is a distinct peck mark right on the white spot! So instead of being a distraction – which might have prevented an attack – it appears that in this case it may be functioning as a deflection, directing the sharp beak (if that is what caused the injury) away from the body of the butterfly. But why is the mark on this butterfly an odd white smudge and not a distinctive eye-spot?
There are of course, many butterflies with high contrast features, which stand out prominently against darker wing backgrounds. I am not talking about eyespots, here, just white marks, like the one in the middle of the wing on the Palmfly (below). Is this bright enough to be distractive? I’m not sure.
Distractive markings are usually thought of as a form of crypsis because, if they work, they prevent detection. Although the distractive mark or marks themselves will be detected, the overall shape or outline of the butterfly might not be. Somehow the extreme apparency of the dot or spot, distracts the attention of the looker away from the animal itself. The distractive mark might work simply by capturing the attention so strongly, that the predator simply does not register (or recognise) the whole organism as a prey item. It just remains puzzled, or mesmerised by the white mark!
It has also been suggested that overall value of a type of camouflage such as this, is not just about whether it prevents the detection of the prey item by predators which had never seen such a thing before, but also how well the markings function in terms of preventing the predator from learning through the experience of multiple encounters (Troscianko et al., 2013).
I have always puzzled about the function of the large white, discal spot on the hindwing of Clouded yellows (Colias croceus). Is this a distractive feature?
Here is a photo (below) of a Lemon Emigrant (Catopsilia pomona pomona) form hilaria, which has a much smaller white shot than a Clouded yellow. In this case, there is a tear from the edge of the wing to the spot; damage from a predator?
I’ll finish with one more definition to puzzle over!
“distractive markings are markings that through (and despite) their relative salience compared to the rest of the coloration or morphology of an animal make it more difficult for a viewer to perceive the characteristics useful for detection or recognition of the animal, hence increasing its net camouflage” (Merilaita et al., 2013)
References and further reading
Dimitrova, M., Stobbe, N., Schaefer, H. M., & Merilaita, S. (2009). Concealed by conspicuousness: distractive prey markings and backgrounds. Proceedings of the Royal Society of London B: Biological Sciences, rspb-2009.
Fraser, S., Callahan, A., Klassen, D., & Sherratt, T. N. (2007). Empirical tests of the role of disruptive coloration in reducing detectability. Proceedings of the Royal Society of London B: Biological Sciences, 274(1615), 1325-1331.
Merilaita, S., Schaefer, H. M., & Dimitrova, M. (2013). What is camouflage through distractive markings?. Behavioral Ecology, 24(5), e1271-e1272.
Olofsson, M., Dimitrova, M., & Wiklund, C. (2013). The white ‘comma’ as a distractive mark on the wings of comma butterflies. Animal behaviour, 86(6), 1325-1331.
Quicke, D. L. (2017). Mimicry, crypsis, masquerade and other adaptive resemblances. John Wiley & Sons.
Stevens, M., Graham, J., Winney, I. S., & Cantor, A. (2008). Testing Thayer’s hypothesis: can camouflage work by distraction?. Biology Letters, 4(6), 648-650.
Stevens, M., Marshall, K. L., Troscianko, J., Finlay, S., Burnand, D., & Chadwick, S. L. (2012). Revealed by conspicuousness: distractive markings reduce camouflage. Behavioral Ecology, 24(1), 213-222.
Stevens, M., Troscianko, J., Marshall, K. L., & Finlay, S. (2013). What is camouflage through distractive markings? A reply to Merilaita et al.(2013). Behavioral Ecology, 24(5), e1272-e1273.
Stevens, M., Winney, I. S., Cantor, A., & Graham, J. (2008). Outline and surface disruption in animal camouflage. Proceedings of the Royal Society B: Biological Sciences, 276(1657), 781-786.
Thayer, GH. (1909). Concealing-coloration in the animal kingdom: an exposition of the laws of disguise through color and pattern: being a summary of Abbott H. Thayer’s discoveries. New York (NY): Macmillan.
Troscianko, J., Lown, A. E., Hughes, A. E., & Stevens, M. (2013). Defeating crypsis: detection and learning of camouflage strategies. PloS one, 8(9), e73733.
Really interesting – thanks