Some colours are the visual equivalent of a scream; from ‘Eat me’, to ‘Don't eat me’, to ‘Come hither’, plants and animals constantly flaunt their talents or warn each other off with pageants of colour. Adriana Briscoe from the University of California, Irvine, USA, explains that other animals sometimes take advantage of the deterrent effect of warning signals and imitate the bright markings to avoid the unwanted attentions of predators. One family that takes advantage of this form of mimicry are the Eueides butterflies that mimic the distinctive yellow bars found on the wings of their toxic distant Heliconius butterfly relatives. However, when Briscoe analysed the yellow pigment in Eueides butterfly wings and other members of the extended family, she found that it was different from the 3-hydroxy-dl-kynurenine (3-OHK) pigment that Keith Brown had found in Heliconius wings, even though the warning markings were extraordinarily similar to our eyes. Why had the butterflies adopted different pigments when both produced a bright yellow warning hue?
Knowing that many animals see a broader palette of colours than are visible to us, Briscoe wondered whether the warning bar on the wings of red postman butterflies – Heliconius erato petiverana, which are members of the Heliconius family – is visible under UV light and was impressed when she discovered that it is. As Heliconius butterflies have an additional light receptor that is sensitive to violet shades, she realised that their 3-OHK-pigmented bar would appear completely different from the yellow shade that we see on Eueides’ wings; ‘this “UV-yellow” is a butterfly form of human purple, which is a mixture of red and blue light’, explains Briscoe. Could the 3-OHK marking be sending coded messages for the eyes of Heliconius butterflies only?
Printing out thousands of paper butterflies with the red postman's distinctive red and black pattern, Susan Finkbeiner added yellow Manila paper covered with a UV filter to produce the correct shade of yellow without the UV component, while she reproduced the true red postman yellow shade with synthetic 3-OHK. In addition, some of the models were painted with yellow UV-reflective paint while others were covered with a UV filter. Then, she tested the attractiveness of the paper imposters to red postman butterflies and hungry birds. Analysing the butterflies’ responses, Finkbeiner, Briscoe and Daniel Osorio were impressed that the red postman butterflies always preferred the fake butterflies with 3-OHK UV in their yellow bands; and when they monitored the butterflies for longer periods, the insects made amorous advances to the 3-OHK models, while they were not seduced by the plain yellow paint substitutes. However, when Finkbeiner posed all four paper models in the forest to find out whether predatory birds had a preference for one pigment over the other, the birds found all of the models equally delectable. In addition, Briscoe made the exciting discovery that the 3-OHK pigment was fluorescent, producing a faint green glow: ‘We thought that the fluorescence might be contributing to the colour signal the butterflies were responding to’, says Briscoe. However, when Dmitry Fishman calculated the strength of the fluorescent message, the fluorescence was too faint to be seen when the butterflies are active in daylight.
Briscoe suspects that toxic Heliconius butterflies adopted the novel UV-reflective 3-OHK pigment, in preference to the yellow pigment used by Eueides and other members of their extended family, to avoid confusion. ‘They [Heliconius] have two problems to solve: avoiding predators like birds and finding appropriate mates’, says Briscoe. She suggests that Heliconius butterflies evolved 3-OHK because it resembles the ancestral yellow sufficiently for birds to learn to give butterflies with yellow warning bars a wide berth, while allowing members of the Heliconius family to distinguish their own species from distant relatives when seeking a mate.
