Sönke Johnsen is fascinated by worlds we cannot see, whether it's the ocean depths or the nocturnal world. But just because these worlds are hidden from our eyes doesn't mean they are invisible to the species that populate them; they are well adapted to dim conditions. One sense thought to be of little use to light deprived creatures was colour vision; we only see the twilight world in shades of grey as coloured light receptors fail when the light is low. That was until Almut Kelber discovered that some nocturnal hawkmoths and geckos are capable of colour vision, even in the dimmest starlight. This remarkable discovery set Johnsen wondering. How would the world appear to nocturnal creatures when bathed in the shifting shades of night and twilight? Would nocturnal creatures benefit from a Technicolor world? Johnsen and his colleagues set about collecting spectra at different times of day in his quest to see the twilight world through hawkmoth eyes(p. 789).
Fortunately, physicists Javier Hernández-Andrés and Raymond Lee Jr, had already collected many of the daylight spectra that Johnsen was curious to investigate from a hawkmoth's perspective. But collecting spectra at night was easier said than done; much of the planet is blanketed in the orange haze of light pollution after dusk, making it almost impossible to record natural light spectra between dusk and sunrise. Turning to the International Dark Sky Association's light pollution maps, Johnsen, Alison Sweeney and Edith Widder headed out to particularly remote and dark sites to collect spectra as twilight passed into night on full- and moonless nights. But the team were defeated when they tried measuring starlight's faint spectrum. Even after heading out to sea to escape land's polluted night skies,their equipment wasn't sensitive enough. The team finally resorted to calculating the spectrum.
With thousands of spectra in hand, Johnsen was amazed at the range of colours that illuminate the world as daylight fails. At the height of the day,the spectrum was fairly white, but as the sun descended beneath the horizon during twilight, the shade changed to a deep blue. Johnsen admits that he was puzzled by this colour change until he learned that the spectrum was caused when ozone absorbs certain wavelengths of light, producing the deep blue hue. Johnsen adds he was most surprised when he analysed the starlight spectrum. It was red.
But how would these light changes affect the hawkmoth's vision? Teaming up with Kelber and Eric Warrant, they calculated how the insect saw coloured flowers under different illuminating colours. The team found that yellow and blue flowers were clearly visible in some lights as sunrise and sunset progressed, and when they tested the insect's view of its preferred colour,white, the flower stood out well at all stages of night and day. Kelber also calculated how much the colours shifted in the insect's visual system as the illuminating colours changed, and found that although the flowers became brighter and dimmer as the light varied, they were still more recognisable against a leafy background when viewed in colour than if the moth was restricted to black and white vision. Johnsen explains that although colour vision is less sensitive than monochrome, the added dimension gives hawkmoths a clearer dusk view.