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Drinking in the dark

Natural History, March, 2003 by Stephan Reebs

Far from city lights, with only the Noon and stars to guide them, people see in shades of gray. That's because our eyes have just two kinds of photoreceptors--rods and cones--and the rods, the only receptors that work well in dim light, do not detect color. Until recently, biologists had assumed that all animals shared the same visual [imitation. But the animal world has a knack for coming up with species whose sensory powers surpass our own. This time, a humble nocturnal moth is our superior.

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The elephant hawkmoth (Deilephila elpenor) locates flowers in the dark of night and feeds on their nectar. When the moth's cousins, the butterflies, seek nectar in the daytime, they rely on color to distinguish and remember flowers particularly rich in nectar. Almut Kelber, Anna Balkenius, and Eric, J. Warrant, all biologists at Lund University in Sweden, thought that having co[or vision would be just as useful for the nocturnal hawkmoth. So they set out to prove for the first time ever that at least one animal can perceive co[or at night.

Under limited light, similar to that of late dusk, Kelber and her colleagues trained sixteen hawkmoths to find the sugar solution placed in the centers of artificial flowers. Some hawkmoths were trained to seek the blue flowers, others the yellow ones. Then the biologists dimmed the room to the level of starlight and presented the moths with a display of variously colored circles (minus the sugar solution). One circle was the animal's training hue (blue or yellow); the rest were various shades of gray. Almost unfailingly the moths chose--that is, touched first--the co[or to which they had been trained. By contrast, six people asked to discriminate among the disks under the same low light failed miserably. Once again, it seems, we are bested--though we do have the brains to prove this fact to ourselves. ("Scotopic colour vision in nocturnal hawkmoths," Nature 419:922-25, October 31, 2002)

Drinking in the dark

Natural History, March, 2003 by Stephan Reebs

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