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First published online February 15, 2006
Journal of Experimental Biology 209, 789-800 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02053

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Crepuscular and nocturnal illumination and its effects on color perception by the nocturnal hawkmoth Deilephila elpenor

Sönke Johnsen^1,*, Almut Kelber², Eric Warrant², Alison M. Sweeney¹, Edith A. Widder³, Raymond L. Lee, Jr⁴ and Javier Hernández-Andrés⁵

¹ Biology Department, Duke University, Durham, NC 27708, USA
² Department of Cell and Organism Biology, Lund University, Sweden
³ Marine Science Division, Harbor Branch Oceanographic Institution, Fort Pierce, FL 34946, USA
⁴ Mathematics and Science Division, US Naval Academy, Annapolis, MD 21402, USA
⁵ Optics Department, University of Granada, Spain

View larger version (21K): [in a new window]   Fig. 1. (A) Spectral reflectance of stimuli (1=100%). (B) Spectral sensitivities of the photoreceptors of Deilephila elpenor assuming fused rhabdoms containing all three photoreceptor types. UV, B and G refer to the photoreceptors with peak absorption wavelengths of 350, 440, and 525 nm, respectively. Solid lines show normalized receptor sensitivities that were used to calculate relative quantum catches. The broken line shows the sensitivity of the green receptor that was used for the achromatic contrast calculations.

View larger version (20K): [in a new window]   Fig. 2. (A) Normalized quantal irradiance during sunset and twilight. The thick line denotes a spectrum taken at sunset (solar elevation –0.6°). The three lines with long-wavelength irradiance greater than at sunset denote solar elevations of 11°, 5.9° and 1.0° (in order of decreasing long-wavelength values). The three lines with 450 nm peak values greater than at sunset denote solar elevations of –3.6°, –6.5° and –9.3° (in order of increasing 450 nm peak values). (B) Normalized quantal irradiance due to three common sources of nocturnal illumination. Spectra in A and B are normalized so that their irradiances integrated from 350 to 700 nm are all equal. (C) Un-normalized spectra. All spectra presented in this study, with the exception of those taken within forests, are freely available from the authors.

View larger version (16K): [in a new window]   Fig. 3. (A) Human-based u'v' chromaticities of daylight, sunset, twilight and nocturnal irradiances. The upper starlight symbols for the Kitt Peak and La Palma starlight data denote the chromaticities during a solar maximum; the lower symbols denote the chromaticities during a solar minimum. For comparison, the chromaticities of a 7° diameter patch of moonless sky (zenith angle 45°) under thin clouds, clear skies and overcast conditions are also shown (Höhn and Büchtermann, 1973). The black line denotes sunset and twilight data from North Carolina. Its symbols show data taken at solar elevation intervals of about 2°. The colored circles next to Kitt Peak starlight and `–11°' show the human-perceived colors at those two chromaticity extremes. The Planckian locus shows the chromaticities of blackbody radiators as a function of temperature. Data points for this locus are every 500 K up to 5000 K, and every 1000 K up to 10000 K, after which each point is labelled. (B) Deilephila-based relative quantum catches for the data shown in A. The three corners depict illuminants that are absorbed by one receptor only. The broken line shows the quantum catches of the spectral colors, with points every 25 nm and numbers every 50 nm. Because 49 of the civil twilight spectra and all 220 forest spectra were not taken at UV wavelengths, their relative quantum catches could not be calculated.

View larger version (10K): [in a new window]   Fig. 4. (A,B) Deilephila-based relative quantum catches for the five different stimuli viewed under various sunset, twilight and nocturnal illuminants. Filled circles represent quantum catches of stimuli at sunset and twilight (solar elevation from 11° to –11°). Solar elevation decreases as data moves from right to left. Quantum catches under the nocturnal illuminants are to the right of those for sunset and twilight and consist of the following: open circles, quantum catches of stimuli under full moonlight; open triangles, quantum catches of stimuli under light polluted night sky; asterisks, quantum catches of stimuli under starlight only. (C) Quantum catches of the five stimuli assuming that D. elpenor has von Kries color constancy and is adapted to a background of green leaves under each illuminant (hence the central location of all the green stimuli). With the exception of light-polluted night skies (triangle), all the data have the same symbols for clarity.

View larger version (25K): [in a new window]   Fig. 5. (A) Achromatic contrast between four stimuli (the white evening primrose, the red hindwing of D. elpenor, the yellow flower, and the blue flower) and a green leaf background. Positive contrast indicates that the object is brighter than the background. (B) Chromatic contrast, defined as the distance between the relative quantum catches of the stimuli and the leaf background. (C) Chromatic contrast assuming that D. elpenor has von Kries color constancy and is adapted to a background of green leaves under each illuminant. (D) Coefficients of variation of the achromatic and chromatic contrasts of the four stimuli when viewed under nautical twilight, moonlight and starlight.

View larger version (16K): [in a new window]   Fig. 6. (A–C) Numbers of photons absorbed by a hypothetical photoreceptor with a given max relative to the number absorbed by the green receptor possessed by D. elpenor (max=525 nm) under three illuminants (nautical twilight, full moon, starlight). (A) Viewing the white evening primrose. (B) Viewing green leaves. (C) Viewing the red hindwing of a conspecific. (D) The variation in relative quantum catches (among nautical twilight, moonlight and starlight), in a fused rhabdom containing all three visual pigments, as a function of the wavelength of the long wavelength receptor. The variation is estimated by the area of the triangle formed by the three points in the Maxwell triangle. As in A–C, the variation at 525 nm is set to 1.

View larger version (25K): [in a new window]   Fig. 7. The variation of sunlight and moonlight relative to starlight as a function of the elevation of the sun or moon, the sky conditions, and the phase of the moon. A, N and C refer to astronomical, nautical and civil twilight, respectively. (Modified from Bond and Henderson, 1963.)