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UV color a hidden signal for butterflies

Feb. 17, 2010
Courtesy Uni­vers­ity of Cal­i­for­nia Ir­vine
and World Science staff

But­terfly ex­perts have sus­pected for more than 150 years that vi­sion plays a key role in ex­plain­ing wing col­or di­vers­ity. Now, bi­ol­o­gists at the Uni­vers­ity of Cal­i­for­nia Ir­vine say they’ve found the first proof of this the­o­ry, at least for nine but­terfly spe­cies.

He­li­co­nius er­a­to but­ter­flies have evolved re­cep­tors in their eyes for de­tect­ing UV col­ors and they form UV-yellow pig­ment on their wings. (Im­age cour­te­sy Bill Ber­thet )


But­terflies that can see ul­tra­vi­o­let (or UV) col­ors al­so have UV-and-yel­low pig­ment on their wings, which only they can rec­og­nize, ac­cord­ing to the sci­en­tists.

The col­ora­t­ion lets but­terflies tell each oth­er apart when preda­tors can’t. This comes in handy be­cause in or­der to dis­cour­age preda­tors, some but­terfly spe­cies have evolved to look out­wardly iden­ti­cal to bad-tasting rel­a­tives. But that could make it hard for but­terflies to tell each oth­er part al­so. Find­ing an ap­pro­pri­ate mate might thus be dif­fi­cult.

The UV col­or-coding solves the prob­lem, sci­en­tists say.

“They’re not wast­ing their time chas­ing af­ter the wrong mate,” said Adri­ana Briscoe, an ecol­o­gist and ev­o­lu­tion­ary bi­ol­o­gist at the uni­vers­ity and lead au­thor of the stu­dy, pub­lished on­line re­cently in the jour­nal Pro­ceed­ings of the Na­tional Acad­e­my of Sci­ences.

Ul­tra­vi­o­let is a col­or that hu­mans can’t see, and only a few an­i­mals can. It comes af­ter vi­o­let at the end of the col­or spec­trum where light has high­er energy. Cer­tain but­terflies have a gene that con­fers UV vi­sion, but only if they have two cop­ies of the gene. 

Of the world’s 14,000 but­terfly spe­cies, only the He­li­co­nius but­terflies, a group of re­lat­ed spe­cies liv­ing in the forests of Mex­i­co and Cen­tral and South Amer­i­ca, are known to have the du­pli­cate gene. The di­verse wing pat­terns of He­li­co­nius but­terflies have al­so gen­er­at­ed much sci­en­tif­ic in­ter­est in re­cent years.

He­li­co­nius but­terflies de­vel­oped the nec­es­sary ex­tra UV-vi­sion gene 12 mil­lion to 25 mil­lion years ago, Briscoe and col­leagues be­lieve. With­in the same time frame, they be­gan dis­play­ing UV-yel­low pig­ment.

Af­ter re­search­ers dis­cov­ered the cop­ied gene, “we wanted to find out why it might be ad­van­ta­geous,” Briscoe said. They ex­am­ined thou­sands of wing-col­or patches and found that but­terflies with just one UV-vi­sion gene had non-UV yel­low wing pig­ment. But the pig­ment was UV in but­terflies with both genes.

“We think that by switch­ing to a new way of mak­ing yel­low, the mi­met­ic [si­mi­lar-look­ing] but­terfly spe­cies were bet­ter able to tell each oth­er apart,” Briscoe said. “We now have strong rea­son to be­lieve that we’ll find oth­er ex­am­ples in which vi­sion and wing col­ors are linked.”

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Butterfly experts have suspected for more than 150 years that vision plays a key role in explaining wing color diversity. Now, biologists at the University of California Irvine say they've found the first proof of this theory, at least for nine butterfly species. Butterflies that can see ultraviolet (or UV) colors also have UV-and-yellow pigment on their wings, which only they can recognize, according to the scientists. The coloration lets butterflies tell each other apart when predators can't. This comes in handy because in order to discourage predators, some butterfly species have evolved to look outwardly identical to bad-tasting relatives. But that could make it hard for butterflies to tell each other part also. Finding an appropriate mate might thus be difficult. The UV color-coding solves the problem, scientists say. “They're not wasting their time chasing after the wrong mate,“ said Adriana Briscoe, an ecologist and evolutionary biologist at the university and lead author of the study, published online recently in the journal Proceedings of the National Academy of Sciences. Ultraviolet is a color that humans can't see, and only a few animals can. It comes after violet at the higher-energy end of the color spectrum. Certain butterflies have a gene that confers UV vision, but only if they have two copies of the gene. Of the world's 14,000 butterfly species, only the Heliconius butterflies, a group of related species living in the forests of Mexico and Central and South America, are known to have the duplicate gene. The diverse wing patterns of Heliconius butterflies have also generated much scientific interest in recent years. Heliconius butterflies developed the necessary extra UV-vision gene 12 million to 25 million years ago, Briscoe and colleagues believe. Within the same time frame, they began displaying UV-yellow pigment. After researchers discovered the copied gene, “we wanted to find out why it might be advantageous,“ Briscoe said. They examined thousands of wing-color patches and found that butterflies with just one UV-vision gene had non-UV yellow wing pigment. But the pigment was UV in butterflies with both genes. “We think that by switching to a new way of making yellow, the mimetic butterfly species were better able to tell each other apart,“ Briscoe said. “We now have strong reason to believe that we'll find other examples in which vision and wing colors are linked.“