Female red postman butterflies see an extra colour in the ultraviolet part of the light spectrum that even the males of their species cannot.
While many animal species can see one colour of UV, the ability to see multiple colours in the UV spectrum is rare. Mantis shrimp and hummingbird hawkmoths may also see multiple colours of UV, according to at California State University, Long Beach, and at the University of California, Irvine.
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Finkbeiner trained 80 Heliconius erato, an American butterfly species, and 120 of two other butterfly species to sip from a feeder placed on a light source that was tuned to a specific UV wavelength. She says that while training butterflies is difficult, the relatively big brains of Heliconius butterflies make them 鈥渢he Einsteins of the butterfly world鈥�.
After training, the butterflies were allowed to fly within an enclosure where two UV light sources were each tuned to a different UV colour. The H. erato males and all the butterflies of the other two species flew towards whichever light was the brightest. When the two lights were equally bright, they landed on the light that had a watered-down honey reward on it about half the time, showing that they couldn鈥檛 tell the difference between the two UV colours, the researchers concluded.
The H. erato females correctly chose the UV colour associated with the honey reward under every testing condition, even if the other UV light was 15 times brighter.
鈥淓ach sex in this butterfly species literally sees the world through different eyes,鈥� says Briscoe. This suggests a division of labour by sex, perhaps similar to that seen in squirrel monkeys, where only females can differentiate between red and green, making them specialists in finding ripe fruits for their group.
In humans, we see this kind of sex-linked difference in colour vision in the greater likelihood of colour blindness in men than women, says Briscoe.
Finkbeiner says that sex-linked vision differences are common in butterflies, but the discovery of the ability to see two UV colours is a first in these insects. This shows that multiple UV wavelength detection is difficult, she says. It doesn鈥檛 help that the researchers can鈥檛 see the two UV colours themselves. 鈥淲ho knows how they appeared to the butterflies,鈥� says Finkbeiner. 鈥淭hey both looked purple to me.鈥�
Journal of Experimental Biology
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