VOLUME 209 (5) MARCH 2006 789 The Journal of Experimental Biology 209, 789-800 Published by The Company of Biologists 2006 doi:10.1242/jeb.02053 Crepuscular and nocturnal illumination and its effects on color perception by the nocturnal hawkmoth Deilephila elpenor Sönke Johnsen1,*, Almut Kelber2, Eric Warrant2, Alison M. Sweeney1, Edith A. Widder3, Raymond L. Lee, Jr4 and Javier Hernández-Andrés5 1Biology Department, Duke University, Durham, NC 27708, USA, 2Department of Cell and Organism Biology, Lund University, Sweden, 3Marine Science Division, Harbor Branch Oceanographic Institution, Fort Pierce, FL 34946, USA, 4Mathematics and Science Division, US Naval Academy, Annapolis, MD 21402, USA and 5Optics Department, University of Granada, Spain *Author for correspondence (e-mail: [email protected]) Accepted 20 December 2005 Summary Recent studies have shown that certain nocturnal insect without von Kries color constancy) of the flowers and and vertebrate species have true color vision under hindwings against a leaf background were determined nocturnal illumination. Thus, their vision is potentially under the various lighting environments. The twilight and affected by changes in the spectral quality of twilight and nocturnal illuminants were substantially different from nocturnal illumination, due to the presence or absence of each other, resulting in significantly different contrasts. the moon, artificial light pollution and other factors. We The addition of von Kries color constancy significantly investigated this in the following manner. First we reduced the effect of changing illuminants on chromatic measured the spectral irradiance (from 300 to 700·nm) contrast, suggesting that, even in this light-limited during the day, sunset, twilight, full moon, new moon, and environment, the ability of color vision to provide reliable in the presence of high levels of light pollution. The signals under changing illuminants may offset the spectra were then converted to both human-based concurrent threefold decrease in sensitivity and spatial chromaticities and to relative quantum catches for the resolution. Given this, color vision may be more common nocturnal hawkmoth Deilephila elpenor, which has color in crepuscular and nocturnal species than previously vision. The reflectance spectra of various flowers and considered. leaves and the red hindwings of D. elpenor were also converted to chromaticities and relative quantum catches. Key words: hawkmoth, Deilephila elpenor, nocturnal vision, color Finally, the achromatic and chromatic contrasts (with and vision, environmental optics. Introduction vision presents additional difficulties for nocturnal species. While multiple visual pigments in certain deep-sea species While the decrease in sensitivity associated with the increase (Cronin and Frank, 1996; Douglas et al., 1998) and multiple in the number of visual channels has little effect on species rod types in amphibians (Makino-Tasaka and Suzuki, 1984) operating during light-saturated diurnal conditions, this have been known for some time, unambiguous evidence sensitivity loss can potentially affect the ability of nocturnal for true color vision under scotopic conditions has only species to function in their light-limited environment. It is recently been acquired (Kelber et al., 2002; Roth and Kelber, primarily for this reason that color vision has generally been 2004). These behavioral studies, which show that the expected to be rare or absent among nocturnal species (Jacobs, nocturnal hawkmoth Deilephila elpenor and the nocturnal 1993). helmet gecko Tarentola chazaliae can discern color under Second, what color are objects when viewed under the night starlight and dim moonlight, respectively, raise at least two sky? Although not perceived by humans, the spectrum of the issues. night sky is not neutral, and depends on multiple factors, First, what is the selective advantage of color vision in these including how far the sun is below the horizon, the presence species that outweighs its costs? Color vision’s detrimental or absence and phase of the moon and, recently, on the level effect on spatial resolution and the additional structural and of light pollution (e.g. Munz and McFarland, 1977; Endler, neurological complexity required for color processing makes 1991; Leinert et al., 1998; McFarland et al., 1999; Cinzano et it a more difficult proposition for all species. However, color al., 2001; Hernández-Andrés et al., 2001; Lee and Hernández- THE JOURNAL OF EXPERIMENTAL BIOLOGY 790 S. Johnsen and others Andrés, 2003). It has long been known that the variation of canopy using a model of color vision for the nocturnal daytime spectra, due to cloud cover, solar elevation, forest hawkmoth, Deilephila elpenor L. canopy and depth (for aquatic species), has a substantial effect on the appearance and visibility of objects and organisms, Measurement of twilight spectra which can be at least partly ameliorated by color vision Fourteen sunset and twilight measurements of spectral (Wyszecki and Stiles, 1982; Endler, 1991; McFarland et al., irradiance under minimal cloud cover were taken on the 1999; Johnsen and Sosik, 2003; Lovell et al., 2005). Less work, beaches of two barrier islands located off the coast of North however, has been done on the appearance of objects during Carolina, USA (Atlantic Beach; 34°42ЈN 76°44ЈW and Cape twilight (reviewed by McFarland et al., 1999; Rickel and Hatteras National Seashore; 35°44ЈN 75°32ЈW, both at sea Genin, 2005), and, to our knowledge, the appearance of objects level) on 11 June, 12 June and 17 July, 2004. The locations under different nocturnal illuminants has received very little were chosen to maximize the view of the sky and minimize the attention. effects of anthropogenic light. Spectra were taken using a This study measures or models spectral irradiance USB2000 spectrometer (Ocean Optics Inc., Dunedin, FL, (300–700·nm) during daylight, sunset, twilight, moonlit nights, USA) that had been modified for increased sensitivity by moonless nights and nights in regions with high light pollution. increasing the width of the entrance slit to 200·␮m and These spectra, in addition to previously published data, are focusing light onto the detector array with a collector lens (L2 then used to calculate the relative quantum catches of the three collector lens, Ocean Optics). The spectrometer was fitted with photoreceptors of D. elpenor under different lighting a 1·mm diameter fiber optic cable that viewed a horizontal slab conditions. In addition to the general illuminants, relative of a Lambertian reflector (Spectralon, Labsphere Inc., North quantum catches of five stimuli (green leaves, three flowers Sutton, NH, USA). Because Lambertian materials reflect light and the red hindwing of D. elpenor) are also calculated. Three evenly in all directions, their radiance is proportional to the different types of contrasts of the latter four stimuli viewed irradiance striking them (Palmer, 1995). This method of against green leaves are then determined: (1) achromatic obtaining the cosine response needed for measuring diffuse contrast, (2) chromatic contrast and (3) chromatic contrast irradiance was chosen because it is more efficient than the assuming von Kries color constancy. Finally, quantum catches typical diffusely transmitting disk (Doxaran et al., 2004). of hypothetical photoreceptors with varying wavelengths of Spectra were taken at solar elevations ranging from +11° to peak absorption are compared to the catches of the long –11° (elevations determined using tables from the United wavelength receptor in D. elpenor under the different States Naval Observatory). At lower solar elevations, the illuminants. integration time of the spectrometer was increased to a maximum of 10·s, with 30 such integrations averaged per measurement. Spectra were taken from 300 to 700·nm and Materials and methods averaged over 5·nm intervals. General approach The goal of this study was to determine the range of spectra Measurement of full moonlight and synthesis of starlight found during sunset, twilight and night. Therefore, rather than spectra measure a large number of spectra under all possible celestial Spectral irradiance under the full moon was measured using and atmospheric conditions, we measured spectra under a spectrometer with a highly sensitive photomultiplier detector various extreme conditions. Both human-based chromaticities (OL-754-PMT, Optronics Laboratories Inc., Orlando, FL, and the relative quantum catches described below have the USA). Spectra were taken on 10 December, 2003 at Harbor property that the value of the mixture of two illuminants falls Branch Oceanographic Institution (Fort Pierce, FL, USA; between the values of the two illuminants alone (Wyszecki 27°26ЈN 80°19ЈW, sea level) during the full moon (elevation and Stiles, 1982). Thus, by measuring the spectra under 69°, moon 98% full). An integrating sphere was used to ensure conditions where one of the various contributors to the a cosine angular response. Data were taken at 5·nm intervals illumination dominates, we can define the boundaries of the from 350 to 700·nm. region where most spectra are found. The following four Preliminary attempts showed that even the OL-754 conditions were thus of particular interest: (1) clear nautical spectrometer was not sensitive enough to measure spectral twilight (solar elevation between –6° and –12°), (2) full moon irradiance on a moonless night. Therefore it was calculated in at high elevation under clear skies, (3) moonless and