Popular press coverage of entomological news: Shimmering wings dazzle boy butterflies www.cbc.ca/storyview/cbc/2003/05/02/butterflies/030502 Bio3323 Entomology Professor J. Houseman February 8, 2004
Butterflies are among the most beautifully adorned creatures in nature, and as a result, have received great admiration from biologists and non-biologists alike. Appreciated for their beautiful colors and patterns, butterfly wings are even more amazing than they appear to the human eyes. In the May 5 th, 2003 news article Shimmering wings dazzle boy butterflies, staff members at cbc.ca reported that the iridescent or shimmering effect of some female butterfly wings lures males in. The article follows from a study published in the scientific journal Nature, by a team of biologists led by Alison Sweeney of Duke University in Durham, North Carolina. The study suggests that it is not just the color of a butterfly s wings that attracts a mate; the wings of some butterflies reflect polarized light and this optical feature may also capture the attention of male suitors (Sweeney et al, 2003). Sweeney and her colleagues found that polarized light bouncing off iridescent wings of female Heliconius cydno butterflies functions as a mating signal. In order to evaluate whether the scientific information presented in the popular press article is accurate, a comparison between the CBC news article and peer reviewed scientific articles was completed and the results are presented in the following paper. Mate searching Mate locating strategies in male butterflies are diverse across species. In some species, males look for females over a large area, while in other species, males perch and wait for females. (Rutowski, 1991). Males often search for females using predominately visual cues but close range courtship involving pheromonal and visual communication also occurs. (Jiggins et al, 2004). This is one point that the article neglects to make as it discusses the wings of butterflies as visual cues. Ringo states in his paper Sexual 2
receptivity in insects that for mating to occur, there has to be an exchange of signals between the female and male. The signals not only involve sexual displays, which he refers to as long range signals but also courtship or close range signals (Ringo,1996). The colorful, shimmering wings that are discussed in this popular press article are just one example amongst many, of sexual advertisement. The article goes on to say that this could be the first demonstration of polarized light utilization in a land based animal, gives a brief rendition of polarization as an optical phenomenon but leaves the reader wondering as to how this relates to butterflies in particular. What is it about the structure of the butterfly wing that allows it to polarize light? What is it about the structure of the butterfly eye that allows it perceive polarized light? Butterfly wing structure and eye structure In his paper, Shedding new light on nature s brightest signals, Darrell Kemp describes the extremely detailed structure of butterfly wings. According to Kemp, the surface of the wings of butterflies consists of thousands of very tiny overlapping scales and it is these structures that are responsible for wing patterns and colorations seen in butterflies. There are two basic ways by which color is generated on butterfly wings. One directs the formation of ordinary color, and the second leads to stunning iridescent color. Iridescent color is not produced by pigmentation. Instead it is produced by the diffraction and scattering of white light by the intricate structures on the surface of each scale. Close examination of a single scale shows a thin film multi-layer structure and it is this structure that is responsible for the phenomenon of polarization (Kemp, 2002). As mentioned by the CBC news article, polarization occurs when white light reflects off a certain surface, for example the scales on wings, and scatters in such a way that the 3
photons from which the light is composed all vibrate in the same direction as opposed to vibrating randomly; in other words, the light waves coming off the wings are in one plane of direction (Sweeney et al, 2003). The article touches on the fact that polarized light is seen quite differently through the eyes of different organisms. It states that to the human eye, polarized light is interpreted as a glare; the eyes of butterflies however, see something completely different- a pattern. The article neglects to mention that, although butterflies are physiologically sensitive to polarization, not all insects are. According to Sweeney, many insects have done away with the polarization sensitive photoreceptors in most parts of their eyes with remaining polarization sensitive ommatidia found only in their dorsal rims. Butterfly eyes, on the other hand are sensitive to polarization in all of their ommatidia (Sweeney et al 2003). Another difference arises between the visual system of butterflies and other insects. Normally, a visual system processes color and polarization separately. In butterflies however, color and polarization are analyzed jointly (Kelber, 1999). Heliconius butterflies The article provides information on the team involved in the particular study at hand, stating that the team is particularly interested in whether the polarizing signals reflected off the wings of female Heliconius cydno butterflies affect mating choices. At this point in the article, information becomes quite limited and scanty. Beyond describing briefly the appearance of H. cydno and stating that it thrives in tropical forests of Central and South America, the article presents little information as to the phylogeny or ecology of the butterfly. This is one area in which the article could be improved. 4
The paper The biology of Heliconius and related genera by Keith Brown provides some excellent information on Heliconius butterflies, otherwise known as Passion Flower or Passion Vine Butterflies (Brown, 1981). These neotropical insects belong to the nymphalid tribe, Heliconiini. They attained their common name of Passion Flower Butterflies through feeding on the Passifloraceae plants (passion flowers). Heliconius butterflies have developed a defense against the alkaloids and cyanogenic glycosides found in these plants. Members of the Heliconiini in turn store the poisonous chemicals derived from this plant into their tissues and as a result, become distasteful to predators (Turner, 1981). Heliconius butterflies that dwell in forest habitats, such as H. cydno, are predominately black with blue, red, orange, yellow, and white warning patterns on extended, long wings. Amongst other things, Heliconius butterflies are a group well known for Mullerian mimicry (Jiggens et al, 2001). In his paper, Jiggens gives the example of H. cydno mimicing the black and white coloration of H. sapho, which itself is inedible. Because it resembles H. sapho, predators learn to avoid H. cydno as well. All the above information would have provided the reader with a more complete and comprehensive portrayal of the species at hand. However, it is understandable that journalists in the media are restricted in the amount of information that they can disseminate to their readers. They are unable to get into too much detail because they run the risk of generating an overall lengthy and drawn out article that may lose the interest of their readers. Popular press articles are structured in such a way as to keep a reader with minimal science background engaged. Journalists are therefore forced to decide as to what information should be reported to the reader and what information can be 5
disregarded. Unfortunately, not all journalists have the necessary expertise to be able to discern significant scientific information from that which is insignificant. Use of polarized light in Heliconius cydno The article s reiteration of the experimental approaches taken by Alison Sweeney and her colleagues and the results that they obtained was well put together. However, it would have been useful for the CBC article to mention that Heliconius butterflies are known to rely on visual cues in sexual selection (Sweeney et al, 2003) To the males being tested, the team displayed the wings of female Heliconius cydno butterflies behind either a neutral or a depolarizing filter and moved them to simulate flight. The neutral filter allows polarized light reflecting off the female butterfly wings to shine through, while the depolarizing filter blocks this polarized light. The scientists reported in the journal Nature that males approached the wings behind the depolarizing filter significantly less often than they approached the wings behind the neutral filters that allows polarized light to shine through. These results indicate that indeed, polarized light may be used as a mating signal in these butterflies (Sweeney et al, 2003). The article also does well in conveying Sweeney s hypothesis as to why H. cydno evolved the ability to use polarized light. Sweeney believes that as H. cydno made the ecological move from meadow habitat to deep forest, it needed to develop alternative strategies to deal with its new environment (Sweeney et al, 2003). The ability to use and detect polarized light may be important for acquiring mates in the deep forest environment where light is greatly scattered and results in patchy light distribution. Polarized light may also be used for food acquisition, an idea put forth by Kelber in his 6
paper Why false colors are seen by butterflies. The color of leaves (for example, shiny or matt) often indicates their quality as a food source. Shiny and matt leaves reflect different amounts of polarized light and the author hypothesizes that a butterfly s ability to see polarized light allows it to discriminate between its food sources as it flies over them (Kelber, 1999). Conclusion After sifting through the scientific information behind the popular press article Shimmering wings dazzle boy butterflies of CBC news, I am able to judge as to whether the article was accurate in its reiteration of the study published in the May 1 st issue of the journal Nature. Although vague in its explanation of polarization in the context of butterfly wings and limited in its background information on the Heliconius cydno butterfly species, the article was sufficient in delivering the scientific findings of Alison Sweeney and her colleagues to the general public. 7
REFERENCES: Brown, Keith.1981.The biology of Heliconius and related genera. Annual Review of Entomology26: 427-56. Jiggins, Estrata, and Rodrigues.2004.Mimicry and the evolution of prematin isolation in Heliconius melpomene linnaeus. Journal of Evolutionary Biology. Jiggins Chris, Naisbit Russell, Coe Rebecca,and Mallet James. 2001.Reproductive isolation caused by color pattern mimicry. Nature. Volume 411. Kelber, Almut.1999.Why false colors are seen by butterflies.nature.volume 402. Kemp, Darrell.2002.Shedding new light on nature s brightest signals. Trends in Ecology and Evolution. Volume 17, No.7 Ringo,John. 1996.Sexual Receptivity in insects. Annual Review of Entomology 41:473-94. Rutowski, Ronald.1991.The evolution of male-mate locating behaviour in Butterflies. American Naturalist. Volume 138,No.5: 1121-1139. Sweeney Alison, Jiggins Chris, and Johnsen Sonke. 2003.Polarized light as a butterfly mating signa. Nature.Volume 423. Turner, John.1981.Adaptation and Evolution in Heliconius: A defense against neodarwinisim. Annual Review of Ecology 12:99-121 8
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